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POAsynchronous glReadPixels with PBO
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  1. POAsynchronous glReadPixels with PBO
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    copied!<p>I want to use two PBOs to read pixel in alternative way. I thought the PBO way will much faster, because glReadPixels returns immediately when using PBO, and a lot of time can be overlapped.</p> <p>Strangely there seems to be not much benefit. Considering some code like:</p> <pre><code> glBindBufferARB(GL_PIXEL_PACK_BUFFER_ARB, 0); Timer t; t.start(); glReadPixels(0,0,1024,1024,GL_RGBA, GL_UNSIGNED_BYTE, buf); t.stop(); std::cout &lt;&lt; t.getElapsedTimeInMilliSec() &lt;&lt; " "; glBindBufferARB(GL_PIXEL_PACK_BUFFER_ARB, pbo); t.start(); glReadPixels(0,0,1024,1024,GL_RGBA, GL_UNSIGNED_BYTE, 0); t.stop(); std::cout &lt;&lt; t.getElapsedTimeInMilliSec() &lt;&lt; std::endl; </code></pre> <p>The result is </p> <pre><code>1.301 1.185 1.294 1.19 1.28 1.191 1.341 1.254 1.327 1.201 1.304 1.19 1.352 1.235 </code></pre> <p>The PBO way is a little faster, but not a satisfactory <strong>immediate-return</strong>。</p> <p>My question is:</p> <ul> <li>What is the factor affecting glReadPixels' performance? Somethimes, the cost of it reaches 10ms, but 1.3ms here.</li> <li><del>Why <strong>immediate-return</strong> costs as much as 1.2ms? Is it too big or just normal?</del></li> </ul> <p>===========================================================================</p> <p>According to comparison with a demo, I found two factors:</p> <ul> <li>GL_BGRA is better than GL_RGBA, 1.3ms=>1.0ms(no PBO), 1.2ms=>0.9ms(with pbo)</li> <li>glutInitDisplayMode(GLUT_RGB|GLUT_ALPHA) rather than GLUT_RGBA, 0.9ms=>0.01ms。That's the performance I want. In my system, GLUT_RGBA=GLUT_RGB=0. GLUT_ALPHA=8</li> </ul> <p>Then another two questions:</p> <ul> <li>Why GL_BGRA is better than GL_RGBA? Is it the case for just specific platform or for all platforms? </li> <li>Why GLUT_ALPHA is so important that it affects PBO performance hugely?</li> </ul>
 

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