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LWLibavSource: unable to return alpha frame of ARGB png image if alpha frame is requested after the main content #8

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AkarinVS opened this issue Sep 8, 2021 · 0 comments
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LWLibavSource: unable to return alpha frame of ARGB png image if alpha frame is requested after the main content #8

AkarinVS opened this issue Sep 8, 2021 · 0 comments
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AkarinVS commented Sep 8, 2021 •
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Using VS R54.

Given a png file (logo11.png) with transparency, this code fails with "lsmas: failed to output a video frame." for the alpha clip:

import vapoursynth as vs
core = vs.core
c = core.lsmas.LWLibavSource('logo11.png')

if True:
  f = c[0].get_frame(0)
  a = f.get_read_array(0)
  print(a[0,1])

f = c[1].get_frame(0) # fails here
a = f.get_read_array(0)
print(a[0,1])

However, if we flip the condition of the if statement, it works.

Also, a test sample movie with alpha (RLE RGBA)
https://user-images.githubusercontent.com/79720032/132518447-659caa63-ce2d-4120-9f76-5700cfc7fcb6.mov
@AkarinVS AkarinVS self-assigned this Sep 8, 2021
@AkarinVS AkarinVS changed the title unable to return alpha frame in certain situations LWLibavSource: unable to return alpha frame in certain situations Sep 8, 2021
@AkarinVS AkarinVS changed the title LWLibavSource: unable to return alpha frame in certain situations LWLibavSource: unable to return alpha frame of ARGB png image if alpha frame is requested after the main content Sep 8, 2021
Asd-g pushed a commit to HomeOfAviSynthPlusEvolution/L-SMASH-Works that referenced this issue Apr 6, 2022
@AkarinVS @Asd-g
VapourSynth/video_output.c: populate alpha make_frame functions earlier
d656d6d 
lsmas used to relying on new_output_video_frame to call determine_colorspace_conversion
to set vs_vohp->make_frame[output_index] for output_index 1 at get_frame time.

However, the guard to determine_colorspace_conversion in new_output_video_frame is
input argument input_pix_fmt_change, which does not take output_index into account.

The end result is that, if the alpha frame is requested before the main clip frames,
then new_output_video_frame notices that input_pix_fmt_change is true, and calls
appropriate functions to set the make_frame function for the output_index 1.

However, if the main clip frame is retrieved first, then new_output_video_frame will
cal determine_colorspace_conversion for output_index 0, which sets results in the flag
vshp->frame_prop_change_flags being cleared (as update_scaler_configuration_if_needed
determined that vshp already matches av_frame's properties.

You might wonder why does the code work if the request order is reversed (alpha first
and then main)? Good question: it is because during vs_setup_video_rendering,
determine_colorspace_conversion is already called for the main clip.

To fix this convoluted mess, we make vs_setup_video_rendering also call
determine_colorspace_conversion for output_index 1 (the alpha clip) as well.

Fixes AkarinVS/L-SMASH-Works#8.

Signed-off-by: akarin <[email protected]>
(cherry picked from commit AkarinVS/L-SMASH-Works@95138e2)
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