Blocky Matte

Hello!

When I first tried to use both chroma and color matte nodes I noticed that the resulting matte was way too blocky. I noticed that only video files result in blocky matte. Stills do not get blocky. I guess Blender has a problem in extracting the right frame or enough info from the video clip to make a correct matte. The workaround for videos is quite simple: convert your video clip to still frames and add it to compositor as a still series. Is this a bug, should I report it as a bug? I never reported a bug and I do not know how to do it. If anyone else is willing to do it I will much appreciate it.
Regards,
Christian.

Video compression is a killer. But converting the vieo to still sshould not eliminate the compression artifacts. What stills did you test on? Were they converted from a video source?

Color or chroma if you like, is heavily compressed in a video stream. The luma component is less compressed as it contains the detail. The problem for keying is that you are trying to derive detail from the wrong channel. Oddly a small amount of blur applied to the color channel split from the luma can improve a key and make the blocks more soft.

There are lots of tutorials and node setups that try and side step this issue. But Blender probably isn’t the best tool for it I’m afraid.

Thanks for pointing me to the other threads but I already read them. For a moment I had the feeling I am the smart guy. I was wrong, totally wrong. I used a high-res(8 MP) jpeg and it all looked ok. But now I rendered a video file to stills and got the same blocky look of matte, no matter the file format: I tried both jpeg, tiff, targa and png. I measured the blocks and they are exactly one pixel in size. I guess the problem is in the way Blender reads the color information. Perhaps it should add antialising/dithering to it.

No that is the way the compressed files are decompressed. If you want to filter the image for post processing then thats what those other threads suggest. If Blender doesn’t do it, it is because you must do that step yourself. If you want the software to do it then pay for that software.

"If you want the software to do it then pay for that software. " Great tip, thank you!

I am not trolling you, this is the same advice I got here ages ago. There is no magic involved, you must do all the hard work yourself, Blender has very few prebuilt tools for this sort of thing. The thread I posted for you earlier really is the best you can do. By all means prove me wrong and show us how much better it could be.

The only way to clean up a key from a heavily compressed signal is to blur the chroma component. Good luck.

I am just guessing but if you are trying to pull a matte on 4:2:2 or 4:2:0 chroma subsampling plates you will get blocky mattes. You need to preblur the image, see http://wiki.blender.org/index.php/Doc:2.4/Tutorials/Composite_Nodes/Setups/Video_compositing

ps. channel mattes work better than chroma or color (usually).

Regele, what does your noodle look like? I can’t say I have much experience working with chroma keying, but the few times I have, the Difference Key, Channel Key and Color Spill nodes have proven quite effective. Could you post a single frame from your sequence, perhaps it’s something that can be resolved?