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• bash : receive single frames from ffmpeg pipe

  30 août 2014, par manu

  I’m trying to achieve single-frame handling in a pipe where the the j2c encoder "kdu_compress" (Kakadu) only accepts single files. To save harddrive space. I didn’t manage to pipe frames directly, so I’m trying to handle them via a bash script, by creating each picture, process it, and overwrite it with the next.

  Here is my approach. Thanks for your advice, I really want to climb this mountain, though I’m a bit fresh here thanks.

  ---

  Is it possible to pipe an ffmpeg output to a bash script and save the individual frame,
  do further commands with the file before the next frame is handled ?

  Best result so far is, that ALL frames are added into the intermediate file, without recognizing the end of a frame.

  I used this ffmpeg setting to pipe, example with .ppm :

  ffmpeg -y  -i "/path/to/source.mov" -an -c:v ppm -updatefirst 1 -f image2 - \
  
   | /path/to/receiver.sh

  and this script as a receiver.sh

  #!/bin/bash  
  
  
  
  while read a; 
  
  do 
  
      cat /dev/null > "/path/to/tempfile.ppm"; #to empty the file first
  
      cat $a >> "/path/to/tempfile.ppm";        #to fill one picture
  
  
  
      kdu_compress -i /path/to/tempfile.ppm -otherparams   #to process this intermediate
  
  
  
  done
  
  exit;

  Thank you very much.

• RGB to YUV conversion with libav (ffmpeg) triplicates image

  17 avril 2021, par José Tomás Tocino

  I'm building a small program to capture the screen (using X11 MIT-SHM extension) on video. It works well if I create individual PNG files of the captured frames, but now I'm trying to integrate libav (ffmpeg) to create the video and I'm getting... funny results.

  


  The furthest I've been able to reach is this. The expected result (which is a PNG created directly from the RGB data of the XImage file) is this :

  


  

  


  However, the result I'm getting is this :

  


  

  


  As you can see the colors are funky and the image appears cropped three times. I have a loop where I capture the screen, and first I generate the individual PNG files (currently commented in the code below) and then I try to use libswscale to convert from RGB24 to YUV420 :

  


  while (gRunning) {
        printf("Processing frame framecnt=%i \n", framecnt);

        if (!XShmGetImage(display, RootWindow(display, DefaultScreen(display)), img, 0, 0, AllPlanes)) {
            printf("\n Ooops.. Something is wrong.");
            break;
        }

        // PNG generation
        // snprintf(imageName, sizeof(imageName), "salida_%i.png", framecnt);
        // writePngForImage(img, width, height, imageName);

        unsigned long red_mask = img->red_mask;
        unsigned long green_mask = img->green_mask;
        unsigned long blue_mask = img->blue_mask;

        // Write image data
        for (int y = 0; y < height; y++) {
            for (int x = 0; x < width; x++) {
                unsigned long pixel = XGetPixel(img, x, y);

                unsigned char blue = pixel & blue_mask;
                unsigned char green = (pixel & green_mask) >> 8;
                unsigned char red = (pixel & red_mask) >> 16;

                pixel_rgb_data[y * width + x * 3] = red;
                pixel_rgb_data[y * width + x * 3 + 1] = green;
                pixel_rgb_data[y * width + x * 3 + 2] = blue;
            }
        }

        uint8_t* inData[1] = { pixel_rgb_data };
        int inLinesize[1] = { in_w };

        printf("Scaling frame... \n");
        int sliceHeight = sws_scale(sws_context, inData, inLinesize, 0, height, pFrame->data, pFrame->linesize);

        printf("Obtained slice height: %i \n", sliceHeight);
        pFrame->pts = framecnt * (pVideoStream->time_base.den) / ((pVideoStream->time_base.num) * 25);

        printf("Frame pts: %li \n", pFrame->pts);
        int got_picture = 0;

        printf("Encoding frame... \n");
        int ret = avcodec_encode_video2(pCodecCtx, &pkt, pFrame, &got_picture);

//                int ret = avcodec_send_frame(pCodecCtx, pFrame);

        if (ret != 0) {
            printf("Failed to encode! Error: %i\n", ret);
            return -1;
        }

        printf("Succeed to encode frame: %5d - size: %5d\n", framecnt, pkt.size);

        framecnt++;

        pkt.stream_index = pVideoStream->index;
        ret = av_write_frame(pFormatCtx, &pkt);

        if (ret != 0) {
            printf("Error writing frame! Error: %framecnt \n", ret);
            return -1;
        }

        av_packet_unref(&pkt);
    }


  


  I've placed the entire code at this gist. This question right here looks pretty similar to mine, but not quite, and the solution did not work for me, although I think this has something to do with the way the line stride is calculated.

  


• How to convert from AV_PIX_FMT_BGRA to PIX_FMT_PAL8 ?

  29 juillet 2014, par Jona

  I’m having a hard time converting my images from AV_PIX_FMT_BGRA to PIX_FMT_PAL8. Unfortunately sws_getCachedContext doesn’t support the conversion to PIX_FMT_PAL8.

  What I’m trying to do is convert my images into a GIF video with higher quality output. It seems that PIX_FMT_PAL8 could potentially provide the higher quality output I’m looking for.

  According to this documentation I need to palettize the pixel data, but I have no clue how to do that.

  When the pixel format is palettized RGB (PIX_FMT_PAL8), the palettized
  image data is stored in AVFrame.data[0]. The palette is transported in
  AVFrame.data[1], is 1024 bytes long (256 4-byte entries) and is
  formatted the same as in PIX_FMT_RGB32 described above (i.e., it is
  also endian-specific). Note also that the individual RGB palette
  components stored in AVFrame.data[1] should be in the range 0..255.
  This is important as many custom PAL8 video codecs that were designed
  to run on the IBM VGA graphics adapter use 6-bit palette components.

  Any help or direction would be appreciated.