Introduction:
- This is a demonstration of a simple way of controlling a first order color bounce on a per material or object basis, without emitting photons or using global illumination. It is not intended to be a physical simulation, but rather a simple way to control an isolated color bounce all the way through post production.

1.) Beauty Pass Setup: The scene is made up of a poly cube for the walls, a subdivided poly sphere, and single spot area light. Generally I work in render layers, and set up my scene with preview settings in the maya software renderer. After I like the position of the key light I switch on final gather to check my reflected light and see what i need for fill, if any. (See my Introduction to Light and Shadow for more information on light setup and settings.)
* It's a good rule to set your shadow color to something above 0, I set my value to 0.1. It's going to be hard to reflect light back into it if not. I have included that in the image as a comparison as well.




- You can see that there is a little of light bleed in the final gather images, but we really need some more for this particular shot. After all, I'm an artist, and I'm more concerned with achieving an aesthetic than I am a simulation. I could play around with emitting photons, adjusting global illumination settings, and smoothing the result with final gather. But my creative side really wants visual feedback to make my adjustments, and wants it fast. I always prefer to adjust my image by compositing at high bit depths, over playing guessing games with numbers and waiting around for long renders.

2.) GI Fake Render Layer: Start off by creating a new render layer for your color bleed isolation. Add your object that the object you whose local color you want to reflect, and the objects you want it reflected onto.
- I recommend duplicating your key light from your beauty pass render layer, and adding the dupe to the this new GI Fake layer. This is because we need to do a little light linking, and we don't want to mess with a good beauty pass. I'm renaming mine to keyLight_Bounce for reference in this tutorial.

3.) Light Linking: Open up the attributes of the keyLight_Bounce. Set its decay rate to no decay and its intesity back to 1. This will give us greater control over the bounce later on. Uncheck "illuminates by default". This will cause your light to not affect anything in the scene. Open up Window > Relationship Editors > Light Linking > Light Centric to assign it to only the object or objects that you want to create the color bounce.



4.) Bounce Object Setup: Keep the original wall material assigned to the walls in this render layer. Create a new surface shader and name it bleedOutput_SG. Assign it to your sphere inside of the GI Fake render layer. Give it a full intensity red for it's local color. Open up the sphere's shapeNode attributes in the attribute editor. Under the render settings tab, create a layer override and uncheck primary visibility. This will let the sphere influence the scene, but not be visible in this pass. I find it helps me to compare these bounce values to the absence of light rather than to a really bright source. They would never seem bright enough, and we don't need really high values for this.
- Setup the render layers globals to preview final gather, and give it a render. Final gather sees objects with surface shader assignments as irradiant objects. You should end up with a image similar to this:

- Ehh, It's alright. I don't really like the uniformity in the light reflection though. Final gather is just illuminating the scene by that constant value we assigned to the local color. What I would really like is to be able to control the amount of light that reflects off of the lightside and off of the darkside individually. Remember that light link we made for this render layer? Let's see what we can do with that now.

5.) Bounce Control Shader: Drag a handful of new nodes into the work area. You will need a surfaceLuminance node, 2 multiply/divide nodes and a ramp node.
- Pipe the output of the surfaceLuminance node into the inputX of one of the multiply/divide nodes. Set that multiply/divide node's operation to power. This will give you falloff and contrast control of the surfaceLuminance data before we multiply it.
- Pipe that multiply/divide node's outputX and pipe that into the vCoord of the ramp. Assign the top color entry list a full intensity red. Assign the bottom color entry list a fully saturated shade of red, and maybe .05 for it's value. As long as it's not black. This is essentially our ambient color, we can tune this later as we adjust these values to control our bleed. The ramp assigns the top color to the fully illuminated surface normals and blends off to the bottom color as we reach surface normals 90 degrees from the light angle. The ramp's interpolation changes how quickly that blend happens.
- Pipe the outColor of the ramp into the input1 of the second multiply/divide node. Leave this node's operation at multiply. This will allow us to steepen the intensity curve even further with values above 1.

- Alright, getting closer! We are reflecting light based on the key light vector now... well, kind of! All that's left to do is to control the reflection amount off of both the light and dark sides.
- You could also switch those colors if you wanted more of a light trasmission effect through a translucent surface.