SSIM filter showing (small) differences in identical(?) streams

[Ian Pilcher]

I am trying to understand how the SSIM and VMAF filters work, with an
eye to finding the "best" compression settings for a video which will
be composed from a series of TIFF images.  Unfortunately, I'm stuck at
the beginning, as I can't get the SSIM filter to behave as expected.

./source contains the original sequence of images.

  $ tiffinfo source/000000.tif
  TIFF Directory at offset 0x473108 (4665608)
    Image Width: 1440 Image Length: 1080
    Bits/Sample: 8
    Sample Format: unsigned integer
    Compression Scheme: None
    Photometric Interpretation: RGB color
    Samples/Pixel: 3
    Rows/Strip: 1
    Planar Configuration: single image plane

I attempt to create a lossless video of the first minute.

  $ ffmpeg -start_number 0 -framerate 60000/1001 -i source/%06d.tif -t
00:01:00 -c:v huffyuv lossless.mkv

The result appears reasonable.

  $ mediainfo lossless.mkv
  General
  Unique ID                                :
235140899628261703308032414639716345340 (0xB0E67D6EF6B78362D9BCF9EA3080A5FC)
  Complete name                            : lossless.mkv
  Format                                   : Matroska
  Format version                           : Version 4
  File size                                : 8.54 GiB
  Duration                                 : 59 s 994 ms
  Overall bit rate                         : 1 223 Mb/s
  Writing application                      : Lavf58.45.100
  Writing library                          : Lavf58.45.100
  ErrorDetectionType                       : Per level 1

  Video
  ID                                       : 1
  Format                                   : HuffYUV
  Format version                           : Version 2
  Codec ID                                 : V_MS/VFW/FOURCC / HFYU
  Duration                                 : 59 s 994 ms
  Bit rate                                 : 1 199 Mb/s
  Width                                    : 1 440 pixels
  Height                                   : 1 080 pixels
  Display aspect ratio                     : 4:3
  Frame rate mode                          : Constant
  Frame rate                               : 59.940 FPS
  Color space                              : RGB
  Bit depth                                : 8 bits
  Scan type                                : Progressive
  Bits/(Pixel*Frame)                       : 12.860
  Stream size                              : 8.37 GiB (98%)
  Writing library                          : Lavc58.91.100 huffyuv
  Default                                  : Yes
  Forced                                   : No

Now let's see what the SSIM filter says.

  $ ffmpeg -i lossless.mkv -start_number 0 -framerate 60000/1001 -i
source/%06d.tif -t 00:01:00 -filter_complex ssim -f null -
  ...
  Input #0, matroska,webm, from 'lossless.mkv':
    Metadata:
      ENCODER         : Lavf58.45.100
    Duration: 00:00:59.99, start: 0.000000, bitrate: 1223104 kb/s
      Stream #0:0: Video: huffyuv (HFYU / 0x55594648), bgr0, 1440x1080,
59.94 fps, 59.94 tbr, 1k tbn, 1k tbc (default)
      Metadata:
        ENCODER         : Lavc58.91.100 huffyuv
        DURATION        : 00:00:59.994000000
  Input #1, image2, from 'source/%06d.tif':
    Duration: 01:47:16.00, start: 0.000000, bitrate: N/A
      Stream #1:0: Video: tiff, rgb24, 1440x1080, 59.94 tbr, 59.94 tbn,
59.94 tbc
  ...
  [Parsed_ssim_0 @ 0x55fb09c738c0] not matching timebases found between
first input: 1/1000 and second input 1001/60000, results may be incorrect!
  ...
  [Parsed_ssim_0 @ 0x55fb09c738c0] SSIM R:0.833774 (7.793009) G:0.835401
(7.835723) B:0.831058 (7.722615) All:0.833411 (7.783532)

That's not what I expected.  My understanding is that the R, G, B, and
All values should all be "1.000000 (inf)".

The "not matching timebases" warning is the obvious thing to look at.
After much searching, I came upon the -video_track_timescale option, but
it seems to only take an integer, and 60 is not the same as 59.94, so it
seems that I simply can't directly compare a video stream with a non-
integer framerate to an image sequence.

