How To Remove Red Tracking Dots From Greenscreen

[Hugh]

Track each dot and move a nearby big of greenscreen over it....

If people interract, see what you can do - but you'll most likely have to resort to painting frame by frame (the clone tool is your friend)

[Cman]

Thanks.
I feared that might be the answer! :scream:

[AAAron]

Track all free points, and assign a clone to the tracker, the points that interact is better to do by hand (as Hugh said in erlier post). But if you have not done that already, prepare for the keying by making a garbage mask around the actor(s) and possible other stuff in the scene.

hope this helps:)

[Hugh]

When I'm cleaning up green-screens, I've got what I find to be quite a useful technique....


The need is to get a clean green-screen that has the same lighting variations over the screen.

Here's a Shake screen-grab from a shot I did a few months back... (thumbnails turned off to protect the innocent)

http://www.brokenpipefilms.com/hugh/images/clean_marked.jpg

Section 1) Create a completely clean back-plate.
To start with, I had to find a big of the screen that was pure green-screen all the way through the sequence. (In the end, I actually had to move this crop around with a tracker. If you move it, though, make sure you use a round() function in the expression - otherwise you'll be losing noise information when you move by less than 1 pixel...)
The straight crop probably won't tile perfectly - this is because the light generally didn't stay the same from one side to the other, so while there was no visible change, if I tiled it and then used an Expand, I could see the edges very clearly. This is the kind of thing that really shows up when a sequence is running.
So, to level off the average colour, I used the group of 5 nodes just below the crop. The idea here is to have one version that is blurred just enough to remove the noise (Blur3 here), and one that is blurred so heavily that the image ends up being a pure colour (I find that a blur value of width*50 is easily enough to do this). This is the one that will give the average colour in the final image.
The idea here is to take away the low-frequency detail in the image and then add back another colour which defines the new average colour. So (having converted to float, so that we keep negative values) we subtract the slightly blurred image from the solid colour. This gives an image (averaging around 0) which, when added to the original, will flatten out the average colour, but keep the noise.
Now, when we tile this (the node in the image above is one I wrote here - you can tile it however you want), we get a nice even colour across the image.

Section 2) Get the colour change across the original image
Even though we've got a solid backplate now, it will probably not match the actual plate overall. Sometimes the lighting on a green-screen can vary hugely from one part to another. What we need to do is get an image that has the average value of the greenscreen from the actual plate.
To do this, I did a very harsh key (as I needed to make sure that EVERYTHING that was not green was included), blurred and expanded it for good measure, and used the inverse of this key on the original plate. At this stage, the image is the green-screen with black holes where the original objects were.
Another trick I use quite a lot is doing a heavy blur followed by an MDiv (divide by alpha) to expand the colour I've already got. This fills in any holes that might have been left from tracking markers on the green-screen, and also pushes the green inside the edges of the object. If you make sure that the image is at float before blurring, you'll find that the MDiv affects a lot more than it would otherwise. (You'll get a reasonable colour anywhere that the blur has given an alpha of anything other than 0)
I have no idea what the Bytes node (Bytes2) is doing there - it's setting it to float, but it's actually already float by that stage. Ignore that one ;)
The SetAlpha is there because, after the MDiv, the colour and alpha channels no longer coincide. Setting the alpha to 1 overall means that this won't come and bite you later.

Section 3) Match the colour of the clean backplate to that of the original plate
This is pretty much the same thing that we did in section 1 - we blur the fine detail out of the whole backplate, get the difference between that and the original image colour that we got in section 2, and then add this difference back to the clean green-screen. The result of this will be a clean green-screen, with the original noise, with the average colour defined from section 2. Do be aware that there will be bright white sections - these will be bits that are covered up by large sections of non-green-screen objects (usually the objects that you want to keep)


This then gives you an image that you can use to reveal sections of when removing tracking markers or other unwanted objects. Section 4 is where, in this script, I do a rough key with a garbage mask to reveal a large chunk of the new clean green-screen. It can also be used for the Reveal input of a QuickPaint node for ones that are so close to your objects that you've got to paint them manually.....


I hope this hasn't been too confusing - it may feel like a lot of work for not much, but it's very useful when you've got to end up with a clean plate. It's also very generic - you can copy it from shot to shot, only having to change the crop amount for the clean green-screen (in section 1) and the key (in sections 2 and 4)

I know this has been very Shake-specific - I have no idea how you'd go about doing this kind of thing in a layer-based compositing tool... If you're using a different node-based compositing tool and aren't sure what something does in here, do let me know and I'll explain a little more....

[beaker]

Dude, Hugh, You wouldn't happen to be related to John Madden would you? :)

Good Tut man!

[Hugh]

What, barging in and knocking everyone flat on their backs? (I only know of John Madden because of the game(s?) with his name on... Or was there something more subtle in your question?)


I'm going to put together a slightly neater version of this one, with nice images and step-by-step shake node screenshots.... Unfortunately, I don't have a green-screen plate I can demo it on (if I used the one that the script above was using, I'd almost certainly lose my job....)

If you or anyone else has a green-screen with markers on that I can use for the proper tutorial, it would be much appreciated.... otherwise I'm going to have to fake one...

[vrljc]

John Madden is notorious for the on screen pen diagrams of football plays. He is often spoofed for it. Your diamgram was just too darn reminiscent! :)

I would be very interested in a new shake tree with actual thumbnails (no production shots of course). Any chance you could also archive the images and shake script into a .zip file for us all?

-jon

[Hugh]

Ahhh - I see now.... ;) Thanks for the clarification!

As soon as I get hold of a reasonably green-screen plate to use, I'll be putting together a better tutorial on this one - fully explained and imaged, with any needed files bundled...


and the scribbles? Well, that was all done in Shake with a quickpaint node. The powers-that-be recently decided that I didn't need my wacom, so it was moused, too....

[Cman]

Thanks Hugh!

I saw the red dots in some footage from Peter Jackon's KingKong video diaries, but the footage is too compressed for a real good key, though maybe you could use it.

I'm not sure which one it is exactly, but when I get a chance to find it I'll post a link.

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