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Spitfire Parade - Crisis
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FFmpeg.Autogen : Issue with Zero-Sized Atom Boxes in MP4 Output
16 juin 2024, par Alexander JanssonI just started learning ffmpeg using ffmpeg.autogen wrapper version 5.1 in c#, and ffmpeg shared libs version 5.1. Im trying to facilitate a class which screenrecords using gdigrab and produces streamable mp4 to a/an buffer/event. Everything seems to work as suposed to with no error except that the outputstream produces atom boxes with 0 in size thus small file size aswell, no data seems to be produced in the boxes, the "debug test mp4 file" is analyzed with MP4Box and the box info is provided in the thread.


To be more specific why does this code produce empty atomboxes, is someone able to make the data produced actually contain any frame data from the gdigrab editing my code ?


`code :


public unsafe class ScreenStreamer : IDisposable
 {
 private readonly AVCodec* productionCodec;
 private readonly AVCodec* screenCaptureAVCodec;
 private readonly AVCodecContext* productionAVCodecContext;
 private readonly AVFormatContext* productionFormatContext;
 private readonly AVCodecContext* screenCaptureAVCodecContext;
 private readonly AVDictionary* productionAVCodecOptions;
 private readonly AVInputFormat* screenCaptureInputFormat;
 private readonly AVFormatContext* screenCaptureInputFormatContext;
 private readonly int gDIGrabVideoStreamIndex;
 private readonly System.Drawing.Size screenBounds;
 private readonly int _produceAtleastAmount;
 public EventHandler OnNewVideoDataProduced;
 private MemoryStream unsafeToManagedBridgeBuffer;
 private CancellationTokenSource cancellationTokenSource;
 private Task recorderTask;

 public ScreenStreamer(int fps, int bitrate, int screenIndex, int produceAtleastAmount = 1000)
 {
 ffmpeg.avdevice_register_all();
 ffmpeg.avformat_network_init();
 recorderTask = Task.CompletedTask;
 cancellationTokenSource = new CancellationTokenSource();
 unsafeToManagedBridgeBuffer = new MemoryStream();
 _produceAtleastAmount = produceAtleastAmount;

 // Allocate and initialize production codec and context
 productionCodec = ffmpeg.avcodec_find_encoder(AVCodecID.AV_CODEC_ID_H264);
 if (productionCodec == null) throw new ApplicationException("Could not find encoder for codec ID H264.");

 productionAVCodecContext = ffmpeg.avcodec_alloc_context3(productionCodec);
 if (productionAVCodecContext == null) throw new ApplicationException("Could not allocate video codec context.");

 // Set codec parameters
 screenBounds = RetrieveScreenBounds(screenIndex);
 productionAVCodecContext->width = screenBounds.Width;
 productionAVCodecContext->height = screenBounds.Height;
 productionAVCodecContext->time_base = new AVRational() { den = fps, num = 1 };
 productionAVCodecContext->pix_fmt = AVPixelFormat.AV_PIX_FMT_YUV420P;
 productionAVCodecContext->bit_rate = bitrate;

 int result = ffmpeg.av_opt_set(productionAVCodecContext->priv_data, "preset", "veryfast", 0);
 if (result != 0)
 {
 throw new ApplicationException($"Failed to set options with error code {result}.");
 }

 // Open codec
 fixed (AVDictionary** pm = &productionAVCodecOptions)
 {
 result = ffmpeg.av_dict_set(pm, "movflags", "frag_keyframe+empty_moov+default_base_moof", 0);
 if (result < 0)
 {
 throw new ApplicationException($"Failed to set dictionary with error code {result}.");
 }

 result = ffmpeg.avcodec_open2(productionAVCodecContext, productionCodec, pm);
 if (result < 0)
 {
 throw new ApplicationException($"Failed to open codec with error code {result}.");
 }
 }

 // Allocate and initialize screen capture codec and context
 screenCaptureInputFormat = ffmpeg.av_find_input_format("gdigrab");
 if (screenCaptureInputFormat == null) throw new ApplicationException("Could not find input format gdigrab.");

 fixed (AVFormatContext** ps = &screenCaptureInputFormatContext)
 {
 result = ffmpeg.avformat_open_input(ps, "desktop", screenCaptureInputFormat, null);
 if (result < 0)
 {
 throw new ApplicationException($"Failed to open input with error code {result}.");
 }

 result = ffmpeg.avformat_find_stream_info(screenCaptureInputFormatContext, null);
 if (result < 0)
 {
 throw new ApplicationException($"Failed to find stream info with error code {result}.");
 }
 }

 gDIGrabVideoStreamIndex = -1;
 for (int i = 0; i < screenCaptureInputFormatContext->nb_streams; i++)
 {
 if (screenCaptureInputFormatContext->streams[i]->codecpar->codec_type == AVMediaType.AVMEDIA_TYPE_VIDEO)
 {
 gDIGrabVideoStreamIndex = i;
 break;
 }
 }

 if (gDIGrabVideoStreamIndex < 0)
 {
 throw new ApplicationException("Failed to find video stream in input.");
 }

 AVCodecParameters* codecParameters = screenCaptureInputFormatContext->streams[gDIGrabVideoStreamIndex]->codecpar;
 screenCaptureAVCodec = ffmpeg.avcodec_find_decoder(codecParameters->codec_id);
 if (screenCaptureAVCodec == null)
 {
 throw new ApplicationException("Could not find decoder for screen capture.");
 }

 screenCaptureAVCodecContext = ffmpeg.avcodec_alloc_context3(screenCaptureAVCodec);
 if (screenCaptureAVCodecContext == null)
 {
 throw new ApplicationException("Could not allocate screen capture codec context.");
 }

 result = ffmpeg.avcodec_parameters_to_context(screenCaptureAVCodecContext, codecParameters);
 if (result < 0)
 {
 throw new ApplicationException($"Failed to copy codec parameters to context with error code {result}.");
 }

 result = ffmpeg.avcodec_open2(screenCaptureAVCodecContext, screenCaptureAVCodec, null);
 if (result < 0)
 {
 throw new ApplicationException($"Failed to open screen capture codec with error code {result}.");
 }
 }

