[FFmpeg-devel] [Vulkan] Clarification on hwdownload transfer formats in hwcontext_vulkan

[SOHAM KUTE via ffmpeg-devel <ffmpeg-devel@ffmpeg.org>]

Hi,

I’m testing a Vulkan filter chain:

hwupload → scale_vulkan → hwdownload → format

and looking into the transfer path implemented in
libavutil/hwcontext_vulkan.c, specifically:

vulkan_transfer_get_formats()

Currently this function only exposes:

- hwfc->sw_format
- AV_PIX_FMT_CUDA (when enabled)

As a result, hwdownload sometimes fails format negotiation even though
the Vulkan → system memory transfer itself works.

I wanted to confirm the intended design:

1. Is the limited format list in vulkan_transfer_get_formats() intentional?
2. Should Vulkan hwdownload advertise more pixel formats, or is the
   expectation that conversion must always happen after hwdownload?
3. If this is not by design, is extending this list something that would
   be acceptable in a patch?

Before attempting any change, I want to make sure I’m not violating an
assumption in the hwcontext API.

Thanks,
Soham
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