As a workaround, I tried extracting the lossless video frames as a
separate image sequence.

  $ ffmpeg -i lossless.mkv -start_number 0 lossless/%06d.png

This created the expected sequence of image files (000000.png -
003595.png).  Since I have both the "source" and the "lossless" streams
as images sequences, I can use ImageMagick to compare them.

  $ for I in `seq -w 0 3595` ; do compare -metric AE source/00${I}.tif
lossless/00${I}.png /tmp/diff.png 2>/dev/null || echo $I ; done

This produces no output, indicating that ImageMagick thinks that the
TIFF files in ./source and the PNG files in ./lossless contain
completely identical image data.  What does the SSIM filter say?

  $ ffmpeg -framerate 60000/1001 -start_number 0 -i lossless/%06d.png
-framerate 60000/1001 -start_number 0 -i source/%06d.tif -t 00:01:00
-filter_complex ssim -f null -
  ...
  Input #0, image2, from 'lossless/%06d.png':
    Duration: 00:00:59.99, start: 0.000000, bitrate: N/A
      Stream #0:0: Video: png, rgb24(pc), 1440x1080, 59.94 fps, 59.94
tbr, 59.94 tbn, 59.94 tbc
  Input #1, image2, from 'source/%06d.tif':
    Duration: 01:47:16.00, start: 0.000000, bitrate: N/A
      Stream #1:0: Video: tiff, rgb24, 1440x1080, 59.94 tbr, 59.94 tbn,
59.94 tbc
  ...
  [Parsed_ssim_0 @ 0x556c7e948980] SSIM R:0.999775 (36.484195)
G:0.999777 (36.508857) B:0.999774 (36.451744) All:0.999775 (36.481536)

Close, but not the "1.000000 (inf)" that I would expect for identical
streams.  I suppose that it may be related to the "rgb24(pc)" vs.
rgb24 pixel formats; it's hard to be sure, as Google isn't turning up
anything that describes what "rgb24(pc)" actually means.

Does anyone know what the heck is going on here?  Am I doing something
wrong?

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[pdr0]