 public void Start()
 {
 recorderTask = Task.Run(() =>
 {
 AVPacket* packet = ffmpeg.av_packet_alloc();
 AVFrame* rawFrame = ffmpeg.av_frame_alloc();
 AVFrame* compatibleFrame = null;
 byte* dstBuffer = null;

 try
 {
 while (!cancellationTokenSource.Token.IsCancellationRequested)
 {
 if (ffmpeg.av_read_frame(screenCaptureInputFormatContext, packet) >= 0)
 {
 if (packet->stream_index == gDIGrabVideoStreamIndex)
 {
 int response = ffmpeg.avcodec_send_packet(screenCaptureAVCodecContext, packet);
 if (response < 0)
 {
 throw new ApplicationException($"Error while sending a packet to the decoder: {response}");
 }

 response = ffmpeg.avcodec_receive_frame(screenCaptureAVCodecContext, rawFrame);
 if (response == ffmpeg.AVERROR(ffmpeg.EAGAIN) || response == ffmpeg.AVERROR_EOF)
 {
 continue;
 }
 else if (response < 0)
 {
 throw new ApplicationException($"Error while receiving a frame from the decoder: {response}");
 }

 compatibleFrame = ConvertToCompatiblePixelFormat(rawFrame, out dstBuffer);

 response = ffmpeg.avcodec_send_frame(productionAVCodecContext, compatibleFrame);
 if (response < 0)
 {
 throw new ApplicationException($"Error while sending a frame to the encoder: {response}");
 }

 while (response >= 0)
 {
 response = ffmpeg.avcodec_receive_packet(productionAVCodecContext, packet);
 if (response == ffmpeg.AVERROR(ffmpeg.EAGAIN) || response == ffmpeg.AVERROR_EOF)
 {
 break;
 }
 else if (response < 0)
 {
 throw new ApplicationException($"Error while receiving a packet from the encoder: {response}");
 }

 using var packetStream = new UnmanagedMemoryStream(packet->data, packet->size);
 packetStream.CopyTo(unsafeToManagedBridgeBuffer);
 byte[] managedBytes = unsafeToManagedBridgeBuffer.ToArray();
 OnNewVideoDataProduced?.Invoke(this, managedBytes);
 unsafeToManagedBridgeBuffer.SetLength(0);
 }
 }
 }
 ffmpeg.av_packet_unref(packet);
 ffmpeg.av_frame_unref(rawFrame);
 if (compatibleFrame != null)
 {
 ffmpeg.av_frame_unref(compatibleFrame);
 ffmpeg.av_free(dstBuffer);
 }
 }
 }
 finally
 {
 ffmpeg.av_packet_free(&packet);
 ffmpeg.av_frame_free(&rawFrame);
 if (compatibleFrame != null)
 {
 ffmpeg.av_frame_free(&compatibleFrame);
 }
 }
 });
 }

 public AVFrame* ConvertToCompatiblePixelFormat(AVFrame* srcFrame, out byte* dstBuffer)
 {
 AVFrame* dstFrame = ffmpeg.av_frame_alloc();
 int buffer_size = ffmpeg.av_image_get_buffer_size(productionAVCodecContext->pix_fmt, productionAVCodecContext->width, productionAVCodecContext->height, 1);
 byte_ptrArray4 dstData = new byte_ptrArray4();
 int_array4 dstLinesize = new int_array4();
 dstBuffer = (byte*)ffmpeg.av_malloc((ulong)buffer_size);
 ffmpeg.av_image_fill_arrays(ref dstData, ref dstLinesize, dstBuffer, productionAVCodecContext->pix_fmt, productionAVCodecContext->width, productionAVCodecContext->height, 1);

 dstFrame->format = (int)productionAVCodecContext->pix_fmt;
 dstFrame->width = productionAVCodecContext->width;
 dstFrame->height = productionAVCodecContext->height;
 dstFrame->data.UpdateFrom(dstData);
 dstFrame->linesize.UpdateFrom(dstLinesize);

 SwsContext* swsCtx = ffmpeg.sws_getContext(
 srcFrame->width, srcFrame->height, (AVPixelFormat)srcFrame->format,
 productionAVCodecContext->width, productionAVCodecContext->height, productionAVCodecContext->pix_fmt,
 ffmpeg.SWS_BILINEAR, null, null, null);

 if (swsCtx == null)
 {
 throw new ApplicationException("Could not initialize the conversion context.");
 }

 ffmpeg.sws_scale(swsCtx, srcFrame->data, srcFrame->linesize, 0, srcFrame->height, dstFrame->data, dstFrame->linesize);
 ffmpeg.sws_freeContext(swsCtx);
 return dstFrame;
 }

 private System.Drawing.Size RetrieveScreenBounds(int screenIndex)
 {
 return new System.Drawing.Size(1920, 1080);
 }

 public void Dispose()
 {
 cancellationTokenSource?.Cancel();
 recorderTask?.Wait();
 cancellationTokenSource?.Dispose();
 recorderTask?.Dispose();
 unsafeToManagedBridgeBuffer?.Dispose();

 fixed (AVCodecContext** p = &productionAVCodecContext)
 {
 if (*p != null)
 {
 ffmpeg.avcodec_free_context(p);
 }
 }
 fixed (AVCodecContext** p = &screenCaptureAVCodecContext)
 {
 if (*p != null)
 {
 ffmpeg.avcodec_free_context(p);
 }
 }

 if (productionFormatContext != null)
 {
 ffmpeg.avformat_free_context(productionFormatContext);
 }

 if (screenCaptureInputFormatContext != null)
 {
 ffmpeg.avformat_free_context(screenCaptureInputFormatContext);
 }

 if (productionAVCodecOptions != null)
 {
 fixed (AVDictionary** p = &productionAVCodecOptions)
 {
 ffmpeg.av_dict_free(p);
 }
 }
 }
 }


I call Start method and wait 8 econds, out of scope I write the bytes to an mp4 file without using the write trailer just to debug the atomboxes. and the mp4 debugging box output I got :


(Full OUTPUT)
https://pastebin.com/xkM4MfG7



(Not full)




