Simon Reeves - 3D Artist

And here it is, from my previous post, here is the ad I have been working over xmas at The Mill, since the start of December. 
I was on the job from the start (which is always favourable) and as such did a variety of things. I spent a bit of time building the bridge through some scripts that I wrote.
These scripts duplicated nulls along the main curve of the bridge with the correct spacing so we could place particles (ICE) instanced objects (the bricks..) on those nulls. Also some lighting and rendering and scene setup and what not! The usual.
Also I was working with my old pal from uni Aidan Gibbons (we made Diversion together at uni)

Just finished on my latest job at the Mill which started from the beginning of December, always a fun schudule over xmas and the new year. So I have a long weekend to go play in the snow.
Will post more about it when I have a link to show.

Trying to  work out what I have done over the last year. So since I have been at The Mill in London since June, I have worked on a few commercials - for 2/3 months I was on one specific one that was will not air now unfortunatly, as you do.
Anyway I worked on this ad for a couple of weeks. I did a wee bit of camera work but mostly I worked on the shaders/lighting/rendering of the ball, also the water splashes from the balls so not too much eh
Here it is:


Mostly I get to work with my old pal from Uni, Aidan who I made Diversion with. And currently we are working on an interesting ad which should be airring in January so I will have something to show from that one.

Falling Strands from Simon Reeves on Vimeo.

Here is another video from testing the strands in XSI ICE, they are fun.

This is the 2nd part of this guide, more 'how' than 'what' from the first part.

I've just realised how long ago it was now when I wrote the first part nearly 6 months, time flies these days :). Check the first part about the different passes you may want to use and why, here.
Compositors
There are a few major compositors, Shake, Combustion, After Effects, Nuke, Fusion and XSI's FxTree.

The most commonly used and standard is probably Shake (although it has now been discontinued by Apple) and as with the 3d side of things, probably even more so, the principles are the same.


Personally my preference is to use the FxTree within XSI. The similarities between FxTree and Shake are a good example of the lack of contrast between compositing apps, including similar nodes layout and workflow. They are what I have most knowledge in, so I will use them as the main apps in this guide.

Instead of using layers like you would in Photoshop (or Combustion/After Effects in fact), Shake and FxTree for instance are node-based so every effect/filter/operator is a node inside a 'tree' which can contain one or many in or outputs. Having this tree, you can always go back and adjust settings in any node as it will not have been applied permanently, frozen or collapsed, and thus non-destructive.

The Tree
Now here is my compositing tree in FxTree, as well a comparison tree in Shake and Fusion, just to show you how much or little they differ.
The nodes are very similar in the different apps, Shake and FxTree are nearly identical and have a few key features. They both have an edit button to bring up the properties of a node and a view button so you can view the tree up until that point.
The small lumps around the nodes are where you plug in other nodes. These vary from node to node, for instance 'Input' nodes don't require an input, like 'Output' nodes don't require an output. And some nodes can have multiple inputs for different reasons.
On the FxTree, the green ones along the top are the inputs, the blue on the site are for mattes, and the red at the bottom is for the output.



FxTree:
Essential Nodes

Now straight into those all important nodes. I'm going to use stuff from the first part of the guide so make sure you've read that too.

Input/Output Nodes

At all the tips of all the branches in the tree, there needs to be some kind of input node. Often it may be an image, a render/pass or a background plate perhaps. But it could also be a plain colour, a gradient or a rotoscope/mask of some kind also.

I've highlighted these on the tree in red:



Also worth mentioning here are Output nodes, this would be at the very base of the tree as the final output for the comp to output to whatever format you want.

So we have are inputs, but how to we stick them together I hear you ask! Well, more nodes......

Math/Layer Nodes

Here on the tree, they are highlighted in yellow:



- 'Over'


In a layer based app like photoshop, you would place the beauty pass with its transparency ontop of the background plate it will lay over the background nicely.
Well the equivalent to that would be the aptly named 'Over' node - Which has two inputs, one for the background element (the left side of the tree in the image), and the other foreground element (the right side). If the foreground element contains an alpha channel (RGBA) it will use that as its transparency (by pre-multiplying, which can be tricky to get your head around so we won't go into that for now).

- 'Add'
In FxTree all the math nodes are selectable in one node named 'Math Composite' which you can see in the image. The node that "Spec_Signs_" is plugged into is set to add.
The first of the math nodes does literally what it says, it adds the pixel values of the two inputs together. We would usually use this to add reflections and specular passes to the comp.
These passes are mostly black apart from the bright highlights/reflections. Adding, effectivly removes the black but doesn't just lay the rest of the image over the background (like an over with an alpha channel) it brightens it.

So if we pick a random pixel on the background image, it will have 3 values from 0-255 for each colour channel (Red, Green and Blue). But for now we will just assume they are the same in each channel, so one value from 0-255 where 0 is black and 255 is white.

Now this is why the black pixels are effectivly removed with an add node, as any pixel value + 0 (black) will remain the original value. But any value greater than 0 will brighten the background pixels:



bg + fg = output
50 + 0 = 50 (no difference)
128 + 205= 255 (as colours will be clamped at 255, pure white)
190 + 55 = 245 (brighter)

Hopefully that didn't sound too complicated, but its also the base for the other math nodes so it should have got a lot covered.

