How to read alpha channel from .webm video using ffmpeg in c++ - c++

Question

Background
I have a .webm file (pix_fmt: yuva420p) converted from .mov video file in order to reduce file size and I would like to read the video data using c++, so I followed using this repo as a reference.
This works perfectly on .mov video.
Problem
By using same repo, however, there is no alpha channel data (pure zeros on that channel) for .webm video but I can get the alpha data from .mov video.
Apparently many people already noticed that after the video conversion, ffmpeg somehow detect video as yuv420p + alpha_mode : 1 and thus alpha channel is not used but there is no one discuss about workaround of this.
I tried forcing pixel format during this part to use yuva420p but that just broke the whole program.
// Set up sws scaler
if (!sws_scaler_ctx) {
auto source_pix_fmt = correct_for_deprecated_pixel_format(av_codec_ctx->pix_fmt);
sws_scaler_ctx = sws_getContext(width, height, source_pix_fmt,
width, height, AV_PIX_FMT_RGB0,
SWS_BILINEAR, NULL, NULL, NULL);
}
I also verified my video that it contains alpha channel using other source so I am sure there is alpha channel in my webm video but I cannot fetch the data using ffmpeg.
Is there a way to fetch the alpha data? Other video format or using other libraries work as well as long as it does have some file compression but I need to access the data in c++.
Note: This is the code I used for converting video to webm
ffmpeg -i input.mov -c:v libvpx-vp9 -pix_fmt yuva420p output.webm

Answer 1

You have to force the decoder.
Set the following before avformat_open_input()
AVCodec *vcodec;
vcodec = avcodec_find_decoder_by_name("libvpx-vp9");
av_fmt_ctx->video_codec = vcodec;
av_fmt_ctx->video_codec_id = vcodec->id;
You don't need to set pixel format or any scaler args.
This assumes that your libavcodec is linked with libvpx.