"
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-
Setting up OpenCV 2.4.6.1 on Ubuntu 12.04.02
12 août 2013, par DamilolaI had previously used OpenCV 2.4.5 with some certain configs and packages on Ubuntu 12.04.1 but had issues upgrading to OpenCV 2.4.6.1 on Ubuntu 12.04.2
I would like to share some ideas (a compilation of noteworthy information gathered from several sources including SO, ubuntu.org, asklinux.org and many other ; and of course by trying several procedures)
Below is what eventually got me through.
NOTE : ensure you uninstall any previous installation of OpenCV, FFMpeg and other dependencies previously installed.
STEP 1 (install ffmpeg and dependencies)
# goto http://sourceforge.net/projects/opencvlibrary/files/opencv-unix/
# download the latest stable opencv such as 2.4.6.1 (http://sourceforge.net/projects/opencvlibrary/files/opencv-unix/2.4.5/opencv-2.4.5.1.tar.gz/download) to current directory (such as home or ~/Document)
# cd /opt
# tar -xvf /OpenCV-2.4.6.1.tar.gz
# cd OpenCV-2.4.6.1
# create a foler under current dir (following previous step, this should be ), called prepare
# cd prepare
# Copy the following script to gedit and save as install.sh to current dir, this should be /prepare
# Check corresponding url used in the script for latest versions of the package and replace as required
# Open terminal and navigate to location used above
# sudo chmod +x install.sh
# ./installecho "Removing any pre-installed ffmpeg, x264, and other dependencies (not all the previously installed dependecies)"
sudo apt-get remove ffmpeg x264 libx264-dev libvpx-dev librtmp0 librtmp-dev libopencv-dev
sudo apt-get update
arch=$(uname -m)
if [ "$arch" == "i686" -o "$arch" == "i386" -o "$arch" == "i486" -o "$arch" == "i586" ]; then
flag=0
else
flag=1
fi
echo "Installing Dependenices"
sudo apt-get install autoconf automake make g++ curl cmake bzip2 python unzip \
build-essential checkinstall git git-core libass-dev libgpac-dev \
libsdl1.2-dev libtheora-dev libtool libva-dev libvdpau-dev libvorbis-dev libx11-dev \
libxext-dev libxfixes-dev pkg-config texi2html zlib1g-dev
echo "downloading yasm (assembler used by x264 and FFmpeg)"
# use git or tarball (not both)
wget http://www.tortall.net/projects/yasm/releases/yasm-1.2.0.tar.gz
tar xzvf yasm-1.2.0.tar.gz
cd yasm-1.2.0
echo "installing yasm"
./configure
make
sudo make install
cd ..
echo 'READ NOTE BELOW which was extracted from http://wiki.serviio.org/doku.php?id=build_ffmpeg_linux'
echo 'New version of x264 contains by default support of OpenCL. If not installed or without sense (example Ubuntu 12.04LTS on VMWare) add to configure additional option --disable-opencl. Without this option ffmpeg could not be configured (ERROR: libx264 not found).'
echo "downloading x264 (H.264 video encoder)"
# use git or tarball (not both)
# git clone http://repo.or.cz/r/x264.git or
git clone git://git.videolan.org/x264.git
cd x264
# wget ftp://ftp.videolan.org/pub/videolan/x264/snapshots/x264-snapshot-20130801-2245-stable.tar.bz2
# tar -xvjf x264-snapshot-20130801-2245-stable.tar.bz2
# cd x264-snapshot-20130801-2245-stable/
echo "Installing x264"
if [ $flag -eq 0 ]; then
./configure --enable-static --disable-opencl
else
./configure --enable-shared --enable-pic --disable-opencl
fi
make
sudo make install
cd ..
echo "downloading fdk-aac (AAC audio encoder)"
# use git or tarball (not both)
git clone --depth 1 git://github.com/mstorsjo/fdk-aac.git
cd fdk-aac
echo "installing fdk-aac"
autoreconf -fiv
./configure --disable-shared
make
sudo make install
cd ..
echo "installing libmp3lame-dev (MP3 audio encoder.)"
sudo apt-get install libmp3lame-dev
echo "downloading libopus (Opus audio decoder and encoder.)
wget http://downloads.xiph.org/releases/opus/opus-1.0.3.tar.gz
tar xzvf opus-1.0.3.tar.gz
cd opus-1.0.3
echo "installing libopus"
./configure --disable-shared
make
sudo make install
cd ..
echo "downloading libvpx VP8/VP9 video encoder and decoder)
# use git or tarball (not both)
git clone --depth 1 http://git.chromium.org/webm/libvpx.git
cd libvpx
# wget http://webm.googlecode.com/files/libvpx-v1.1.0.tar.bz2 (this seems not to be update, but can still be used if the fedoraproject link below is not available))
# wget http://pkgs.fedoraproject.org/repo/pkgs/libvpx/libvpx-v1.2.0.tar.bz2/400d7c940c5f9d394893d42ae5f463e6/libvpx-v1.2.0.tar.bz2
# tar xvjf libvpx-v1.2.0.tar.bz2
# cd libvpx-v1.2.0
echo "installing libvpx"
./configure --disable-examples
make
sudo make install
cd ..
sudo ldconfig
echo "downloading ffmpeg"
# git clone http://repo.or.cz/r/ffmpeg.git
git clone git://source.ffmpeg.org/ffmpeg.git
cd ffmpeg/
# wget http://ffmpeg.org/releases/ffmpeg-2.0.tar.bz2
# tar -xvjf ffmpeg-2.0.tar.bz2
# cd ffmpeg-2.0/
echo "installing ffmpeg"
if [ $flag -eq 0 ]; then
./configure --enable-gpl --enable-libass --enable-libfdk-aac --enable-libopus --enable-libfaac --enable-libmp3lame --enable-libopencore-amrnb --enable-libopencore-amrwb --enable-libtheora --enable-libvorbis --enable-libx264 --enable-libxvid --enable-nonfree --enable-postproc --enable-version3 --enable-x11grab --enable-libvpx
else
./configure --enable-gpl --enable-libass --enable-libfdk-aac --enable-libopus --enable-libfaac --enable-libmp3lame --enable-libopencore-amrnb --enable-libopencore-amrwb --enable-libtheora --enable-libvorbis --enable-libx264 --enable-libxvid --enable-nonfree --enable-postproc --enable-version3 --enable-x11grab --enable-libvpx --enable-shared
fi
make
sudo make install
hash -r
cd .. # move up one level to prepare folder
cd .. # move up one level to opencv folder
echo "Checking to see if you're using your new ffmpeg"
ffmpeg 2>&1 | head -n1
sudo ldconfigSTEP 2 (Install OpenCV and necessary packages)
echo "Installing Dependenices"
sudo apt-get install libopencv-dev
sudo apt-get install libtiff4-dev libjpeg-dev libjasper-dev
echo "installing Video I/O libraries, support for Firewire video cameras and video streaming libraries"
sudo apt-get install libav-tools libavcodec-dev libavformat-dev libswscale-dev libdc1394-22-dev libxine-dev libgstreamer0.10-dev libgstreamer-plugins-base0.10-dev libv4l-dev v4l-utils v4l-conf
echo "installing the Python development environment and the Python Numerical library"
sudo apt-get install python-dev python-numpy
echo "installing the parallel code processing library (the Intel tbb library)"
sudo apt-get install libtbb-dev
echo "installing the Qt dev library"
sudo apt-get install libqt4-dev libgtk2.0-dev
echo "installing other dependencies (if need be it would upgrade current version of the packages)"