- 'Multiply'
The Multiply in the tree above is the 'Math Composite' which the 'AO_' is plugged into.
Kind of the opposite to add, using Multiply removes the white from an image. Now for multiply I've found it best to consider the pixel values normalised, so the proper range of 0-255 is now 0.0 to 1.0
So when multiplying a Ambient Occlusion pass for instance, we want to remove all the white and keep the darker areas.
Remember, black = 0.0 (0), white = 1.0 (255)

bg x fg = output
50 x 0.0 (0) = 0 (black)
128 x 1.0 (255)= 128 (no difference)
190 x 0.22 (55) = 41 (darker)




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

Update September 2009:
This is a wee bit old now. So personanly my workflow has probably changed a bit, but in general hopefully this is still a useful guide :) (also this page my be a little messy, mostly copy+paste from the old blog)

Before I start I should say that this is based on what I know and learnt from people at uni, and at work, and the internet.
Other people you may learn from prefer more or less passes etc. for more control or to save on excess files - so just keep in mind there are usually different ways to do things all with their own advantages and disadvantages (and don't blame me).

In this tutorial I'll be using examples from a couple of shots from our short film 'Diversion'. Directed by myself and Aidan Gibbons


Types of Passes

If you want very fine control in post over every element of the scene, you could render the the objects in tons of passes with a different one for every element in the render, ambient texture, different light passes, shadows, reflection, refraction, GI the list goes on.but generally thats not really necessary.

So if you like you may want to include lighting/shadows/specular/GI all in this one pass and maybe just add a couple of additional passes to this 'Beauty' pass.

For compositing with live action in 'Diversion', we generally used a standard set of passes on all shots, and then on some shots we needed some other specific passes to help out with mattes, masks and the like - which I'll try and mention a couple of later.


So here is an example from excitingly named 'Shot14'.
First of all, here is the background plate from the footage:



Don't think there's much to explain on that one.

Beauty (RGBA could be named 'Diffuse' or 'RGB' perhaps)


This is the base for the cg elements in the shot, and it can vary a lot what is included in this pass.
Our standard beauty pass included shadows cast onto the objects but not shadows cast onto the plate (separate pass for them) but as there isn't any shadows in this shot, I won't go into why now.
When we light the diffuse pass, I find it helps to make it a bit brighter than you want in the final shot, especially to take into account the 'Ambiant Occlusion' (or 'Dirtmap') pass which adds the darker areas in this shot.

Specular Pass (RGB, Alpha channel not needed)


This pass is for specular only, and is layered over the top of the beauty pass with 'screen' (or you could 'add' (additive) it). Screen or Add basically remove all the black in the pass, leaving just the highlights (the specular in fact.)
This is the same for a Reflection pass, and if you want to render a light in a separate pass - as again you just want to hide all the black pixels values and keep the white values.


Reflection Pass (RGB, Alpha channel not needed)


Whoops, I seem to have got a little ahead of myself there. Anyway this reflection pass, is on the plate, so instead of compositing it onto the last two passes, we can lay it straight onto the plate. Smashing.

Reflection Matte Pass (RGB, Alpha channel not needed)


Now you can't just slap the reflection onto the plate in this instance, because there are already reflections on the floor that would be occluded by the signs were actually there (they would block them).
So I found a good way to create a matte for where the reflections will be is to make a pass where the signs materials to a constant white material, and have that as the only thing reflecting in the scene.
N.B This needs to be inverted in post truthfully in the film I made the objects black, and rendered them on a white surface and a white environment - but now I realise that is way to overcomplicated as you can just invert it.

You can multiply this pass onto the plate before the main reflections to matte out those reflections you don't want.



Another note about the reflections I'm afraid. If you look at the plate, you can see the differences between the soaked floor, and the standing water/puddles, the puddles are sharp reflections while where the water has soaked in, the reflections are blurred.
So before those passes are rendered I painted a matte based on the plate - and used a camera projection to apply that texture onto the floor, to control the amount of blur for the reflection.
Hopefully that made sense.


Ambient Occlusion (RGB, Alpha channel not needed)


This pass effects the CG elements as well as the plate (although often you may want to render 2 separate AO Passes), so to layer it onto both, before this we need to layer the CG passes we have so far, over the plate (including the reflections).
For this pass, we just want to keep the darker values, so to remove/hide the white, we will use 'Multiply'.

ID Pass (RGBA)





This should come earlier in the comp, I mainly use it to colour correct the diffuse pass before anything else is layered on top (so maybe earlier in this guide? But its a bit special so I thought I'd leave it till now). This is a very useful pass for making multiple matte's (masks) in one pass.

By giving certain objects materials that are either, 100% Red, 100% Blue or 100% Green we can extract just one of these channels (R, G, or B) in post as a matte to do colour correction on specific objects or whatever you please. As you can see by this gif below, which cycles through the RGB, then Red, then Green, then Blue channels.



If you're feeling fancy you can use the A channel too, currently the A channel will show the objects that are in the R,G,B channels (as they have an alpha of 1 / 100%), but if you assign everything else a black material with an alpha of 1, you could specifically assign some objects with a black material with alpha 0.
Hope that made sense, don't have any images for that at the moment. :)


Here is the final Graded shot:


If you want to watch the shot, and indeed the film, here it is:

Diversion (2005) from Simon Reeves on Vimeo.

Hopefully I will write another part to this if its useful about the actually comping process, Multiplying/Adding/Over using ID mattes and so on.
I personally use the FxTree within XSI but Shake/Nuke/Combustion are all the same principles so it shouldn't be a problem.

If you see any mistakes or have any comments please email me or use the comments thing on here - Cheers.

Bear with me here. But I will backdate and add guides and tutorials on here that I have made before which are currently a bit missing. And then I'll try and post some new helpful stuff also.

Cheers!

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