Implementing render-to-vertex-array, glReadPixels fails (invalid operation)

Question

I'm trying to copy vertex data from a texture to a vertex buffer, and then draw the vertex buffer. As far as I know the best way to do this is to bind the texture to a fbo, and use glReadPixels to copy it to a vbo. However, I can't seem to get this working: glReadPixels fails with the error "invalid operation".

Corrections, examples and alternate methods welcome. :)

Here's the relevant code:

glEnable(GL_TEXTURE_2D)

w, h = 32, 32

vbo = glGenBuffers(1)
glBindBuffer(GL_ARRAY_BUFFER, vbo)
glBufferData(GL_ARRAY_BUFFER, sizeof(c_float)*w*h*4, None, GL_STREAM_COPY)
glBindBuffer(GL_ARRAY_BUFFER, 0)

fbo = glGenFramebuffersEXT(1)
glBindFramebufferEXT(GL_FRAMEBUFFER_EXT, fbo)

tex = glGenTextures(1)
glBindTexture(GL_TEXTURE_2D, tex)
# tex params here
glTexImage2D(GL_TEXTURE_2D, 0, GL_RGBA32F, w, h, 0, GL_RGBA, GL_FLOAT, None)
glFramebufferTexture2DEXT(GL_FRAMEBUFFER_EXT, GL_COLOR_ATTACHMENT0_EXT, GL_TEXTURE_2D, tex, 0)

assert glCheckFramebufferStatusEXT(GL_FRAMEBUFFER_EXT) == 36053

glReadBuffer(GL_COLOR_ATTACHMENT0_EXT)
glBindBuffer(GL_PIXEL_PACK_BUFFER, vbo)
glReadPixels(0, 0, w, h, GL_RGBA, GL_FLOAT, None) # invalid operation?

GetTexImage can save you a couple of instructions (no need for FBO) — kvark, Apr 22 '11 at 14:02
The code looks good to me. Are you sure the error happens exactly after glReadPixels? — kvark, Apr 22 '11 at 14:07
Yes, pyopengl checks for errors automatically, and it's glReadPixels that causes the error. — doeke, Apr 22 '11 at 14:24

score 1 · Accepted Answer · edited May 26 '11 at 08:40

1

I've solved the issue myself.

The last argument to ReadPixels is used as an offset instead of a pointer in this case, and is not automatically cast by pyopengl, use:

glReadPixels(0, 0, w, h, GL_RGBA, GL_FLOAT, c_void_p(0)) # works!

edited May 26 '11 at 08:40

Mia Clarke

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answered May 25 '11 at 14:02

doeke

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There seem to be a few places in pyopengl where c_void_p(0) is required. glVertexPointer() when using vbo's is another. – Ted Middleton Feb 22 '12 at 18:08

Implementing render-to-vertex-array, glReadPixels fails (invalid operation)

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