Thanks for the commit, Andreas. It is indeed too bad that the result of the write/read roundtrip test depends on the specific version of the external x264 library.

At any rate, I uploaded the version 2.0.1-1 of the Debian package octave-video, which builds correctly on all architectures:
https://buildd.debian.org/status/package.php?p=octave-video

P.S.: I just spotted a spelling error in your commit : “realtive” should read “relative”

Thanks for your feedback, Andreas.

It seems that the results effectively depend on the version of the x264 library. I just tried again the example that I include in my first message on a ppc64el system (this time with libx264-160) and I get:

max_rel_err = 0.021516

The new resulting file is much more similar to the one obtained on an amd64 system:

2c2
< Complete name                            : rainbow-x86_64.mp4
---
> Complete name                            : rainbow-ppc64le-x264-160.mp4
6c6
< File size                                : 11.9 KiB
---
> File size                                : 11.5 KiB
8,9c8,9
< Overall bit rate                         : 24.3 kb/s
< Writing application                      : Lavf58.45.100
---
> Overall bit rate                         : 23.6 kb/s
> Writing application                      : Lavf58.76.100
22c22,23
< Bit rate                                 : 21.6 kb/s
---
> Bit rate                                 : 20.9 kb/s
> Maximum bit rate                         : 450 kb/s
32,34c33,35
< Bits/(Pixel*Frame)                       : 0.024
< Stream size                              : 10.6 KiB (89%)
< Writing library                          : x264 core 161
---
> Bits/(Pixel*Frame)                       : 0.023
> Stream size                              : 10.2 KiB (89%)
> Writing library                          : x264 core 160 r3011 cde9a93

Also, it is visually okay (I am attaching the result to this bug report). However, there is still a much higher absolute error due to compression.

(file #52344)

The unit test of function VideoWriter of package video fails on a ppc64el system.  This is probably due a MP4 compression problem, as written in lines 253 and 254 of VideoWriter.m:

%!   # this doesn't work well due to compression...
%!   # FIXME: a better idea to test write/read roundtrip

I tested the code below in both amd64 and ppc64el systems (it is essentially the same as in the failing unit test):

fn = fullfile (tempdir(), sprintf ("rainbow-%s.mp4", uname ().machine));
width = 200;
height = 150;
nframes = 120;
p = permute (rainbow (width), [3 1 2]);
raw_video = zeros (height, width, 3, nframes);
w = VideoWriter (fn);
for k=1:nframes
  ps = circshift (p, k * 6);
  img = uint8 (255 * repmat (ps, height, 1));
  raw_video (:, :, :, k) = img;
  writeVideo (w, img);
endfor
close (w)
r = VideoReader (fn);
max_rel_err = -Inf;
for k=1:size (raw_video, 4)
  img = readFrame (r);
  d = double (img) - raw_video(:,:,:,k);
  rel_err = sum (abs(d(:)))/numel(d)/255;
  if rel_err > max_rel_err
    max_rel_err = rel_err;
  endif
endfor
close (r);
max_rel_err

It yields the following:

On amd64:

max_rel_err =  0.0081331

On ppc64el:

max_rel_err =  0.046289

The resulting rainbow-*.mp4 are attached to this bug report.

I think that the threshold 0.015 for the assert test should be increased.