sudo apt-get install patch subversion ruby librtmp0 librtmp-dev libfaac-dev libmp3lame-dev libopencore-amrnb-dev libopencore-amrwb-dev libvpx-dev libxvidcore-dev
echo installing optional packages"
sudo apt-get install libdc1394-utils libdc1394-22-dev libdc1394-22 libjpeg-dev libpng-dev libtiff-dev libjasper-devSTEP 3 (run ldconfig)
# Open a new terminal window
# Open /etc/ld.so.conf and check,
# if the paths "/usr/lib" and "/usr/local/lib" including the quote exist in the file. If not, add them manually or by
sudo echo "/usr/local/lib" >> /etc/ld.so.conf
sudo echo "/usr/lib" >> /etc/ld.so.conf
# execute the following
sudo ldconfigSTEP 4
# still ensure you haven't close the new terminal window open in STEP 3
# execute the following
mkdir build
cd build
cmake -D CMAKE_BUILD_TYPE=RELEASE -D CMAKE_INSTALL_PREFIX=/usr/local -D BUILD_SHARED_LIBS=OFF -D WITH_TBB=ON -D BUILD_NEW_PYTHON_SUPPORT=ON -D WITH_V4L=ON -D INSTALL_C_EXAMPLES=ON -D INSTALL_PYTHON_EXAMPLES=ON -D BUILD_EXAMPLES=ON -D WITH_QT=ON -D WITH_OPENGL=ON ..
make
sudo make install
# add the following to user environment variable ~/.bashrc
export LD_LIBRARY_PATH=${LD_LIBRARY_PATH}:/usr/local/lib
export PKG_CONFIG_PATH=${PKG_CONFIG_PATH}:/usr/local/lib/pkgconfig
# execute the following
sudo ldconfig
# start to use and enjoy opencv, it should have been install into these locations
# /usr/local/include/, /usr/local/share/, /usr/local/lib/STEP 5 (install v4l : Note : installing v4l-utils after opencv installation works for Ubuntu 12.04.2 & OpenCV 2.4.6.1)
# still ensure you haven't close the new terminal window open in STEP 3
# goto http://www.linuxtv.org/downloads/v4l-utils
# download the latest v4l such as v4l-utils-0.9.5.tar.bz2
# copy the downloaded file to the current terminal dir (following previous step, this should be /prepare)
# execute the following
tar -xvjf v4l-utils-0.9.5.tar.bz2
cd v4l-utils-0.9.5/
./configure
make
sudo make install
cd ..
cd .. # (to go to )
sudo ldconfigWorth Noting
# To check the path where opencv & other lib files are stored, do:
pkg-config --cflags opencv
(output will come as)
-I/usr/include/opencv
pkg-config --libs opencv
(output will come as)
-lopencv_core -lopencv_imgproc -lopencv_highgui -lopencv_ --ml -lopencv_video -lopencv_features2d -lopencv_calib3d -lopencv_objdetect -lopencv_contrib -lopencv_legacy -lopencv_flann
# The above paths are needed to compile your opencv programs, as given in the next illustration.
# write a simple C program to test, by saving below program in a file named DisplayImage.c
#include
#include <opencv2></opencv2>highgui/highgui.hpp>
int main(int argc, char *argv[]) {
IplImage* img=0; /* pointer to an image */
printf("Hello\n");
if(argv[1] != 0)
img = cvLoadImage(argv[1], 0); // 1 for color
else
printf("Enter filename\n");
if(img != 0) {
cvNamedWindow("Display", CV_WINDOW_AUTOSIZE); // create a window
cvShowImage("Display", img); // show image in window
cvWaitKey(0); // wait until user hits a key
cvDestroyWindow("Display");
}
else
printf("File not found\n");
return 0;
}
# write a simple C++ program to test, by saving below program in a file named DisplayImage.cpp
#include
#include <opencv2></opencv2>opencv.hpp>
#include <opencv2></opencv2>highgui/highgui.hpp>
using namespace cv;
int main( int argc, char** argv )
{
Mat image;
image = imread( argv[1], 1 );
if( argc != 2 || !image.data )
{
printf( "No image data \n" );
return -1;
}
namedWindow( "Display Image", CV_WINDOW_AUTOSIZE );
imshow( "Display Image", image );
waitKey(0);
return 0;
}
# To compile & run :
g++ `pkg-config --cflags --libs opencv` && ./a.out img
or
g++ -I/usr/include/opencv -I/usr/local/include -lopencv_core -lopencv_highgui -lopencv_ml -lopencv_imgproc -lopencv_video -lopencv_features2d -lopencv_calib3d -lopencv_objdetect -lopencv_contrib -lopencv_legacy -lopencv_flann -lopencv_nonfree && ./a.out img
where “img” is the name of any image with extension within the same folder .
You should be able to see “Hello” and the image in a different window.
If this runs, Congrats! now you can run any C/C++ program with opencv lib.
# Now lets simplify the above big command by making a shortcut for it:
go to your local home directory(cd /home/) and open the .bashrc file using gedit(the file will be hidden). Append the following to the file:
alias gcv="g++ -I/usr/include/opencv -I/usr/local/include -lopencv_core -lopencv_highgui -lopencv_ml -lopencv_imgproc -lopencv_video -lopencv_features2d -lopencv_calib3d -lopencv_objdetect -lopencv_contrib -lopencv_legacy -lopencv_flann -lopencv_nonfree"
and save. Close the terminal and open it again.(as this process requires relogin of the terminal)
# Now, go to directory containing a sample program & do
gcv && ./a.out
or
gcv
./a.out input_img.jpgAs you can see the commands now become similar to $cc filename.c, $./a.out which are used normally for compiling and executing C/C++ programs.
Some ways to check whether all lib files are installed-
apt-cache search opencvreturns :
libcv-dev - Translation package for libcv-dev
libcv2.3 - computer vision library - libcv* translation package
libcvaux-dev - Translation package for libcvaux-dev
libcvaux2.3 - computer vision library - libcvaux translation package
libhighgui-dev - Translation package for libhighgui-dev
libhighgui2.3 - computer vision library - libhighgui translation package
libopencv-calib3d-dev - development files for libopencv-calib3d
libopencv-calib3d2.3 - computer vision Camera Calibration library
libopencv-contrib-dev - development files for libopencv-contrib
libopencv-contrib2.3 - computer vision contrib library
libopencv-core-dev - development files for libopencv-core
libopencv-core2.3 - computer vision core library
libopencv-dev - development files for opencv
libopencv-features2d-dev - development files for libopencv-features2d
libopencv-features2d2.3 - computer vision Feature Detection and Descriptor Extraction library
libopencv-flann-dev - development files for libopencv-flann
libopencv-flann2.3 - computer vision Clustering and Search in Multi-Dimensional spaces library
libopencv-gpu-dev - development files for libopencv-gpu
libopencv-gpu2.3 - computer vision GPU Processing library
libopencv-highgui-dev - development files for libopencv-highgui
libopencv-highgui2.3 - computer vision High-level GUI and Media I/O library
libopencv-imgproc-dev - development files for libopencv-imgproc
libopencv-imgproc2.3 - computer vision Image Processing library
libopencv-legacy-dev - development files for libopencv-legacy