Ian Pilcher wrote

> I am trying to understand how the SSIM and VMAF filters work, with an
> eye to finding the "best" compression settings for a video which will
> be composed from a series of TIFF images.  Unfortunately, I'm stuck at
> the beginning, as I can't get the SSIM filter to behave as expected.
>
> ./source contains the original sequence of images.
>
>   $ tiffinfo source/000000.tif
>   TIFF Directory at offset 0x473108 (4665608)
>     Image Width: 1440 Image Length: 1080
>     Bits/Sample: 8
>     Sample Format: unsigned integer
>     Compression Scheme: None
>     Photometric Interpretation: RGB color
>     Samples/Pixel: 3
>     Rows/Strip: 1
>     Planar Configuration: single image plane
>
> I attempt to create a lossless video of the first minute.
>
>   $ ffmpeg -start_number 0 -framerate 60000/1001 -i source/%06d.tif -t
> 00:01:00 -c:v huffyuv lossless.mkv
>
> The result appears reasonable.
>
>   $ mediainfo lossless.mkv
>   General
>   Unique ID                                :
> 235140899628261703308032414639716345340
> (0xB0E67D6EF6B78362D9BCF9EA3080A5FC)
>   Complete name                            : lossless.mkv
>   Format                                   : Matroska
>   Format version                           : Version 4
>   File size                                : 8.54 GiB
>   Duration                                 : 59 s 994 ms
>   Overall bit rate                         : 1 223 Mb/s
>   Writing application                      : Lavf58.45.100
>   Writing library                          : Lavf58.45.100
>   ErrorDetectionType                       : Per level 1
>
>   Video
>   ID                                       : 1
>   Format                                   : HuffYUV
>   Format version                           : Version 2
>   Codec ID                                 : V_MS/VFW/FOURCC / HFYU
>   Duration                                 : 59 s 994 ms
>   Bit rate                                 : 1 199 Mb/s
>   Width                                    : 1 440 pixels
>   Height                                   : 1 080 pixels
>   Display aspect ratio                     : 4:3
>   Frame rate mode                          : Constant
>   Frame rate                               : 59.940 FPS
>   Color space                              : RGB
>   Bit depth                                : 8 bits
>   Scan type                                : Progressive
>   Bits/(Pixel*Frame)                       : 12.860
>   Stream size                              : 8.37 GiB (98%)
>   Writing library                          : Lavc58.91.100 huffyuv
>   Default                                  : Yes
>   Forced                                   : No
>
> Now let's see what the SSIM filter says.
>
>   $ ffmpeg -i lossless.mkv -start_number 0 -framerate 60000/1001 -i
> source/%06d.tif -t 00:01:00 -filter_complex ssim -f null -
>   ...
>   Input #0, matroska,webm, from 'lossless.mkv':
>     Metadata:
>       ENCODER         : Lavf58.45.100
>     Duration: 00:00:59.99, start: 0.000000, bitrate: 1223104 kb/s
>       Stream #0:0: Video: huffyuv (HFYU / 0x55594648), bgr0, 1440x1080,
> 59.94 fps, 59.94 tbr, 1k tbn, 1k tbc (default)
>       Metadata:
>         ENCODER         : Lavc58.91.100 huffyuv
>         DURATION        : 00:00:59.994000000
>   Input #1, image2, from 'source/%06d.tif':
>     Duration: 01:47:16.00, start: 0.000000, bitrate: N/A
>       Stream #1:0: Video: tiff, rgb24, 1440x1080, 59.94 tbr, 59.94 tbn,
> 59.94 tbc
>   ...
>   [Parsed_ssim_0 @ 0x55fb09c738c0] not matching timebases found between
> first input: 1/1000 and second input 1001/60000, results may be incorrect!
>   ...
>   [Parsed_ssim_0 @ 0x55fb09c738c0] SSIM R:0.833774 (7.793009) G:0.835401
> (7.835723) B:0.831058 (7.722615) All:0.833411 (7.783532)
>
> That's not what I expected.  My understanding is that the R, G, B, and
> All values should all be "1.000000 (inf)".
>
> The "not matching timebases" warning is the obvious thing to look at.
> After much searching, I came upon the -video_track_timescale option, but
> it seems to only take an integer, and 60 is not the same as 59.94, so it
> seems that I simply can't directly compare a video stream with a non-
> integer framerate to an image sequence.
>
> As a workaround, I tried extracting the lossless video frames as a
> separate image sequence.
>
>   $ ffmpeg -i lossless.mkv -start_number 0 lossless/%06d.png
>
> This created the expected sequence of image files (000000.png -
> 003595.png).  Since I have both the "source" and the "lossless" streams
> as images sequences, I can use ImageMagick to compare them.
>
>   $ for I in `seq -w 0 3595` ; do compare -metric AE source/00${I}.tif
> lossless/00${I}.png /tmp/diff.png 2>/dev/null || echo $I ; done
>
> This produces no output, indicating that ImageMagick thinks that the
> TIFF files in ./source and the PNG files in ./lossless contain
> completely identical image data.  What does the SSIM filter say?
>
>   $ ffmpeg -framerate 60000/1001 -start_number 0 -i lossless/%06d.png
> -framerate 60000/1001 -start_number 0 -i source/%06d.tif -t 00:01:00
> -filter_complex ssim -f null -
>   ...
>   Input #0, image2, from 'lossless/%06d.png':
>     Duration: 00:00:59.99, start: 0.000000, bitrate: N/A
>       Stream #0:0: Video: png, rgb24(pc), 1440x1080, 59.94 fps, 59.94
> tbr, 59.94 tbn, 59.94 tbc
>   Input #1, image2, from 'source/%06d.tif':
>     Duration: 01:47:16.00, start: 0.000000, bitrate: N/A
>       Stream #1:0: Video: tiff, rgb24, 1440x1080, 59.94 tbr, 59.94 tbn,
> 59.94 tbc
>   ...
>   [Parsed_ssim_0 @ 0x556c7e948980] SSIM R:0.999775 (36.484195)
> G:0.999777 (36.508857) B:0.999774 (36.451744) All:0.999775 (36.481536)
>
> Close, but not the "1.000000 (inf)" that I would expect for identical
> streams.  I suppose that it may be related to the "rgb24(pc)" vs.
> rgb24 pixel formats; it's hard to be sure, as Google isn't turning up
> anything that describes what "rgb24(pc)" actually means.
>
> Does anyone know what the heck is going on here?  Am I doing something
> wrong?

