[FFmpeg-devel] [PATCH 1/1] libswscale: force a minimum size of the slide for bayer sources

[Chema Gonzalez]

Bayer sources are read in groups of 2 lines (e.g. for a
BGGR flavor, the first row contains only B and G samples,
while the second row contains only G and R samples). They
need to be read as a whole.

Tested:

``
$ 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
```

We can see that 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  ................
```

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()`.

The solution was suggested by Anton Khirnov. We set the `dst_slice_align`
value to 2 for Bayer conversions.

```
$ ./ffmpeg -y -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: 0000 0000 0000 0000 0000 0000 0000 0000  ................
00000060: ffff ffff ffff ffff ffff ffff ffff ffff  ................
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  ................
```

We can see the slicing at work, though: The demosaicing does not
carry through different slices (workers). Compare to forcing a
single worker:

```
$ ./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.alt.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  ................
$ xxd /tmp/image.raw.alt.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  ................
```
---
 libswscale/swscale_unscaled.c | 1 +
 1 file changed, 1 insertion(+)

diff --git a/libswscale/swscale_unscaled.c b/libswscale/swscale_unscaled.c
index 8838cc8b53..9af2e7ecc3 100644
--- a/libswscale/swscale_unscaled.c
+++ b/libswscale/swscale_unscaled.c
@@ -2095,6 +2095,7 @@ void ff_get_unscaled_swscale(SwsContext *c)
         c->convert_unscaled = rgbToPlanarRgbWrapper;
 
     if (isBayer(srcFormat)) {
+        c->dst_slice_align = 2;
         if (dstFormat == AV_PIX_FMT_RGB24)
             c->convert_unscaled = bayer_to_rgb24_wrapper;
         else if (dstFormat == AV_PIX_FMT_RGB48)
-- 
2.37.3