libopencv-legacy2.3 - computer vision legacy library
libopencv-ml-dev - development files for libopencv-ml
libopencv-ml2.3 - computer vision Machine Learning library
libopencv-objdetect-dev - development files for libopencv-objdetect
libopencv-objdetect2.3 - computer vision Object Detection library
libopencv-video-dev - development files for libopencv-video
libopencv-video2.3 - computer vision Video analysis library
opencv-doc - OpenCV documentation and examples
python-opencv - Python bindings for the computer vision library -
A systematic approach to making Web Applications accessible
22 février 2012, par silviaWith the latest developments in HTML5 and the still fairly new ARIA (Accessible Rich Interface Applications) attributes introduced by the W3C WAI (Web Accessibility Initiative), browsers have now implemented many features that allow you to make your JavaScript-heavy Web applications accessible.
Since I began working on making a complex web application accessible just over a year ago, I discovered that there was no step-by-step guide to approaching the changes necessary for creating an accessible Web application. Therefore, many people believe that it is still hard, if not impossible, to make Web applications accessible. In fact, it can be approached systematically, as this article will describe.
This post is based on a talk that Alice Boxhall and I gave at the recent Linux.conf.au titled “Developing accessible Web apps – how hard can it be ?” (slides, video), which in turn was based on a Google Developer Day talk by Rachel Shearer (slides).
These talks, and this article, introduce a process that you can follow to make your Web applications accessible : each step will take you closer to having an application that can be accessed using a keyboard alone, and by users of screenreaders and other accessibility technology (AT).
The recommendations here only roughly conform to the requirements of WCAG (Web Content Accessibility Guidelines), which is the basis of legal accessibility requirements in many jurisdictions. The steps in this article may or may not be sufficient to meet a legal requirement. It is focused on the practical outcome of ensuring users with disabilities can use your Web application.
Step-by-step Approach
The steps to follow to make your Web apps accessible are as follows :
- Use native HTML tags wherever possible
- Make interactive elements keyboard accessible
- Provide extra markup for AT (accessibility technology)
If you are a total newcomer to accessibility, I highly recommend installing a screenreader and just trying to read/navigate some Web pages. On Windows you can install the free NVDA screenreader, on Mac you can activate the pre-installed VoiceOver screenreader, on Linux you can use Orca, and if you just want a browser plugin for Chrome try installing ChromeVox.
1. Use native HTML tags
As you implement your Web application with interactive controls, try to use as many native HTML tags as possible.
HTML5 provides a rich set of elements which can be used to both add functionality and provide semantic context to your page. HTML4 already included many useful interactive controls, like <a>, <button>, <input> and <select>, and semantic landmark elements like <h1>. HTML5 adds richer <input> controls, and a more sophisticated set of semantic markup elements like such as <time>, <progress>, <meter>, <nav>, <header>, <article> and <aside>. (Note : check browser support for browser support of the new tags).
Using as much of the rich HTML5 markup as possible means that you get all of the accessibility features which have been implemented in the browser for those elements, such as keyboard support, short-cut keys and accessibility metadata, for free. For generic tags you have to implement them completely from scratch.
What exactly do you miss out on when you use a generic tag such as <div> over a specific semantic one such as <button> ?
- Generic tags are not focusable. That means you cannot reach them through using the [tab] on the keyboard.
- You cannot activate them with the space bar or enter key or perform any other keyboard interaction that would be regarded as typical with such a control.
- Since the role that the control represents is not specified in code but is only exposed through your custom visual styling, screenreaders cannot express to their users what type of control it is, e.g. button or link.
- Neither can screenreaders add the control to the list of controls on the page that are of a certain type, e.g. to navigate to all headers of a certain level on the page.
- And finally you need to manually style the element in order for it to look distinctive compared to other elements on the page ; using a default control will allow the browser to provide the default style for the platform, which you can still override using CSS if you want.
Example :
Compare these two buttons. The first one is implemented using a <div> tag, the second one using a <button> tag. Try using a screenreader to experience the difference.
<style> .custombutton cursor : pointer ; border : 1px solid #000 ; background-color : #F6F6F6 ; display : inline-block ; padding : 2px 5px ; </style> <div class="custombutton" onclick="alert(’sent !’)"> Send </div>
<button onclick="alert(’sent !’)"> Send </button>
2. Make interactive elements keyboard accessible
Many sophisticated web applications have some interactive controls that just have no appropriate HTML tag equivalent. In this case, you will have had to build an interactive element with JavaScript and <div> and/or <span> tags and lots of custom styling. The good news is, it’s possible to make even these custom controls accessible, and as a side benefit you will also make your application smoother to use for power users.
The first thing you can do to test usability of your control, or your Web app, is to unplug the mouse and try to use only the [TAB] and [ENTER] keys to interact with your application.
Try the following :
- Can you reach all interactive elements with [TAB] ?
- Can you activate interactive elements with [ENTER] (or [SPACE]) ?
- Are the elements in the right tab order ?
- After interaction : is the right element in focus ?
- Is there a keyboard shortcut that activates the element (accesskey) ?
No ? Let’s fix it.
2.1. Reaching interactive elements
If you have an element on your page that cannot be reached with [TAB], put a @tabindex attribute on it.
Example :
