"Bluescreen removal" the noobie way

I know there is a nice fancy-looking word for this process, but I’m too lazy to dig it up right now (“chroma key” maybe?).

I just wanted to test some functions in the node compositor and tried to setup a simple bluescreened testscene i can filter.

http://static.flickr.com/102/291741691_0adccd3926.jpg

In the nodetree on the top i’ve tried to filter out the blue color from the rendered image, but no matter what i do i can’t get rid of the shadows :-/

So i figured: why trying to remove something when i can just tell Blender what should remain … i.e Red&Blue.

In this tree i replace the background with another color (red), but an image or other render should work just as well i think.

I’m sure there is a better/faster way of doing this [1], but i figured i’ll post my finding here anyway.

Werner

[1] No i didn’t even search the forum before doing this … it’s kinda hard to do that without internet access. But I’ll do that after i’ve finished this post :wink:

EDIT: Found some related threads :wink:

If you set your bluescreen in the scene to be “shadeless” in the materials options that should stop shadows and light from affecting the colour.

lol, well so much for nodes.

Usually the only reason to use color as a compositing source is when you have live-video to deal with that used it. Otherwise, you just use Alpha.

@lukus & sundialsvc4:

Just imagine the first image to be a live-action shot. I didn’t have my digicam around to make a real one, so I had to improvise a bit using a virtual scene (including shadows/shading for a bit of realism) :wink:

Werner

One thing that works really well is to set the world as an alpha image file. I just made a 10 x 10 pixel alpha .png and set to to world settings. I found in previous versions that this way I wouldn’t get that annoying shade halo around objects with a transparent background. I have not tried with the new version yet so I don’t know if this is the case.

To be honest I haven’t really gotten in to using blenders compositing nodes yet, but the way it is generally done is with garbage mattes. What you do is a roughly mask out objects that you dont want to be visible before you use the cromakey to remove the green. This mask can be done in blender with a black bezier plane or two in the 3d-view while leaving the background (=what sould remain visible in the end composite) white.

Hm, thanks for the tip, that’s a really good technique although you need to make an extra scene & simple render for this (not a biggie) or am I wrong?

Werner

Blender looks promising for this kind of work but still official compositors are easer to use at lest it was last time I checked.

I confess… I love my Macintosh, and the tools that are available for that. You can do “very good” video editing strictly within Blender, of course, but there are advantages to using it simply to produce footage that you can then integrate into the other (live) material using the tool that you’re already using for that live material.

You’re probably going to need to concentrate first upon taking your live-footage and cleaning it up. In other words, apply the chroma-keying filters so that you are looking strictly at “the live footage with the background subtracted out.” Study that footage carefully, by hand for “artifacts,” and clean them up. If there are extraneous objects, matte them out. A pro-grade video editor can do that easily enough.

Be especially careful of cases where a reflection of the chroma background has “splashed” onto an object you want to keep. It can easily render such a thing “translucent.” Likewise, similarly-colored objects, on clothing and such, can pop up as “holes.”

Ideally, you would do all of the “chroma” processing first, replacing that with a pure alpha-channel mask, before proceeding. I like to preview all that stuff against several garish, brightly-colored background planes in the video editor, playing it slowly forward (or, to help break my eyes from “watching it,” slowly backwards) until the whole piece has been visually inspected. The garish backgrounds help to point out the holes and such. It’s time-consuming, exasperating work, but it must be done and done well, because the client sees all. :rolleyes:

Obviously, when you are shooting … when you are setting up the live shots, working out camera angles and so-forth … is the best time to catch these “issues,” i.e. before you start to roll, but you can’t always control what happens next.

Oh, just to go back to my first post … bluescreening is now even easier with the new matte nodes in 2.43 (no release yet):
Manual/Compositing Nodes Mattes

EDIT: wait a second, i need to re-add them to the wiki first :slight_smile:

Werner

I look forward to writing a How to Use section, Werner, once you get it worked out :wink: Check outt he garbage matte technique video from I think Siggraph, I think the post is on BlenderNation. He integrates a blue screen shot of a spaceship model over an earth that is perfect. Garbage matte is used to block out what isnt blue screened.

Btw, I think you want the shadows to be there, so that when you alpha over, the shadow comes with it. That’s why the lighting in the blue screen shots and the real background has to be the same; so that there is no mismatch. That is the purpose of shooting the chrome ball with the live action.

I’ve re-added the deleted text for the matte-nodes to the wiki-page.

I wont have much time to dig far into the new nodes (yet), but a (simple) tutorial or example woudl get me started faster for sure … do you stil have the link, i can’t seem to find the news entry you are referring to. :-/

I’ve seen quite some examples where the shadow was re-added after bluescreening by using OnlyShadow planes and other geometry as well. Whatever works best i assume :slight_smile:

Werner

yes, i even saved the vid on my PC. Bob Holcomb, Siggraph 06 presentation. BlenderNation. quicktime file. 53 meg. excellent. goes thru garbage mat, chroma keying to remove spill, etc. shows node layout. http://www.blendernation.com/2006/08/09/siggraph-video-compositing/#more-1046

There is/was also http://www.weirdhat.com/blender/compositing2/ which I have also saved on my PC

Off to airport, out of touch for a week. njoy!

BTW, the newest complete CVS build(they fixed that armature issue), they have chroma key, and matte nodes.
Awesome.

Ya know, Hoehrer, this is a very good and often-called-for use of Blender. Could you put this in the wiki?

Yeah, that is a good idea … but why don’t we just make a pratical example fro the new matte nodes (especially the chroma key node)? They where added for exactly this purpose. I’ll try to come up with some nice example.

http://mediawiki.blender.org/index.php/Manual/Compositing_Nodes_Mattes#Chroma_Key_Node

Werner

On a related note: Did anybody try the “Chroma Key” node in the recent CVS builds? I can’t get it to work at all. Feeding it a bluescreened image and tweaking the options doesn’t affect the output at all (output=white matte output and original image). :-/

The “Difference Key” node seems to work (at least in some cases … i need to play with it a bit to check this)

Werner

Hoehrer,

I wrote the chroma key nodes and I’m curious as to the problem your having. If you can, send me the image to bob_holcomb ‘at’ hotmail.com. I’ll be happy to help you out and try to make those nodes better.

cheers,

bob

Werner, I just rewrote one of the keying nodes (Luma key is now Channel Key). It should be able to pull a good matte from your image. And the UI is pretty easy to use :wink:

Cheers,

Bob