“3D Compositing Guide – part2” by Simon Reeves
So multiplying white pixels has no effect, but for anything less than 1, it darkens the pixels.
– Other Math Nodes Multiply, Add and Over are the main layering nodes that you’ll need to use, while there are other common nodes, including ‘Screen’, ‘Subtract’ etc. I won’t go into these, have a play for yourself if you want to see what they do 
(Slightly) Channel re-ordering
These mostly concern extracting/switching mattes and alpha channels from different passes and colour correcting.
These can be found under some quite different names, and with different choices in the different apps. For instance, FxTree gives us a few nodes that with different levels of complexity:
‘Swap Component’
‘Component Parser’
‘Pixel Parser’
Personally I tend to use the ‘Component Parser’. A little more advanced than Swap Component as it gives you the chance to write a simple expression, and also multiple inputs so you can pick channels from more than one source.
You can use these nodes to change the alpha to a solid colour, effectively removing it. By setting the alpha to 1 (black) or 0 (white) – which is handy if you’re adding a pass (reflection pass for instance) but don’t want to add its alpha channel as well – which could cause those evil nasty black edges.
Often a you may have an image/pass that doesn’t have the alpha channel you want, so you can replace the original using another image. As well as alpha replacement, you can replace the alpha with the values from another channel (eg. green). And from the first part of the guide , this is why I have rendered an ID Pass.
First of all we plug the ID pass (input node) into the component parser. In the component parser settings, we decide what channel we want to use, red for now, so we type ‘r’ into the alpha’s input area. It doesn’t matter what we type for the R, G + B channels because all we’re going to use is the new alpha. Now we can plug this into the matte input of some other node, a colour correct for the beauty pass for instance – so we can colour correct a specific part of the render.
Copy and paste those nodes a couple of times and change the alpha inputs from ‘r’ to ‘g’ and ‘b’ respectively to use those mattes also.
In the image above, you can see the nodes in action:
The beauty pass on the top left in its original state.
The ID Pass on the top right, again in its original state.
Below that, the ID pass’s red channel has been extracted and set to be used in all the channels, but importantly, the alpha.
This plugs into the colour correct’s matte input, so that only the frame of the triangle sign is affected, and turned a lovely green colour, a fine improvment.
Thats about all for now, might add a little bit so feedback at this point would be welcome.
Bit Depths
I was also planning on writing a wee bit about bit depth, but through rather very little research I found a couple of really great, clear sources and explainations. Though I’d like to quickly mention that often these 2 links refer to 24bit (16 million colours) which is 8 bits per channel, and thats how it is usually reffered to in 3d software. ie When you save a png it will ask if you want to save in 8bit, or 16bit, not 24bit and 32.
Cambridge In Colour
Wikipedia – Bit Depth
Other Resources
Books:
The Art and Science of Digital Compositing – The holy grail book!
Web:
Andrew Whitehurst – Matching Real Lighting
Jostein Finnekåsa – Compositing Tutorial
Compositing Forum on Cgtalk
Thanks.
Simon Reeves ,www.simonreeves.com
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