OpenGL 4 Shading Language Cookbook - Third Edition

How to do it...

For the first pass, enable the framebuffer object that we set up earlier, and render the scene normally. In the fragment shader, send the ambient component and the diffuse + specular component to separate outputs:

layout( location = 0 ) out vec4 Ambient; 
layout( location = 1 ) out vec4 DiffSpec; 
 
void shade( ) {
  // Compute the shading model, and separate out the ambient 
  // component. 
  Ambient = ...;   // Ambient 
  DiffSpec = ...;  // Diffuse + specular 
} 
void main() { shade(); } 

In the second pass, we'll render our shadow volumes. We want to set up the stencil buffer so that the test always succeeds, and that front faces cause an increment, and back faces cause a decrement:

glClear(GL_STENCIL_BUFFER_BIT); glEnable(GL_STENCIL_TEST); ...