[FFmpeg-devel] [PATCH 3/3] lavfi/vf_libvmaf: add warning when color ranges differ

[Chema Gonzalez]

The VMAF filter uses the pixel values without considering
the color ranges. This is incorrect. Patch adds a warning
so at least the user knows it.

Let's see an example.

(1) Let's get FR and LR versions of the same image.

```
$ ffmpeg -y -i /tmp/lena.490x490.ppm -vf scale="out_range=full" -pix_fmt yuv420p /tmp/lena.full.y4m
$ xxd /tmp/lena.full.y4m |head
00000000: 5955 5634 4d50 4547 3220 5734 3930 2048  YUV4MPEG2 W490 H
00000010: 3439 3020 4632 353a 3120 4970 2041 303a  490 F25:1 Ip A0:
00000020: 3020 4334 3230 6a70 6567 2058 5953 4353  0 C420jpeg XYSCS
00000030: 533d 3432 304a 5045 4720 5843 4f4c 4f52  S=420JPEG XCOLOR
00000040: 5241 4e47 453d 4655 4c4c 0a46 5241 4d45  RANGE=FULL.FRAME
00000050: 0a72 7271 7070 706f 6f6e 6d6d 6c6d 6d6d  .rrqpppoonmmlmmm
00000060: 6c6e 6e6d 6d6e 6e6e 6d6c 6d6d 6d6d 6d6d  lnnmmnnnmlmmmmmm
00000070: 6d6e 6d6b 6c6d 6e6e 6d6c 6d6d 6e6e 6f6f  mnmklmnnmlmmnnoo
00000080: 6f6f 6e6e 6e6e 6f70 7172 7375 7676 7370  oonnnnopqrsuvvsp
00000090: 6d69 6662 5e59 534d 4845 3d35 302e 2d2c  mifb^YSMHE=50.-,
```

```
$ ffmpeg -y -i /tmp/lena.490x490.ppm -vf scale="out_range=limited" -pix_fmt yuv420p /tmp/lena.limited.y4m
$ xxd /tmp/lena.limited.y4m | head
00000000: 5955 5634 4d50 4547 3220 5734 3930 2048  YUV4MPEG2 W490 H
00000010: 3439 3020 4632 353a 3120 4970 2041 303a  490 F25:1 Ip A0:
00000020: 3020 4334 3230 6a70 6567 2058 5953 4353  0 C420jpeg XYSCS
00000030: 533d 3432 304a 5045 4720 5843 4f4c 4f52  S=420JPEG XCOLOR
00000040: 5241 4e47 453d 4c49 4d49 5445 440a 4652  RANGE=LIMITED.FR
00000050: 414d 450a 7272 7170 7070 6f6f 6e6e 6e6d  AME.rrqpppoonnnm
00000060: 6e6e 6e6d 6f6e 6e6e 6e6e 6e6e 6d6e 6e6e  nnnmonnnnnnnmnnn
00000070: 6e6e 6e6e 6f6e 6c6d 6e6f 6e6e 6d6e 6e6f  nnnnonlmnonnmnno
00000080: 6f6f 6f6f 6f6f 6f6f 6f6f 7071 7273 7576  oooooooooopqrsuv
00000090: 7673 706e 6a68 6461 5c57 524e 4b44 3d39  vspnjhda\WRNKD=9
```

Note that the 2x images are the same. Only difference is the range,
and the precision issues related to range conversion.

(2) Let's calculate the VMAF score:
```
$ ./ffmpeg -filter_threads 1 -filter_complex_threads 1 -i /tmp/lena.full.y4m -i /tmp/lena.limited.y4m -lavfi libvmaf="model=path=/usr/share/model/vmaf_v0.6.1neg.json" -report -f null -
...
[Parsed_libvmaf_0 @ 0x3cc9b40] VMAF score: 85.530109
```

As we are comparing an image with itself, we expect the score to
be close to 100. Issue here is that the VMAF filter just uses the
pixel values, ignoring the color ranges.

Proposed solution is to add a warning.
```
$ ./ffmpeg -filter_threads 1 -filter_complex_threads 1 -i /tmp/lena.full.y4m -i /tmp/lena.limited.y4m -lavfi libvmaf="model=path=/us
r/share/model/vmaf_v0.6.1neg.json" -report -f null -
...
[Parsed_libvmaf_0 @ 0x3cc9b40] Warning: distorted and reference frames use different color ranges (pc != tv)
...
[Parsed_libvmaf_0 @ 0x3cc9b40] VMAF score: 85.530109
```

Tested:

Ran fate.
```
$ make fate -j
...
TEST    seek-lavf-ppmpipe
TEST    seek-lavf-pgmpipe
TEST    seek-lavf-mxf_opatom
```
---
 libavfilter/vf_libvmaf.c | 7 +++++++
 1 file changed, 7 insertions(+)

diff --git a/libavfilter/vf_libvmaf.c b/libavfilter/vf_libvmaf.c
index 2586f37d99..d330658abb 100644
--- a/libavfilter/vf_libvmaf.c
+++ b/libavfilter/vf_libvmaf.c
@@ -141,6 +141,13 @@ static int do_vmaf(FFFrameSync *fs)
     if (ctx->is_disabled || !ref)
         return ff_filter_frame(ctx->outputs[0], dist);
 
+    if (dist->color_range != ref->color_range) {
+        av_log(ctx, AV_LOG_WARNING, "Warning: distorted and reference "
+               "frames use different color ranges (%s != %s)\n",
+               av_color_range_name(dist->color_range),
+               av_color_range_name(ref->color_range));
+    }
+
     err = copy_picture_data(ref, &pic_ref, s->bpc);
     if (err) {
         av_log(s, AV_LOG_ERROR, "problem during vmaf_picture_alloc.\n");
-- 
2.39.2