CODEWITHSUNDEEP
c++openglrenderingtexturesfbo
kbirk
kbirk

Reputation: 4022

OpenGL MRT glDrawBuffers() argument numbers and behavior

I have a total of 5 render targets. I use the same shader to write to the first 4, then in a seperate pass write to the last one.

Before calling rendering to the first 4 targets I call: 

GLenum drawBuffers[] = { GL_COLOR_ATTACHMENT0, GL_COLOR_ATTACHMENT1,  GL_COLOR_ATTACHMENT2, GL_COLOR_ATTACHMENT3};
glDrawBuffers(4, drawBuffers);

However, when I call it for second pass and only want to write to the last one, the 5th target, why does the following give strange results?

GLenum drawBuffers[] = { GL_COLOR_ATTACHMENT4 };
glDrawBuffers(1, drawBuffers);

Why do I have to instead use:

GLenum drawBuffers[] = { GL_NONE, GL_NONE, GL_NONE, GL_NONE, GL_COLOR_ATTACHMENT4 };
glDrawBuffers(5, drawBuffers);

Is this simply how glDrawBuffers() works, or is being caused by something else?

EDIT: fixed code with regards Jarrods comment

Upvotes: 1

Views: 2508

Answers (3)

tanglei
tanglei

Reputation: 347

I find that there's something wrong in glDrawBuffers(). for exmple tmp = {attement_color0, attachement_color2} glDrawBuffers(tmp). in shader: gl_fragdata[0]=... gl_fragdata[2]=...

or u can use layout location to define the attments output. But sometimes, at least in my PC, it does not work. I mean the attachment_color2 does NOT have the exact output.

Upvotes: 0

Nicol Bolas
Nicol Bolas

Reputation: 473174

Yes, this is simply how glDrawBuffers works. But there's a reason for that.

Fragment shaders write color outputs that map to certain "color numbers". These have no relation to GL_COLOR_ATTACHMENT''i'' numbers. The entire point of glDrawBuffers is to map those fragment color numbers to actual buffers in the framebuffer.

Upvotes: 3

sleep
sleep

Reputation: 4944

http://www.opengl.org/sdk/docs/man/xhtml/glDrawBuffers.xml

The 2nd parameter must be of the type const GLenum* i.e. a pointer "to an array of symbolic constants specifying the buffers into which fragment colors or data values will be written".

So just passing GL_COLOR_ATTACHMENT4 as the 2nd param is the wrong type. You need to pass a pointer to an array of GLEnum.

Upvotes: 1

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