So, what I'm doing right now is creating a texture backed FBO/renderbuffer, rendering, then reading back with glGetTexImage().

My problem is that my component values are being clipped to [0.0f,1.0f]. I'm using a float_pixel texture. Is there anyway I can get values outside of [0.0f,1.0f] back?

***Edit: Cripes. So I messed up. My values were being clipped because I was using glColor to draw. Once I stick my floats in a texture, I get the right output.

Anyhow, I'm still interested in hearing what the best way to do the readback is.

The only real good way of doing this is to figure out a way of not having to do it... The performance hit you get is MASSIVE. It's way lighter to monopolize a texture unit for a one pixel texture, and use a texture sampler in your shader, than reading the single pixel over the bus and pushing it back as a uniform.

If it's that bad for a single pixel, you can imagine the rest.

On the other hand, if you really have to. You don't have to make the FBO a texture target. You can create a regular FBO and use glReadPixels instead. This way, you won't be locking up a texture unit.

You can use pixel buffer objects (ARB_pixel_buffer_object) to get asynchronous pixel readback, if you can cope with the latency required you shouldn't see a major performance hit I think. Examples in the PBO spec.

Just to clarify, the rendering I'm doing isn't for anything visible. This is all offscreen and not used for graphics. The reason I want to do the readback is because I want to sample (potentially) thousands of points in the frame for calculation of a vector field. The floats I get out of the readback need to update the state of my application, not just the graphics.

So yeah, I want pretty much all the data in the frame. And no, I'm not bound by synchronizing with the graphics.

OneSadCookie Wrote:You can use pixel buffer objects (ARB_pixel_buffer_object) to get asynchronous pixel readback, if you can cope with the latency required you shouldn't see a major performance hit I think. Examples in the PBO spec.

I looked at the PBO route, but I wasn't clear on how to get the pixels out of GL and back into my application layer. I can do this to get the pixels into a PBO float array, but then what?

glMapBufferARB... as much time later as possible.

how do I check if it's done?

You don't; glMapBufferARB will block if it's not.

For a 480x360 frame buffer backed by a float32 rgba texture I get:
glGetTexImage: 40.1FPS max
glMapBufferARB: 76.8FPS max

so far so good I guess.

EDIT:
ew... well, if up the buffer to 512x512 it starts to barf about 2/3rds the way in... if I push it higher it barfs on all the bytes.