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List:       mesa3d-users
Subject:    Re: [Mesa3d-users] Fastest way to grab pixel values in to memory
From:       Brian Paul <brianp () vmware ! com>
Date:       2009-07-31 14:48:33
Message-ID: 4A730441.20102 () vmware ! com
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Srimal Jayawardena wrote:
> Hi all
> 
> 
> I'm working on a Image recognition project where I need to do
> calculations on different views (more than 100 frames)
> of a 3d model within an order of less than 1 sec, in order to
> calculate a particular optimization function.
> 
> I am hoping to use OpenGL to obtain 2D pixel values from different 3D
> view points, in the hope that the 3d h/w processing will do things a
> lot faster for me.
> 
> I need to render 100frames of 20,000 triangles
> in less than 1sec.
> 
> At the moment I am using glReadPixels() to read transformed pixel
> values into memory.
> 
> This is a bit slow and it takes around 10sec to grab 100 frames .
> 
> Is  there a faster way? Or would you recommend a
> better approach?
> 
> I've heard of buffer object ? but am not quite sure how they can
> be used to grab pixel data into memory.
> 
> Rendering the pixels into some virtual buffer is fine with my application
> as long as reading the pixel values happens in less than 1 sec.
> 
> 
> Any pointers would be much appreciated.
> 
> Thanks in advance

Which hardware/driver are you using?

The only thing I can suggest is trying different format/type 
parameters to glReadPixels().  A combo such as 
GL_BGRA/GL_UNSIGNED_INT_8_8_8_8 might be a better match for the 
hardware layout and allow the driver to avoid swizzling.  But this can 
vary from one gpu/driver to another.

-Brian

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