setup for post-render control over AO and lights

hey everyone.

I have a huge scene for a still that will take around an hour to render, so I’d like to have some control over the lighting after the render via passes and compositing. I don’t know much about this so I was wondering what would be the best approach to setting up something where I can control the intensity and color of the AO, and the intensity and color of a few different lights in my scene.

Let’s say I have 2 basic lamps, both casting soft raytraced shadows and some AO. I have no idea how to split up the layers/passes and the blender composite nodes scare me! I know this will require the employment of floating point images.

Would this be terribly complicated? Too complicated to explain in one post? I would really appreciate it if someone could tell me how to set this up.

-Thanks so much.

Hi OrchidFace

I guess one cannot give a very short answer. First up, I am not sure if it is possibly to really influence the light energy in composite nodes. You can do that in Luxrender, though, which totally gives the control you need.

But in Blender you could do the following:

  1. Put the objects you need control over on different layers. Put the ground plane on the last one.
  2. Go to ‘Render Layers’. On top select all layers. Then create different new render layers. They will be numbered. Then select only individual layers for them (see screenshot)
  3. Enable different passes for each layer (AO, diffuse, … what you want, really)
  4. Enable composite nodes.
  5. Create different input-nodes (Space - Add - Render Layer) and assign the individual layers you already created.
  6. Go crazy with Brightness/Contrast, RGB-Curves, Blur (for AO), Tonemapping and so on. Use the screenshot as a guideline.
  7. Recomposite the layers with the Mix-Node (Space - Color - Mix) or the Alpha over Node (for that you will need the Alpha output from your RenderLayers)

I hope this is in anyway helpful (I know I did not answer your actual question, though)…
cheers, sloane

ps you will probably want to go through the wiki-documentation.


wow thankyou so much for taking the time! The screenie of the flow really helps. I’m beginning to think my scene is just too complicated for me to set this up (because I suck too much with compositing). What are your mix nodes set to? I can’t see because all your stuff is minimized.

I read through the wiki… clear as mud haha. I was told though that with a floating point image adjusting the brightness has the exact same effect as adjusting the light intensity in 3d, so if I could get as far as you with the composite I think I could adjust the light intensities. The problem is I want to have this thing done my monday (for the weekend challenge), gonna do an all nighter tonight but I doubt I can grasp this stuff in a night.

From the wiki:

AO is a geometry-based dirt shader, making corners darker. It is separately enabled in the World settings and computed as a separate pass. When enabled, it has one of three Modes (Add, Subtract, Both), and variable Energy level (which changes the intensity of the shading). The third variable is the amount of Ambient light that the material receives. If it does not receive any, then ambient occlusion does not affect it. Based on these variables, Blender computes an AO pass. If you call it out as a separate pass and wish to composite it back into your image, you will need to enable the Color and Diffuse pass as well.
To configure your noodle, consider the example image above.

  • . First, depending on the AO mode do one of the following: If AO mode is Add: directly use the AO pass. If AO mode is Sub: Calculate AO - 1, or if AO mode is Both: Calculate 2*AO - 1
  • . Multiply the output of Step 1 with the AO energy level
  • . Multiply the output of Step 2 with the material’s ambience value. If you have materials which receive different ambience light levels (0.5 is the default), one would have to create an ambience map based on Object ID)
  • . Multiply the output of Step 3 with the color pass
  • . Add the output of Step 4 to the diffuse pass

If shadows, colored ambient light, specularity, reflections, and/or refractions are involved they have to be added to the diffuse pass before adding the converted AO pass."