Colour hues of imported videos are sometimes unexpectedly different

[triska]

OS and Version (please complete the following information):

• Operating System: Debian Trixie
• Shotcut Version: shotcut-linux-x86_64-26.4.26.txz from the project download page

Describe the bug
Colour hues of imported videos are sometimes unexpectedly different even for identical displayed clip properties.

Steps To Reproduce

1. Download green.mp4 from https://www.metalevel.at/shotcut/green.mp4
2. Create a copy of green.mp4 with: ffmpeg -i green.mp4 -colorspace 1 copy.mp4
3. Start Shotcut, create a new project and append both videos to the timeline.
4. The properties of the two videos show the same value for Color space: ITU-R BT.709.
5. Yet the hue of the green differs visibly and measurably between the two clips. In the clip corresponding to green.mp4, the green value is measured as 225, whereas in that of copy.mp4 it is 238.

Expected behavior

I would expect the green hue to be exactly the same in the two clips, since the -colorspace option of ffmpeg is meant to set the frame attribute that was, according to the shown properties, apparently implicitly already taken to be bt709 for green.mp4, and explicitly set to that same value for copy.mp4.

If the difference in hues is unavoidable, I would expect one of the clip "Properties" to show a difference between one of the attributes pertaining to colours.

Screenshots

green.mp4:

copy.mp4:

Identical video properties for both clips, in particular ITU-R BT.709:

[ddennedy]

Your green.mp4 does not specify a colorspace. When you run ffprobe to get detailed stream info (in Shotcut Properties > menu > More Information), it shows:

color_range=unknown
color_space=unknown
color_transfer=unknown
color_primaries=unknown

So Shotcut has to guess, and it uses a heuristic about whether it is SD or HD. 1280x720 is HD and assumes BT.709. I think what you observe is that Shotcut and ffmpeg behave differently when they have to guess. Whatever wrote green.mp4 should specify the color properties to avoid this in the first place.

[triska]

Thank you for looking into this!

I think the -colorspace option for ffmpeg does specify the colour space of the input file?

So for both videos, BT.709 is used, and still they look different?

[ddennedy]

Shotcut, or rather MLT, is probably doing something wrong when it has to guess. I suggest to avoid making it guess. It might be awhile until someone finds a fix.

[triska]

Yes that's OK! Until this issue is addressed, I can definitely work around it by explicitly setting the parameter with ffmpeg. The original file was created with a popular proprietary program, and it apparently does not by itself add the parameter.

If it's OK for you, I keep this issue open until it is resolved, also in the hope that someone becomes interested in looking into it, and thus help everyone who runs into the same problem.