Here we have a <span> tag that works as a link (don’t do this – it’s just a simple example). The first one cannot be reached using [TAB] but the second one has a tabindex and is thus part of the tab order of the HTML page.
(Note : since we experiment lots with the tabindex in this article, to avoid confusion, click on some text in this paragraph and then hit the [TAB] key to see where it goes next. The click will set your keyboard focus in the DOM.)
Click
<style> .customlink text-decoration : underline ; cursor : pointer ; </style> <span class="customlink" onclick="alert(’activated !’)"> Click </span>
Click <style> .customlink text-decoration : underline ; cursor : pointer ; </style> <span class="customlink" onclick="alert(’activated !’)" tabindex="0"> Click </span>
You set @tabindex=0 to add an element into the native tab order of the page, which is the DOM order.
2.2. Activating interactive elements
Next, you typically want to be able to use the [ENTER] and [SPACE] keys to activate your custom control. To do so, you will need to implement an onkeydown event handler. Note that the keyCode for [ENTER] is 13 and for [SPACE] is 32.
Example :
Let’s add this functionality to the <span> tag from before. Try tabbing to it and hit the [ENTER] or [SPACE] key.
Click <span class="customlink" onclick="alert(’activated !’)" tabindex="0"> Click </span>
<script><br />
function handlekey(event) {<br />
var target = event.target || event.srcElement;<br />
if (event.keyCode == 13 || event.keyCode == 32) { target.onclick(); }<br />
}<br />
</script>
Click<span class="customlink" onclick="alert(’activated !’)" tabindex="0" onkeydown="handlekey(event) ;"> Click </span> <script> function handlekey(event) var target = event.target || event.srcElement ; if (event.keyCode == 13 || event.keyCode == 32) target.onclick() ; </script>Note that there are some controls that might need support for keys other than [tab] or [enter] to be able to use them from the keyboard alone, for example a custom list box, menu or slider should respond to arrow keys.
2.3. Elements in the right tab order
Have you tried tabbing to all the elements on your page that you care about ? If so, check if the order of tab stops seems right. The default order is given by the order in which interactive elements appear in the DOM. For example, if your page’s code has a right column that is coded before the main article, then the links in the right column will receive tab focus first before the links in the main article.
You could change this by re-ordering your DOM, but oftentimes this is not possible. So, instead give the elements that should be the first ones to receive tab focus a positive @tabindex. The tab access will start at the smallest non-zero @tabindex value. If multiple elements share the same @tabindex value, these controls receive tab focus in DOM order. After that, interactive elements and those with @tabindex=0 will receive tab focus in DOM order.
Example :
The one thing that always annoys me the most is if the tab order in forms that I am supposed to fill in is illogical. Here is an example where the first and last name are separated by the address because they are in a table. We could fix it by moving to a <div> based layout, but let’s use @tabindex to demonstrate the change.
Firstname :
Address :
Lastname :
City :
<table class="customtabs"> <tr> <td>Firstname : <input type="text" id="firstname"> </td> <td>Address : <input type="text" id="address"> </td> </tr> <tr> <td>Lastname : <input type="text" id="lastname"> </td> <td>City : <input type="text" id="city"> </td> </tr> </table>Click here to test this form,
then [TAB] :Firstname :
Address :
Lastname :
City :
<table class="customtabs"> <tr> <td>Firstname : <input type="text" id="firstname" tabindex="10"> </td> <td>Address : <input type="text" id="address" tabindex="30"> </td> </tr> <tr> <td>Lastname : <input type="text" id="lastname" tabindex="20"> </td> <td>City : <input type="text" id="city" tabindex="40"> </td> </tr> </table>Be very careful with using non-zero tabindex values. Since they change the tab order on the page, you may get side effects that you might not have intended, such as having to give other elements on the page a non-zero tabindex value to avoid skipping too many other elements as I would need to do here.
2.4. Focus on the right element
Some of the controls that you create may be rather complex and open elements on the page that were previously hidden. This is particularly the case for drop-downs, pop-ups, and menus in general. Oftentimes the hidden element is not defined in the DOM right after the interactive control, such that a [TAB] will not put your keyboard focus on the next element that you are interacting with.
The solution is to manage your keyboard focus from JavaScript using the .focus() method.
Example :
Here is a menu that is declared ahead of the menu button. If you tab onto the button and hit enter, the menu is revealed. But your tab focus is still on the menu button, so your next [TAB] will take you somewhere else. We fix it by setting the focus on the first menu item after opening the menu.
<script><br />
function displayMenu(value) {<br />
document.getElementById("custommenu").style.display=value;<br />
}<br />
</script><div id="custommenu" style="display:none ;"> <button id="item1" onclick="displayMenu(’none’) ;">Menu item1</button> <button id="item2" onclick="displayMenu(’none’) ;">Menu item2</button> </div> <button onclick="displayMenu(’block’) ;">Menu</button> <script> function displayMenu(value) document.getElementById("custommenu").style.display=value ; </script><script><br />
function displayMenu2(value) {<br />
document.getElementById("custommenu2").style.display=value;<br />
document.getElementById("item1").focus();<br />
}<br />
</script><div id="custommenu" style="display:none ;"> <button id="item1" onclick="displayMenu(’none’) ;">Menu item1</button> <button id="item2" onclick="displayMenu(’none’) ;">Menu item2</button> </div> <button onclick="displayMenu(’block’) ;">Menu</button> <script> function displayMenu(value) document.getElementById("custommenu").style.display=value ; document.getElementById("item1").focus() ; </script>You will notice that there are still some things you can improve on here. For example, after you close the menu again with one of the menu items, the focus does not move back onto the menu button.