Different timebase means there is potential to compare different frames.

Use settb

ffmpeg -framerate 60000/1001 -i lossless.mkv -start_number 0 -framerate
60000/1001 -i source/%06d.tif -t 00:01:00 -lavfi
"[0:v]settb=1/AVTB,setpts=PTS-STARTPTS[main];[1:v]settb=1/AVTB,setpts=PTS-STARTPTS[ref];[main][ref]ssim"
-f null -



For the 2nd image sequence png vs. tiff, one is rgb24(pc) , one is rgb24 .
There might be some metadata that is skewing the results. Need more info



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[Carl Eugen Hoyos-2]

Am Sa., 27. Feb. 2021 um 04:53 Uhr schrieb pdr0 <[hidden email]>:

> ffmpeg -framerate 60000/1001 -i lossless.mkv -start_number 0
> -framerate 60000/1001 -i source/%06d.tif -t 00:01:00 -lavfi
> "[0:v]settb=1/AVTB,setpts=PTS-STARTPTS[main];[1:v]settb=1/AVTB,setpts=PTS-STARTPTS[ref];[main][ref]ssim"
> -f null -

I suspect you have to remove the first "framerate",
the mkv demuxer doesn't know this option.

> For the 2nd image sequence png vs. tiff, one is rgb24(pc) , one is rgb24 .

Iirc, the ssim filter ignores this metadata difference.

Carl Eugen
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[Paul B Mahol]

On Sat, Feb 27, 2021 at 9:37 AM Carl Eugen Hoyos <[hidden email]> wrote:

> Am Sa., 27. Feb. 2021 um 04:53 Uhr schrieb pdr0 <[hidden email]>:
>
> > ffmpeg -framerate 60000/1001 -i lossless.mkv -start_number 0
> > -framerate 60000/1001 -i source/%06d.tif -t 00:01:00 -lavfi
> >
> "[0:v]settb=1/AVTB,setpts=PTS-STARTPTS[main];[1:v]settb=1/AVTB,setpts=PTS-STARTPTS[ref];[main][ref]ssim"
> > -f null -
>
> I suspect you have to remove the first "framerate",
> the mkv demuxer doesn't know this option.
>
> > For the 2nd image sequence png vs. tiff, one is rgb24(pc) , one is rgb24
> .
>
> Iirc, the ssim filter ignores this metadata difference.
>

but not the swscale filter


Full log would show this.


> Carl Eugen
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[Ian Pilcher]

On 2/26/21 9:52 PM, pdr0 wrote:
> Different timebase means there is potential to compare different frames.
>
> Use settb
>
> ffmpeg -framerate 60000/1001 -i lossless.mkv -start_number 0 -framerate
> 60000/1001 -i source/%06d.tif -t 00:01:00 -lavfi
> "[0:v]settb=1/AVTB,setpts=PTS-STARTPTS[main];[1:v]settb=1/AVTB,setpts=PTS-STARTPTS[ref];[main][ref]ssim"
> -f null -

That seems to work.  It gets rid of the "not matching timebases"
warning.  It does cause the duration option (-t 00:01:00) to not work,
but I was able to use the "-frames:v" option instead.  (In fact, I don't
need specify the framerate of the image sequence at all.)

  $ ffmpeg -i lossless.mkv -start_number 0 -i source/%06d.tif -frames:v
3596 -lavfi
"[0:v]settb=1/AVTB,setpts=PTS-STARTPTS[main];[1:v]settb=1/AVTB,setpts=PTS-STARTPTS[ref];[main][ref]ssim"
-f null -
  ...
  Input #0, matroska,webm, from 'lossless.mkv':
    Metadata:
      ENCODER         : Lavf58.45.100
    Duration: 00:00:59.99, start: 0.000000, bitrate: 1223104 kb/s
      Stream #0:0: Video: huffyuv (HFYU / 0x55594648), bgr0, 1440x1080,
59.94 fps, 59.94 tbr, 1k tbn, 1k tbc (default)
      Metadata:
        ENCODER         : Lavc58.91.100 huffyuv
        DURATION        : 00:00:59.994000000
  Input #1, image2, from 'source/%06d.tif':
    Duration: 04:17:10.96, start: 0.000000, bitrate: N/A
      Stream #1:0: Video: tiff, rgb24, 1440x1080, 25 tbr, 25 tbn, 25 tbc
  ...
  [Parsed_ssim_4 @ 0x562a600ac780] SSIM R:0.999888 (39.506142)
G:0.999889 (39.531716) B:0.999887 (39.466715) All:0.999888 (39.501442)

