Rendering Ambient/Diffuse [Color], Specular and occlusion passes

This process uses mental ray. The objective is to achieve a GI look without GI rendering time.

This does not work directly with transparent images, you will have to use a work-around for that.

We will first create a flat lit diffuse pass using only ambient lighting then overlay an occlusion pass [a grayscale or tinted image based on environment map) to represent the shadowing caused by the environment.

The method is called Ambient Occlusion to describe the occlusion or blocking of the ambient light amount to simulate GI shadows.

This is similar to rendering your scene with a skylight, but it can be faster than global illumination.

We need to make 3 passes:

ambient color pass for main diffuse rendering

ambient occlusion pass to attenuate the diffuse pass

specular pass so we can also control highlights

Scene

Put your model on a plane

In the environment map, have something simple like a sky gradient.

Ambient pass

Use only the scene’s ambient light ... if you already have lights set up, turn them all off.

No advanced lighting

No skylights

In Environment, turn up ambient value to 140

The result of this first pass should be a very flat looking render with no cast shadows.

Use the default scanline renderer to render.

Specular pass

Turn on, or set up, all your lights, probably point lights to achieve specular highlights.

Don’t worry at all about the general lighting, only highlights.

No skylight.

Use render elements to render out the specular pass.

Use the default scanline renderer to render.

The result should be very dark except where you wanted highlights.

Occlusion pass

Now, turn on mental ray.

Go Standard/mental ray to start a new material

 

In the surface slot, add the Ambient/Reflective Occlusion [base] shader

 

Your sample settings can go up to about 64.

Try using type 0 or 1 to see the results.

mode 0 goes to a grayscale image that can later be tweaked in PSD for color.

mode 1 reflects the environment colors of your scene

The result of this pass will be a single color image with shading.

Compositing in PSD

1 Put the Ambient [Diffuse] layer at the bottom.

2 Using Overlay blending mode, place the Occlusion layer next.

3 Finally add the Specular layer with Screen blending mode.