Transform feedback without a framebuffer?

Question

I'm interested in using a vertex shader to process a buffer without producing any rendered output. Here's the relevant snippet:

glUseProgram(program);

GLuint tfOutputBuffer;
glGenBuffers(1, &tfOutputBuffer);
glBindBuffer(GL_ARRAY_BUFFER, tfOutputBuffer);
glBufferData(GL_ARRAY_BUFFER, sizeof(double)*4*3, NULL, GL_STATIC_READ);

glEnable(GL_RASTERIZER_DISCARD_EXT);

glBindBufferBase(GL_TRANSFORM_FEEDBACK_BUFFER, 0, tfOutputBuffer);
glBeginTransformFeedbackEXT(GL_TRIANGLES);

glBindBuffer(GL_ARRAY_BUFFER, positionBuffer);
glEnableVertexAttribArray(positionAttribute);
glVertexAttribPointer(positionAttribute, 4, GL_FLOAT, GL_FALSE, sizeof(double)*4, 0);

glBindBuffer(GL_ELEMENT_ARRAY_BUFFER, elementBuffer);

glDrawElements(GL_TRIANGLES, 1, GL_UNSIGNED_INT, 0);

This works fine up until the glDrawElements() call, which results in GL_INVALID_FRAMEBUFFER_OPERATION. And glCheckFramebufferStatusEXT(GL_FRAMEBUFFER_EXT); returns GL_FRAMEBUFFER_UNDEFINED.

I presume this is because my GL context does not have a default framebuffer, and I have not bound another FBO. But, since I don't care about the rendered output and I've enabled GL_RASTERIZER_DISCARD_EXT, I thought a framebuffer shouldn't be necessary.

So, is there a way to use transform feedback without a framebuffer, or do I need to generate and bind a framebuffer even though I don't care about its contents?

What did you expect `glCheckFramebufferStatus (...)` to return if you have no FBO bound and no default framebuffer? *GL_FRAMEBUFFER_UNDEFINED is returned if target is the default framebuffer, but the default framebuffer does not exist.* — Andon M. Coleman, Jun 09 '14 at 02:02
I was just surprised to get a framebuffer-related error at all, since I was under the impression that `GL_RASTERIZER_DISCARD_EXT` meant no framebuffer should be necessary. So I got more information about the framebuffer situation, and included that information here in case it might help answer the question. — smokris, Jun 09 '14 at 02:13

score 5 · Accepted Answer · edited Jun 20 '20 at 09:12

This is actually perfectly valid behavior, as-per the specification.

OpenGL 4.4 Core Specification - 9.4.4 Effects of Framebuffer Completeness on Framebuffer Operations

A GL_INVALID_FRAMEBUFFER_OPERATION error is generated by attempts to render to or read from a framebuffer which is not framebuffer complete. This error is generated regardless of whether fragments are actually read from or written to the framebuffer. For example, it is generated when a rendering command is called and the framebuffer is incomplete, even if GL_RASTERIZER_DISCARD is enabled.

What you need to do to work around this is create an FBO with a 1 pixel color attachment and bind that. You must have a complete FBO bound or you get GL_INVALID_FRAMEBUFFER_OPERATION and one of the rules for completeness is that at least 1 complete image is attached.

OpenGL 4.3 actually allows you to skirt around this issue by defining an FBO with no attachments of any sort (see: GL_ARB_framebuffer_no_attachments). However, because you are using the EXT form of FBOs and Transform Feedback, I doubt you have a 4.3 implementation.

I see. Thanks. (Yes, unfortunately, at the moment I'm limited to OpenGL 2.1 + some extensions.) — smokris, Jun 09 '14 at 02:23

Transform feedback without a framebuffer?

1 Answers1

This is actually perfectly valid behavior, as-per the specification.