So this gets me to the same result that I got when I compared the
derived PNG image sequence to the original TIFF sequence.

  $ ffmpeg -start_number 0 -i lossless/%06d.png -start_number 0 -i
source/%06d.tif -frames:v 3596 -filter_complex ssim -f null -
  ...
  [Parsed_ssim_0 @ 0x5630c77ad440] SSIM R:0.999888 (39.506142)
G:0.999889 (39.531716) B:0.999887 (39.466715) All:0.999888 (39.501442)

Now that I know about the -frames:v option, I decided to try omitting
the last frame from the comparison.

  $ ffmpeg -start_number 0 -i lossless/%06d.png -start_number 0 -i
source/%06d.tif -frames:v 3595 -filter_complex ssim -f null -
  ...
  [Parsed_ssim_0 @ 0x560797482440] SSIM R:1.000000 (inf) G:1.000000
(inf) B:1.000000 (inf) All:1.000000 (inf)

  $ ffmpeg -i lossless.mkv -start_number 0 -i source/%06d.tif -frames:v
3595 -lavfi
"[0:v]settb=1/AVTB,setpts=PTS-STARTPTS[main];[1:v]settb=1/AVTB,setpts=PTS-STARTPTS[ref];[main][ref]ssim"
-f null -
  ...
  [Parsed_ssim_4 @ 0x5571f0a97780] SSIM R:1.000000 (inf) G:1.000000
(inf) B:1.000000 (inf) All:1.000000 (inf)

That gets me the expected score.

I don't understand why I have to omit the final frame.  Both ImageMagick
and ffmpeg show the images as identical.

  $ compare -metric AE lossless/003595.png source/003595.tif /tmp/diff.png
  0

  $ ffmpeg -i lossless/003595.png -i source/003595.tif -filter_complex
ssim -f null -
  ...
  [Parsed_ssim_0 @ 0x561a6aae1540] SSIM R:1.000000 (inf) G:1.000000
(inf) B:1.000000 (inf) All:1.000000 (inf)

This is definitely good enough for my purposes, however.  I'm going to
declare victory and move on.

Thanks for your help!

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[Paul B Mahol]

On Sun, Feb 28, 2021 at 5:31 PM Ian Pilcher <[hidden email]> wrote:

