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Despill question

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Ingi Fiev
Despill question
on Sep 15, 2017 at 9:58:15 pm

Hello,
I read in a book recently what algorithm uses ultimatte for its blue despill.
It goes like this:
If green is equal to or less than red, hold blue to the level of green. If green is greater than red, let blue exceed green only by the amount green exceeds red.
Which I translate by (in no particular language), it works very well with nodes:
despilled blue=
if R>=G and B>=G then RGG else RGB;
$Gr = G+(G-R);
if G>R and B>Gr then RGGr else RGB;

Does anyone know an algorithm for green despill? (Or even others for blue despill)

Thanks!


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Simon Ubsdell
Re: Despill question
on Sep 16, 2017 at 1:10:26 pm
Last Edited By Simon Ubsdell on Sep 16, 2017 at 1:12:21 pm

Here's the simplest version of a green despill:

if G>R then R else G

or you could use:

if G>B then B else G

A more complex arrangement is to use an average of both red and blue as a reference:

if G> ave(R, B) then ave(R, B) else G

But there are many others options that give useful results, depending on the colour composition of your shot.

My Fusion keyer tutorial looks at this question:





Simon Ubsdell
tokyo productions
hawaiki


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Ingi Fiev
Re: Despill question
on Sep 16, 2017 at 3:19:09 pm

Hi,
Thanks for the answer, watched the tutorial!

I am aware of these despill techniques averaging red and blue channels for green despill and works very well in most cases. However what I am looking for is a more complex algorithm that can be effective in more situations and that has specific rules based on natural saturation etc, like that blue despill rule I found in a book. A rule that takes out the green while inflicting the least possible damage to colour automatically.
Where can I found such formulas? In Siggraph papers?

Ingi


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Simon Ubsdell
Re: Despill question
on Sep 16, 2017 at 3:49:33 pm

[Ingi Fiev] "Where can I found such formulas? In Siggraph papers?"

They tend to be what is known as "intellectual property" and as such not generally given away for free, sadly.

As a developer of a keying plug-in (Hawaiki Keyer) I've helped come up with some pretty interesting recipes too but I can't really share them for obvious reasons.

However, once you understand the basic principles, which you already do, you can get a long way with coming up with your own proprietary solutions ... and then you'll want to keep those a secret too.

Simon Ubsdell
tokyo productions
hawaiki


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Ingi Fiev
Re: Despill question
on Sep 16, 2017 at 4:43:08 pm

Yeah I was pretty sure getting that blue despill formula was a bit of luck, probably won't happen again any time soon.

Thanks for your time,
Ingi


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Simon Ubsdell
Re: Despill question
on Sep 17, 2017 at 1:30:12 pm
Last Edited By Simon Ubsdell on Sep 17, 2017 at 1:55:05 pm

One fairly simple thing to bear in mind is that your basic CDK is in fact a very useful map of where the green exceeds the other channels, and hence where the spill is a problem.

With this in mind you can use the CDK as an Effect Mask for any sort of colour correction of your foreground, to boost or reduce specific colours or to adjust brightness and/or saturation.

I show this method in the tutorial I referred to and in many ways it is the most useful way of handling despill.

You can adjust the density of the CDK "map" as required (using either a dedicated node or the Low/High controls for the Effect Mask itself) and the method you use for the CDK itself* is going to affect which colours are targeted by the eventual process.

You can achieve outstanding results with this method and you can really fine tune how it affects different areas of your foreground by adjusting the map itself in any number of different ways.

This can ultimately be a whole lot more useful than looking for a "perfect all-in-one" despill routine.

*If your CDK has a method of adjusting the balance of red and green then it's much easier to target the colours you want to suppress. Therefore try:

G minus mix(R, B)

... rather than the standard:

G minus max(R, B)

Again, there are lots of different and exciting ways of refining the CDK method that I urge you to experiment with.

Simon Ubsdell
tokyo productions
hawaiki


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