[FFmpeg-devel] Bug on Bayer conversions

[Chema Gonzalez]

Hi,

I found an issue while playing with Bayer pixel format conversions.

```
$ echo -ne '\x00\x00\x00\x00\x00\x00\x00\x00\x00\x00\x00\x00\x00\x00\x00\x00\x00\x00\x00\x00\x00\x00\x00\x00\x00\x00\x00\x00\x00\x00\x00\x00\xff\xff\xff\xff\xff\xff\xff\xff\xff\xff\xff\xff\xff\xff\xff\xff\xff\xff\xff\xff\xff\xff\xff\xff\xff\xff\xff\xff\xff\xff\xff\xff'
> image.raw
$ xxd image.raw
00000000: 0000 0000 0000 0000 0000 0000 0000 0000  ................
00000010: 0000 0000 0000 0000 0000 0000 0000 0000  ................
00000020: ffff ffff ffff ffff ffff ffff ffff ffff  ................
00000030: ffff ffff ffff ffff ffff ffff ffff ffff  ................
```

And then:
```
$ ffmpeg -y -f rawvideo -pixel_format bayer_bggr8 -s 8x8 -i image.raw
-f rawvideo -pix_fmt rgb24 -video_size 8x8 image.raw.rgb
...
Assertion srcSliceH > 1 failed at libswscale/swscale_unscaled.c:1310
Aborted (core dumped)ated 2 times
```

The issue relates to the ffmpeg parallelization.
```
$ ffmpeg -y -filter_threads 1 -f rawvideo -pixel_format bayer_bggr8 -s
8x8 -i image.raw -f rawvideo -pix_fmt rgb24 -video_size 8x8
image.raw.rgb
...
frame=    1 fps=0.0 q=-0.0 Lsize=       0kB time=00:00:00.00
bitrate=N/A speed=   0x    eed=N/A
video:0kB audio:0kB subtitle:0kB other streams:0kB global headers:0kB
muxing overhead: 0.000000%
$ xxd image.raw.rgb
00000000: 0000 0000 0000 0000 0000 0000 0000 0000  ................
00000010: 0000 0000 0000 0000 0000 0000 0000 0000  ................
00000020: 0000 0000 0000 0000 0000 0000 0000 0000  ................
00000030: 0000 0000 0000 0000 0000 0000 0000 0000  ................
00000040: 0000 0000 0000 0000 0000 0000 0000 0000  ................
00000050: 7f00 3f7f 0000 7f00 3f7f 0000 0000 0000  ..?.....?.......
00000060: ffff ffff ffff 7fbf ff7f ffff 7fbf ff7f  ................
00000070: ffff ffff ffff ffff ffff ffff ffff ffff  ................
00000080: ffff ffff ffff ffff ffff ffff ffff ffff  ................
00000090: ffff ffff ffff ffff ffff ffff ffff ffff  ................
000000a0: ffff ffff ffff ffff ffff ffff ffff ffff  ................
000000b0: ffff ffff ffff ffff ffff ffff ffff ffff  ................
```

FYI:
```
$ grep processor /proc/cpuinfo  |wc -l
64
```

Problem seems to be that `ff_sws_slice_worker()`
[libswscale/swscale.c:1222] tries to slice the input to parallelize
the scaling task, in my case in 16 different jobs (gdb'ing the process
shows `nb_threads == nb_jobs == 16`). The 8x8 input is therefore
divided in eight 8x1 slices (1-pixel height), which eventually breaks
in `bayer_to_rgb24_wrapper()` as it asserts `srcSliceH > 1`. The
problem is the same in the 3 Bayer conversion functions
(`bayer_to_rgb24_wrapper()`, `bayer_to_rgb48_wrapper()`, and
`bayer_to_yv12_wrapper()`.

Wondering about the right solution: We could just enforce `nb_threads
= nb_jobs = 1` for all Bayer inputs. That may be the simplest
solution. Or we could make sure that `nb_threads` (and `nb_jobs`) are
capped at `input_height / 2` (to ensure at least 2 pixels per thread).

Any suggestions?

Thanks,
-Chema