> On 2/26/21 9:52 PM, pdr0 wrote:
> > Different timebase means there is potential to compare different frames.
> >
> > Use settb
> >
> > ffmpeg -framerate 60000/1001 -i lossless.mkv -start_number 0 -framerate
> > 60000/1001 -i source/%06d.tif -t 00:01:00 -lavfi
> >
> "[0:v]settb=1/AVTB,setpts=PTS-STARTPTS[main];[1:v]settb=1/AVTB,setpts=PTS-STARTPTS[ref];[main][ref]ssim"
> > -f null -
>
> That seems to work.  It gets rid of the "not matching timebases"
> warning.  It does cause the duration option (-t 00:01:00) to not work,
> but I was able to use the "-frames:v" option instead.  (In fact, I don't
> need specify the framerate of the image sequence at all.)
>
>   $ ffmpeg -i lossless.mkv -start_number 0 -i source/%06d.tif -frames:v
> 3596 -lavfi
> "[0:v]settb=1/AVTB,setpts=PTS-STARTPTS[main];[1:v]settb=1/AVTB,setpts=PTS-STARTPTS[ref];[main][ref]ssim"
>
> -f null -
>   ...
>   Input #0, matroska,webm, from 'lossless.mkv':
>     Metadata:
>       ENCODER         : Lavf58.45.100
>     Duration: 00:00:59.99, start: 0.000000, bitrate: 1223104 kb/s
>       Stream #0:0: Video: huffyuv (HFYU / 0x55594648), bgr0, 1440x1080,
> 59.94 fps, 59.94 tbr, 1k tbn, 1k tbc (default)
>       Metadata:
>         ENCODER         : Lavc58.91.100 huffyuv
>         DURATION        : 00:00:59.994000000
>   Input #1, image2, from 'source/%06d.tif':
>     Duration: 04:17:10.96, start: 0.000000, bitrate: N/A
>       Stream #1:0: Video: tiff, rgb24, 1440x1080, 25 tbr, 25 tbn, 25 tbc
>   ...
>   [Parsed_ssim_4 @ 0x562a600ac780] SSIM R:0.999888 (39.506142)
> G:0.999889 (39.531716) B:0.999887 (39.466715) All:0.999888 (39.501442)
>
> So this gets me to the same result that I got when I compared the
> derived PNG image sequence to the original TIFF sequence.
>
>   $ ffmpeg -start_number 0 -i lossless/%06d.png -start_number 0 -i
> source/%06d.tif -frames:v 3596 -filter_complex ssim -f null -
>   ...
>   [Parsed_ssim_0 @ 0x5630c77ad440] SSIM R:0.999888 (39.506142)
> G:0.999889 (39.531716) B:0.999887 (39.466715) All:0.999888 (39.501442)
>
> Now that I know about the -frames:v option, I decided to try omitting
> the last frame from the comparison.
>
>   $ ffmpeg -start_number 0 -i lossless/%06d.png -start_number 0 -i
> source/%06d.tif -frames:v 3595 -filter_complex ssim -f null -
>   ...
>   [Parsed_ssim_0 @ 0x560797482440] SSIM R:1.000000 (inf) G:1.000000
> (inf) B:1.000000 (inf) All:1.000000 (inf)
>
>   $ ffmpeg -i lossless.mkv -start_number 0 -i source/%06d.tif -frames:v
> 3595 -lavfi
> "[0:v]settb=1/AVTB,setpts=PTS-STARTPTS[main];[1:v]settb=1/AVTB,setpts=PTS-STARTPTS[ref];[main][ref]ssim"
>
> -f null -
>   ...
>   [Parsed_ssim_4 @ 0x5571f0a97780] SSIM R:1.000000 (inf) G:1.000000
> (inf) B:1.000000 (inf) All:1.000000 (inf)
>
> That gets me the expected score.
>
> I don't understand why I have to omit the final frame.  Both ImageMagick
> and ffmpeg show the images as identical.
>
>   $ compare -metric AE lossless/003595.png source/003595.tif /tmp/diff.png
>   0
>
>   $ ffmpeg -i lossless/003595.png -i source/003595.tif -filter_complex
> ssim -f null -
>   ...
>   [Parsed_ssim_0 @ 0x561a6aae1540] SSIM R:1.000000 (inf) G:1.000000
> (inf) B:1.000000 (inf) All:1.000000 (inf)
>
> This is definitely good enough for my purposes, however.  I'm going to
> declare victory and move on.
>

Try ssim=shortest=1

Most likely inputs have not same number of frames.

>
> Thanks for your help!
>
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[Ian Pilcher]

On 2/28/21 12:57 PM, Paul B Mahol wrote:

> On Sun, Feb 28, 2021 at 5:31 PM Ian Pilcher <[hidden email]> wrote:
>>    $ ffmpeg -i lossless.mkv -start_number 0 -i source/%06d.tif -frames:v
>> 3596 -lavfi
>> "[0:v]settb=1/AVTB,setpts=PTS-STARTPTS[main];[1:v]settb=1/AVTB,setpts=PTS-STARTPTS[ref];[main][ref]ssim"
>>
>> -f null -
>
> Try ssim=shortest=1
>
> Most likely inputs have not same number of frames.

They don't have the same number of frames.  That's why I need to specify
the duration, either with "-t 00:01:00" or with "-frames:v 3596".  The
correct number of frames *is* 3596, however, so I don't understand why
I have to limit the comparison to only 3595 frames to get the correct
result.

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