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+--- Thread: Best way to readback float pixels? (/thread-2750.html)
Best way to readback float pixels? - kelvin - Feb 27, 2008 04:43 PM
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.
Best way to readback float pixels? - sohta - Feb 27, 2008 06:55 PM
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.
Best way to readback float pixels? - OneSadCookie - Feb 27, 2008 07:06 PM
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.
Best way to readback float pixels? - kelvin - Feb 27, 2008 07:27 PM
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.
Best way to readback float pixels? - kelvin - Feb 27, 2008 07:31 PM
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?
Code:
glBindBuffer(GL_PIXEL_PACK_BUFFER_ARB, pbo);
glReadPixels(0, 0, m_width, m_height, GL_RGBA, GL_FLOAT, NULL);Best way to readback float pixels? - OneSadCookie - Feb 27, 2008 07:44 PM
glMapBufferARB... as much time later as possible.
Best way to readback float pixels? - kelvin - Feb 27, 2008 07:48 PM
how do I check if it's done?
Best way to readback float pixels? - OneSadCookie - Feb 27, 2008 08:23 PM
You don't; glMapBufferARB will block if it's not.
Best way to readback float pixels? - kelvin - Feb 27, 2008 10:23 PM
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.