CODEWITHSUNDEEP
openglopengl-4
Tarquiscani
Tarquiscani

Reputation: 699

Multiple output buffers with the default framebuffer

I have a shader that I use primarily with an FBO that has two color attachments. In this fragment shader so there are two output buffers.

in vec2 vs_tex_coords;
flat in uint vs_id;

out vec4 fs_color;
out uint fs_id;

uniform sampler2D u_texture;

void main()
{
     fs_color = texture(u_texture, vs_tex_coords)
     fs_id = vs_id;
}

Now, since I'm lazy, I'd like to reuse the same fragment shader also with the default framebuffer. In this case obviously there is only one color attachment (i.e. the screen color buffer). What happens with the second color output? Is it simply ignored or could it cause troubles? Does the standard mention anything about such a case?

I tested that behavior on two devices and it seemed to work fine. It simply ignored the second output buffer. But what can I say about other implementations?

Upvotes: 1

Views: 1089

Answers (1)

Nicol Bolas
Nicol Bolas

Reputation: 473302

Now, I'm going to assume that, somewhere in your main OpenGL code, you are assigning those fragment shader output variables to specific output locations. Because if you aren't, then you're already relying on the implementation-defined output location ordering, and that's not reliable.

The following assumes that fs_color is assigned to the output location 0, and fs_id is assigned to the output location 1. Feel free to switch the numbers as appropriate for your needs.

Given that, the question you're asking is really about the framebuffer's draw buffers state. This is the array that specifies which output locations map to which color images in the framebuffer. All framebuffers, default and FBO alike, have this mapping table. But each framebuffer's table is separate.

So, if you didn't make a glDrawBuffers-equivalent call for the default framebuffer to set up the mapping from output locations to framebuffer images, the default is used. By default, the draw buffer state assigns location 0 to write to the GL_BACK buffer (or GL_FRONT if no back buffer exists). The specification is actually a bit underspecified here, as the state of the other locations is not stated. My guess would be that they would be GL_NONE (and thus the FS writes are ignored).

Upvotes: 4

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