chromakey does not observe existing transparency

[Gavin]

Summary of the bug:

The chromakey filter does not observe the existing alpha transparency in the source AVFrame when generating the output.

How to reproduce:

% ffmpeg -i video.mxf -i green_screen4.png -filter_complex "[1:v]chromakey=color=0x00ff00:similarity=0.24:blend=0.01[ckout];[0:v][ckout]overlay[out]" -map "[out]" output.mp4


ffmpeg version git-2020-06-08-d29aaf1 (Windows nightly automated build)

I have created a test image (attached):

• There is a transparent circle in each of the 4 corners of the image
• There is a semi-transparent pink circle in the middle of the image.

Compare the input image to the output image and note the differences.

[Gavin]

Green screen test clip with semi-transparency

[Elon Musk]

And why it should? Use two chromakeys and blend filter with or mode for alpha plane.

[Gavin]

Replying to richardpl:

And why it should? Use two chromakeys and blend filter with or mode for alpha plane.

I'm not experienced with chroma key, so I'll turn that around and ask why it shouldn't respect alpha transparency? That is not meant to be a witty retort.

Can you please elaborate on the solution you propose? I don't understand.

[Gavin]

I believe I have the necessary changes to make this work. I'll submit in the next few hours using the correct channels.

[pdr0]

Here is a workaround using blend on the alpha channel

(using "gray" as the background and your chromkey arguments)

ffmpeg -f lavfi -i color=c=gray:s=640x400 -i green_screen4.png -filter_complex "[1:v]extractplanes=a[origa];[1:v]chromakey=color=0x00ff00:similarity=0.24:blend=0.01,format=gbrap,split[ckout1][ckout2];[ckout1]extractplanes=a[cka];[origa][cka]blend=all_mode=darken,format=gbrap[blend];[ckout2][blend]mergeplanes=0x00010210:gbrap[merge]" -map [merge] merge.png

[Gavin]

Proposed patch

[Gavin]

I have submitted a patch to the devel mailing list for review. Patch is now attached to ticket.