There are some people who know a great deal about GLSL, so I hope they come set the record straight, but based on what I've seen, you should be able to, in your fragment shader, do something like this (pseudocode, I'll leave the actual GLSL up to the people who know it better):

out vec4 fragment_color
vec4 final_color
final_color = flat_color
If this fragment is in shadow
    final_color = shadow_color
If this fragment is an edge
    final_color = edge_color
If this fragment is a highlight
    final_color = highlight_color
fragment_color = final_color

By using ifs in this way, you get something like multiple passes. Does that make sense?

Edit: Also, hopefully this question on SO helps dispel some misunderstanding.

There are some people who know a great deal about GLSL, so I hope they come set the record straight, but based on what I've seen, you should be able to, in your fragment shader, do something like this (pseudocode, I'll leave the actual GLSL up to the people who know it better):

out vec4 fragment_color
vec4 final_color
final_color = flat_color
If this fragment is in shadow
    final_color = shadow_color
If this fragment is an edge
    final_color = edge_color
If this fragment is a highlight
    final_color = highlight_color
fragment_color = final_color

By using ifs in this way, you get something like multiple passes. Does that make sense?

Edit: Also, hopefully this question on SO helps dispel some misunderstanding.

There are some people who know a great deal about GLSL, so I hope they come set the record straight, but based on what I've seen, you should be able to, in your fragment shader, do something like this (pseudocode, I'll leave the actual GLSL up to the people who know it better):

out vec4 fragment_color
vec4 final_color
final_color = flat_color
If this fragment is in shadow
    final_color = shadow_color
If this fragment is an edge
    final_color = edge_color
If this fragment is a highlight
    final_color = highlight_color
fragment_color = final_color

By using ifs in this way, you get something like multiple passes. Does that make sense?

There are some people who know a great deal about GLSL, so I hope they come set the record straight, but based on what I've seen, you should be able to, in your fragment shader, do something like this (pseudocode, I'll leave the actual GLSL up to the people who know it better):

out vec4 fragment_color
vec4 final_color
final_color = flat_color
If this fragment is in shadow
    final_color = shadow_color
If this fragment is an edge
    final_color = edge_color
If this fragment is a highlight
    final_color = highlight_color
fragment_color = final_color

By using ifs in this way, you get something like multiple passes. Does that make sense?

Edit: Also, hopefully this question on SO helps dispel some misunderstanding.