Also, after opening the menu, you may prefer not to move the focus onto the first menu item but rather just onto the menu <div>. You can do so by giving that div a @tabindex and then calling .focus() on it. If you do not want to make the div part of the normal tabbing order, just give it a @tabindex=-1 value. This will allow your div to receive focus from script, but be exempt from accidental tabbing onto (though usually you just want to use @tabindex=0).
Bonus : If you want to help keyboard users even more, you can also put outlines on the element that is currently in focus using CSS”s outline property. If you want to avoid the outlines for mouse users, you can dynamically add a class that removes the outline in mouseover events but leaves it for :focus.
2.5. Provide sensible keyboard shortcuts
At this stage your application is actually keyboard accessible. Congratulations !
However, it’s still not very efficient : like power-users, screenreader users love keyboard shortcuts : can you imagine if you were forced to tab through an entire page, or navigate back to a menu tree at the top of the page, to reach each control you were interested in ? And, obviously, anything which makes navigating the app via the keyboard more efficient for screenreader users will benefit all power users as well, like the ubiquitous keyboard shortcuts for cut, copy and paste.
HTML4 introduced so-called accesskeys for this. In HTML5 @accesskey is now allowed on all elements.
The @accesskey attribute takes the value of a keyboard key (e.g. @accesskey="x") and is activated through platform- and browser-specific activation keys. For example, on the Mac it’s generally the [Ctrl] key, in IE it’ the [Alt] key, in Firefox on Windows [Shift]-[Alt], and in Opera on Windows [Shift]-[ESC]. You press the activation key and the accesskey together which either activates or focuses the element with the @accesskey attribute.
Example :
<script><br />
var button = document.getElementById('accessbutton');<br />
if (button.accessKeyLabel) {<br />
button.innerHTML += ' (' + button.accessKeyLabel + ')';<br />
}<br />
</script><button id="accessbutton" onclick="alert(’sent !’)" accesskey="e"> Send </button> <script> var button = document.getElementById(’accessbutton’) ; if (button.accessKeyLabel) button.innerHTML += ’ (’ + button.accessKeyLabel + ’)’ ; </script>Now, the idea behind this is clever, but the execution is pretty poor. Firstly, the different activation keys between different platforms and browsers make it really hard for people to get used to the accesskeys. Secondly, the key combinations can conflict with browser and screenreader shortcut keys, the first of which will render browser shortcuts unusable and the second will effectively remove the accesskeys.
In the end it is up to the Web application developer whether to use the accesskey attribute or whether to implement explicit shortcut keys for the application through key event handlers on the window object. In either case, make sure to provide a help list for your shortcut keys.
Also note that a page with a really good hierarchical heading layout and use of ARIA landmarks can help to eliminate the need for accesskeys to jump around the page, since there are typically default navigations available in screen readers to jump directly to headings, hyperlinks, and ARIA landmarks.
3. Provide markup for AT
Having made the application keyboard accessible also has advantages for screenreaders, since they can now reach the controls individually and activate them. So, next we will use a screenreader and close our eyes to find out where we only provide visual cues to understand the necessary interaction.
Here are some of the issues to consider :
- Role may need to get identified
- States may need to be kept track of
- Properties may need to be made explicit
- Labels may need to be provided for elements
This is where the W3C’s ARIA (Accessible Rich Internet Applications) standard comes in. ARIA attributes provide semantic information to screen readers and other AT that is otherwise conveyed only visually.
Note that using ARIA does not automatically implement the standard widget behavior – you’ll still need to add focus management, keyboard navigation, and change aria attribute values in script.
3.1. ARIA roles
After implementing a custom interactive widget, you need to add a @role attribute to indicate what type of controls it is, e.g. that it is playing the role of a standard tag such as a button.
Example :
This menu button is implemented as a <div>, but with a role of “button” it is announced as a button by a screenreader.
<div tabindex="0" role="button">Menu</div>
ARIA roles also describe composite controls that do not have a native HTML equivalent.
Example :
This menu with menu items is implemented as a set of <div> tags, but with a role of “menu” and “menuitem” items.
<div role="menu"> <div tabindex="0" role="menuitem">Cut</div> <div tabindex="0" role="menuitem">Copy</div> <div tabindex="0" role="menuitem">Paste</div> </div>
3.2. ARIA states
Some interactive controls represent different states, e.g. a checkbox can be checked or unchecked, or a menu can be expanded or collapsed.
Example :
The following menu has states on the menu items, which are here not just used to give an aural indication through the screenreader, but also a visual one through CSS.
<style> .custombutton[aria-checked=true]:before content : "\2713 " ; </style> <div role="menu"> <div tabindex="0" role="menuitem" aria-checked="true">Left</div> <div tabindex="0" role="menuitem" aria-checked="false">Center</div> <div tabindex="0" role="menuitem" aria-checked="false">Right</div> </div>
3.3. ARIA properties
Some of the functionality of interactive controls cannot be captured by the role attribute alone. We have ARIA properties to add features that the screenreader needs to announce, such as aria-label, aria-haspopup, aria-activedescendant, or aria-live.
Example :
The following drop-down menu uses aria-haspopup to tell the screenreader that there is a popup hidden behind the menu button together with an ARIA state of aria-expanded to track whether it’s open or closed.
<script><br />
var button = document.getElementById("button");<br />
var menu = document.getElementById("menu");<br />
var items = document.getElementsByClassName("menuitem");<br />
var focused = 0;<br />
function showMenu(evt) {<br />
evt.stopPropagation();<br />
menu.style.visibility = 'visible';<br />
button.setAttribute('aria-expanded','true');<br />
focused = getSelected();<br />
items[focused].focus();<br />
}<br />
function hideMenu(evt) {<br />
evt.stopPropagation();<br />
menu.style.visibility = 'hidden';<br />
button.setAttribute('aria-expanded','false');<br />
