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• 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 to make transparent background of image after rotating using ffmpeg in Android ?

  18 juin 2020, par Mit Shah

  The issue I am facing is, rotating the image in app, it works as expected. But after saving the file, the difference is seen. The image is rotated but with black background instead of transparent. I have looked several places for solution but didn't succeed.

  



  I have tried this things :

  



  

  • In command, c=none, c=black@, c=black@0, c=000000, c=0x000000

  


  • With both extensions jpg and png.

  


  



  But failed to get the required output.

  



  For practical understanding, here are the two images stating Input and Output.

  



  Help me out. Thanks

  



  Input :
  


  



  Output :
  


  


• How to use the FFmpeg blackdetect filter for the following image

  17 septembre 2021, par dinesh47

  I am using FFmpeg to find the blackdetect from a mp4 video file. but for some reasons file with very low light or poor background at night get detected as black frame For sample i have attached the frame which detected as black

  


  My ffmpeg filter setting is "blackdetect=d=121:pix_th=0.00"

  


  my file has a resolution of 1280*720 and frame rate of 30 which uses decoder format of 4:2:0 YUV


  


  There are 2 Questions

  


  1.Why the ffmpeg lib detects this as black frame ?

  


  2.How to over come this ?