Recherche avancée

Sur d’autres sites (7959)

• Different filesizes for images generated using octave and python

  17 décembre 2020, par lakshayg

  I am using python (scikit-image) and octave to generate 200 images as follows

  


  Python3

  


  import numpy as np
from skimage.io import imsave

images = [255*np.ones((100,100), dtype=np.uint8),  # white
             np.zeros((100,100), dtype=np.uint8)]  # black

for i in range(200): # save alternating black and white images
    imsave('%04d.png'%(i+1), images[i%2])


  


  Octave

  


  pkg load image;

im1 = 255*ones(100,100); # white
im2 = zeros(100,100);    # black
for i=1:200
    name = sprintf('%04d.png', i);
    if mod(i,2) == 0
        imwrite(im1, name);
    else
        imwrite(im2, name);
    end
end


  


  Next, I use ffmpeg to generate two videos (alternating white and black frames) from these two sets of images using the following command

  


  ffmpeg -r 10 -loglevel quiet \
       -i ./%04d.png -c:v libx264 \
       -preset ultrafast -crf 0 ./out.mkv


  


  

  1. Sizes of image files generated by both these codes are different.

  


  


  

  • Octave white : 192 bytes, black : 98 bytes

  


  • Python white : 120 bytes, black : 90 bytes

  


  


  

  2. Sizes of video files generated from these octave and python images are significantly different from each other.

  


  


  

  • Octave filesize : 60 kilobytes

  


  • Python filesize : 116 kilobytes

  


  


  Why do we have this apparently very strange behavior ?

  


  EDIT

  


  Since it was suggested that the behavior might be due to octave and python using different bit-depths to store the images, I changed the octave code to use 8 bit numbers

  


  im1 = uint8(255*ones(100,100)); # white
im2 = uint8(zeros(100,100));    # black


  


  and now the image file sizes are nearly the same

  


  

  • Octave white : 118 bytes, black : 90 bytes

  


  • Python white : 120 bytes, black : 90 bytes

  


  


  but the problem is still the same for video files, octave : 60K, python : 116K

  


• How do I chromakey an enclosed shape (or background) in ffmpeg ? [closed]

  31 janvier, par JustAnotherCodemonkey

  I have an animation with a fully contiguous 0xFFFFFF white background I want to remove. The issue is that the foreground contains the same exact color. These parts are all completely enclosed and contained within at least several pixels of a different shade at its weakest.

  


  All I need is to be able to essentially, for each frame, make all pixels that are white and connected by white to 0,0, transparent. Chromakey and its siblings remove those bits from the foreground as well which is unacceptable and backgroundkey, well you can imagine how that goes.

  


  I'm somewhat proficient with ffmpeg and can handle severe filter shenanigans, I've just thought about this for like 10 min and decided I seemingly don't have the creativity to piece together a clever solution. I just don't want to have to spend some time sitting down and manually writing out a program to do this frame-by-frame.

  


• How to utilize hardware decode for audio ?

  20 mars 2014, par jAckOdE

  I have a buffer that contain packets read by ffmpeg from a video file encoded using H264/AAC
  According to Apple document, audio stream encoded in AAC can be decode with hardware support,

  how to decode the audio stream with hardware support ?

  UPDATE : I use Audio Queue Service to output the audio. Right now i decode AAC packet using ffmpeg and send LPCM audio to AQS. According to the Apple document, I can send directly AAC audio to AQ and it will take care of decoding task. Does it decode with hardware ? Do i need, and how to set Audio Queue's parameter to enable audio hardware decoding ?