button.focus();<br />
}<br />
function getSelected() {<br />
for (var i=0; i < items.length; i++) {<br />
if (items[i].getAttribute('aria-checked') == 'true') {<br />
return i;<br />
}<br />
}<br />
}<br />
function setSelected(elem) {<br />
var curSelected = getSelected();<br />
items[curSelected].setAttribute('aria-checked', 'false');<br />
elem.setAttribute('aria-checked', 'true');<br />
}<br />
function selectItem(evt) {<br />
setSelected(evt.target);<br />
hideMenu(evt);<br />
}<br />
function getPrevItem(index) {<br />
var prev = index - 1;<br />
if (prev < 0) {<br />
prev = items.length - 1;<br />
}<br />
return prev;<br />
}<br />
function getNextItem(index) {<br />
var next = index + 1;<br />
if (next == items.length) {<br />
next = 0;<br />
}<br />
return next;<br />
}<br />
function handleButtonKeys(evt) {<br />
evt.stopPropagation();<br />
var key = evt.keyCode;<br />
switch(key) {<br />
case (13): /* ENTER */<br />
case (32): /* SPACE */<br />
showMenu(evt);<br />
default:<br />
}<br />
}<br />
function handleMenuKeys(evt) {<br />
evt.stopPropagation();<br />
var key = evt.keyCode;<br />
switch(key) {<br />
case (38): /* UP */<br />
focused = getPrevItem(focused);<br />
items[focused].focus();<br />
break;<br />
case (40): /* DOWN */<br />
focused = getNextItem(focused);<br />
items[focused].focus();<br />
break;<br />
case (13): /* ENTER */<br />
case (32): /* SPACE */<br />
setSelected(evt.target);<br />
hideMenu(evt);<br />
break;<br />
case (27): /* ESC */<br />
hideMenu(evt);<br />
break;<br />
default:<br />
}<br />
}<br />
button.addEventListener('click', showMenu, false);<br />
button.addEventListener('keydown', handleButtonKeys, false);<br />
for (var i = 0; i < items.length; i++) {<br />
items[i].addEventListener('click', selectItem, false);<br />
items[i].addEventListener('keydown', handleMenuKeys, false);<br />
}<br />
</script><div class="custombutton" id="button" tabindex="0" role="button" aria-expanded="false" aria-haspopup="true"> <span>Justify</span> </div> <div role="menu" class="menu" id="menu" style="display : none ;"> <div tabindex="0" role="menuitem" class="menuitem" aria-checked="true"> Left </div> <div tabindex="0" role="menuitem" class="menuitem" aria-checked="false"> Center </div> <div tabindex="0" role="menuitem" class="menuitem" aria-checked="false"> Right </div> </div> [CSS and JavaScript for example omitted]3.4. Labelling
The main issue that people know about accessibility seems to be that they have to put alt text onto images. This is only one means to provide labels to screenreaders for page content. Labels are short informative pieces of text that provide a name to a control.
There are actually several ways of providing labels for controls :
- on img elements use @alt
- on input elements use the label element
- use @aria-labelledby if there is another element that contains the label
- use @title if you also want a label to be used as a tooltip
- otherwise use @aria-label
I’ll provide examples for the first two use cases - the other use cases are simple to deduce.
Example :
The following two images show the rough concept for providing alt text for images : images that provide information should be transcribed, images that are just decorative should receive an empty @alt attribute.
Image by Noah Sussman<img src="texture.jpg" alt=""> <img src="lolcat.jpg" alt="shocked lolcat titled ’HTML cannot do that !"> <img src="texture.jpg" alt="">
When marking up decorative images with an empty @alt attribute, the image is actually completely removed from the accessibility tree and does not confuse the blind user. This is a desired effect, so do remember to mark up all your images with @alt attributes, even those that don’t contain anything of interest to AT.
Example :
In the example form above in Section 2.3, when tabbing directly on the input elements, the screen reader will only say "edit text" without announcing what meaning that text has. That’s not very useful. So let’s introduce a label element for the input elements. We’ll also add checkboxes with a label.
<label>Doctor title :</label> <input type="checkbox" id="doctor"/> <label>Firstname :</label> <input type="text" id="firstname2"/>
<label for="lastname2">Lastname :</label>
<input type="text" id="lastname2"/><label>Address :
<input type="text" id="address2">
</label>
<label for="city2">City :
<input type="text" id="city2">
</label>
<label for="remember">Remember me :</label>
<input type="checkbox" id="remember">In this example we use several different approaches to show what a different it makes to use the <label> element to mark up input boxes.
The first two fields just have a <label> element next to a <input> element. When using a screenreader you will not notice a difference between this and not using the <label> element because there is no connection between the <label> and the <input> element.
In the third field we use the @for attribute to create that link. Now the input field isn’t just announced as "edit text", but rather as "Lastname edit text", which is much more useful. Also, the screenreader can now skip the labels and get straight on the input element.
In the fourth and fifth field we actually encapsulate the <input> element inside the <label> element, thus avoiding the need for a @for attribute, though it doesn’t hurt to explicity add it.
Finally we look at the checkbox. By including a referenced <label> element with the checkbox, we change the screenreaders announcement from just "checkbox not checked" to "Remember me checkbox not checked". Also notice that the click target now includes the label, making the checkbox not only more usable to screenreaders, but also for mouse users.
4. Conclusions
This article introduced a process that you can follow to make your Web applications accessible. As you do that, you will noticed that there are other things that you may need to do in order to give the best experience to a power user on a keyboard, a blind user using a screenreader, or a vision-impaired user using a screen magnifier. But once you’ve made a start, you will notice that it’s not all black magic and a lot can be achieved with just a little markup.
You will find more markup in the WAI ARIA specification and many more resources at Mozilla’s ARIA portal. Now go and change the world !
Many thanks to Alice Boxhall and Dominic Mazzoni for their proof-reading and suggested changes that really helped improve the article !
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