Please skip unsupported ffmpeg decoders

[lumag]

fluster reports that hw-accelerated ffmpeg decoders are available, although the hardware doesn't support it. For example, on my testing machine (Snapdragon laptop) only software VP8 decoders are available. For GStreamer it is correctly detected, but for ffmpeg all encoders are marked as available. This affects e.g. running fluster from scripts to generate summary report, where it will happily skip unavailable GSreamer decoders, but will attempt (and fail) all unavailable ffmpeg decoders:

$ ./fluster.py l -c -d VP8

List of available test suites:

VP8-TEST-VECTORS
    Codec: VP8
    Description: VP8 Test Vector Catalogue from https://github.com/webmproject/vp8-test-vectors
    Test vectors: 61

List of available decoders:

VP8
    FFmpeg-VP8: FFmpeg VP8 SW decoder... ✔️
    FFmpeg-VP8-CUDA: FFmpeg VP8 CUDA decoder... ✔️
    FFmpeg-VP8-VAAPI: FFmpeg VP8 VAAPI decoder... ✔️
    FFmpeg-VP8-v4l2m2m: FFmpeg VP8 v4l2m2m decoder... ✔️
    GStreamer-VP8-D3D11-Gst1.0: GStreamer VP8 D3D11 decoder for GStreamer 1.0... ❌
    GStreamer-VP8-Libav-Gst1.0: GStreamer VP8 Libav decoder for GStreamer 1.0... ❌
    GStreamer-VP8-NVDEC-Gst1.0: GStreamer VP8 NVDEC decoder for GStreamer 1.0... ❌
    GStreamer-VP8-NVDECSL-Gst1.0: GStreamer VP8 NVDECSL decoder for GStreamer 1.0... ❌
    GStreamer-VP8-V4L2-Gst1.0: GStreamer VP8 V4L2 decoder for GStreamer 1.0... ❌
    GStreamer-VP8-V4L2SL-Gst1.0: GStreamer VP8 V4L2SL decoder for GStreamer 1.0... ❌
    GStreamer-VP8-VA-Gst1.0: GStreamer VP8 VA decoder for GStreamer 1.0... ❌
    GStreamer-VP8-VAAPI-Gst1.0: GStreamer VP8 VAAPI decoder for GStreamer 1.0... ❌
    GStreamer-VP8-libvpx-Gst1.0: GStreamer VP8 libvpx decoder for GStreamer 1.0... ✔️
    ccdec-VP8: VP8 cros-codecs decoder... ❌
    libvpx-VP8: VP8 reference decoder... ❌

[lumag]

And then:

$ ./fluster.py r -j 1 -s -so iris.md -d FFmpeg-VP8-v4l2m2m GStreamer-VP8-V4L2-Gst1.0
****************************************************************************************************
Running test suite VP8-TEST-VECTORS with decoder FFmpeg-VP8-v4l2m2m
Using 1 parallel job(s)
****************************************************************************************************

[TEST SUITE      ] (DECODER           ) TEST VECTOR               ... RESULT
----------------------------------------------------------------------
[VP8-TEST-VECTORS] (FFmpeg-VP8-v4l2m2m) vp80-00-comprehensive-001 ... Error
[VP8-TEST-VECTORS] (FFmpeg-VP8-v4l2m2m) vp80-00-comprehensive-002 ... Error
[VP8-TEST-VECTORS] (FFmpeg-VP8-v4l2m2m) vp80-00-comprehensive-003 ... Error
[VP8-TEST-VECTORS] (FFmpeg-VP8-v4l2m2m) vp80-00-comprehensive-004 ... Error
[VP8-TEST-VECTORS] (FFmpeg-VP8-v4l2m2m) vp80-00-comprehensive-005 ... Error
[VP8-TEST-VECTORS] (FFmpeg-VP8-v4l2m2m) vp80-00-comprehensive-006 ... Error
[....]

Ran 0/61 tests successfully               in 8.733 secs
Skipping decoder GStreamer-VP8-V4L2-Gst1.0 because it cannot be run
Generating summary for test suite VP8-TEST-VECTORS and decoders FFmpeg-VP8-v4l2m2m:

[rgonzalezfluendo]

Unlike GStreamer, FFmpeg doesn't have a method to discover available hardware video acceleration backends.

For instance:

ffmpeg -hwaccels               # Show available HW acceleration methods (aka `AVHWDeviceType` + `HWAccel`[1])
ffmpeg -init_hw_device list    # List hardware devices (aka `AVHWDeviceType`)

Only show backend enabled when building FFmpeg. No backends available at run-time.

IMHO, this would be a good feature for FFmpeg. I have spoken about it before with @ndufresne

[1] HWAccel was deleted in commit 5633f9a. So -hwaccels == -init_hw_device list