Instanced rendering with multiple color attachments - opengl

Question

Before I implement this, I want to know whether it is possible or not:
I want to use instanced rendering along with multiple color attachments. For each instance or a geometry, i want to write the color data to different color attachments of FBO. Based on my value of gl_instance I want to write to particular color attachment. In other words, my 1st instance should go to 1st color attachment and 2nd instance should go into 2nd attachment so my VS would be:
in vec4 position;
uniform mat4 u_mvp0;
uniform mat4 u_mvp1;
flat out instance_id;
main(){
mat4 mvp;
if(gl_Instance_ID == 0)
mvp = u_mvp1;
if(gl_Instance_ID == 1)
mvp = u_mvp0;
gl_Position = mvp * position;
instance_id = gl_Instance_ID;
}
FS:
out vec4 Color0;
out vec4 Color1;
flat in instance_id;
main(){
if(instance_id == 0)
Color0 = vec4(1,1,1,1);
if(instance_id == 1)
Color1 = vec4(1,0,0,1);
}
Can we do conditional writing to color attachment? Can we assign value of gl_Instance_ID to any variable inside shader? or Can we use gl_Instance_ID in fragment shader?

Answer 1

Can we do conditional writing to color attachment?
Not in the way you mean.
If the glDrawBuffers state says that a fragment output will be routed to a color buffer, then one will always be routed to it. If you don't write to that value, then the value written to the attachment will be undefined.
So while you can change the value you write, you cannot choose to write nothing to an attachment. That choice is made based on the glDrawBuffers state.
It seems very much like what you want is layered rendering.
Can we assign value of gl_Instance_ID to any variable inside shader?
... yes. Why would you think otherwise? It's just an input value filled in by the implementation.
Can we use gl_Instance_ID in fragment shader?
No. It only exists within the vertex shader. If you need any other shader stage to access it, then your VS needs to pass the value to it.

Answer 2

debonair, I think you are slightly misunderstanding Nicol Bolas. I'll try to rephrase his answer.
The code in your fragment shader will work. You can decide which color attachment output gets what.
What Nicol is saying is that you can't not generate an output. Your fragment shader will always output Color0 and Color1 (Color2, 3, …). If you don't assign a value then the output will be garbage, but something will be written.
So you should have a default value for when the instance id doesn't match the color attachment and assign that to the non-matching outputs.
Hope this helps.