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POCalculate average of pixels in the front buffer of the gpu without copying the front buffer back to system memory
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  1. POCalculate average of pixels in the front buffer of the gpu without copying the front buffer back to system memory
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    copied!<p>I am preparing to build a clone of the ambilight for my pc. For this purpose I need a way to calculate the average color of several areas of the screen. </p> <p>The fastest way I have found so far is the following:</p> <pre><code> pd3dDevice-&gt;CreateOffscreenPlainSurface(ddm.Width, ddm.Height, D3DFMT_A8R8G8B8, D3DPOOL_SCRATCH/*D3DPOOL_SYSTEMMEM*/, &amp;pSurface, nullptr) pd3dDevice-&gt;GetFrontBufferData(0, pSurface); D3DLOCKED_RECT lockedRect; pSurface-&gt;LockRect(&amp;lockedRect, nullptr, D3DLOCK_NO_DIRTY_UPDATE|D3DLOCK_NOSYSLOCK|D3DLOCK_READONLY); memcpy(pBits, (unsigned char*) lockedRect.pBits, dataLength); pSurface-&gt;UnlockRect(); //calculate average over of pBits </code></pre> <p>However it involes copying the whole front buffer back to the system memory which takes 33 ms on average. Obviuosly 33ms is no way near the speed that I need for a decent update rate therefore I am looking for a way to calculate the average over a region of the front buffer directly on the gpu without copying the front buffer back to the system memory.</p> <p>edit: the bottleneck in the code snippet is <code>pd3dDevice-&gt;GetFrontBufferData(0, pSurface);</code>. The memcpy has no visible effect on performance.</p> <p><strong>edit:</strong></p> <p>Based on user3125280's answer i cooked up a pice of code that should take the top left corner of the screen and average it. However the result is always 0. What am I missing? Also notice that <code>pSurface</code> is now in video memory and thus <code>GetFrontBufferData</code> is just a memcpy in video ram which is super fast.</p> <pre><code> pd3dDevice-&gt;CreateOffscreenPlainSurface(1, 1, D3DFMT_A8R8G8B8, D3DPOOL_SCRATCH, &amp;pAvgSurface, nullptr); pd3dDevice-&gt;CreateOffscreenPlainSurface(ddm.Width, ddm.Height, D3DFMT_A8R8G8B8, D3DPOOL_DEFAULT, &amp;pSurface, nullptr); pd3dDevice-&gt;GetFrontBufferData(0, pSurface); RECT r; r.right = 100; r.bottom = 100; r.left = 0; r.top = 0; pd3dDevice-&gt;StretchRect(pSurface, &amp;r, pAvgSurface, nullptr, D3DTEXF_LINEAR); D3DLOCKED_RECT lockedRect; pAvgSurface-&gt;LockRect(&amp;lockedRect, nullptr, D3DLOCK_NO_DIRTY_UPDATE|D3DLOCK_NOSYSLOCK|D3DLOCK_READONLY); unsigned int color = -1; memcpy((unsigned char*) &amp;color, (unsigned char*) lockedRect.pBits, 4); //FIXME there has to be a better way than memcopy pAvgSurface-&gt;UnlockRect(); </code></pre> <p><strong>edit2:</strong> Apparantly <code>GetFrontBufferData</code> requires the target to reside in the system memory. So I am back to square one.</p> <p><strong>edit3:</strong> According to <a href="http://www.virtualdub.org/blog/pivot/entry.php?id=356" rel="nofollow">this</a> the following should be possible in DX11.1:</p> <ul> <li>Create a Direct3D 11.1 device. (Maybe earlier works too -- I haven't tried. I'm not sure there's a reason to use a D3D10/10.1/11 device anyway.)</li> <li>Find the IDXGIOutput you want to duplicate, and call DuplicateOutput() to get an IDXGIOutputDuplication interface.</li> <li>Call AcquireNextFrame() to wait for a new frame to arrive.</li> <li>Process the received texture.</li> <li>Call ReleaseFrame().</li> <li>Repeat.</li> </ul> <p>However due to my non existing knowledge of DirectX I am having a hard time implementing it.</p> <p><strong>edit4:</strong> DuplicateOutput is not supported in operating systems older than Windows 8 :(</p> <p><strong>edit5:</strong> I did some experiments with the classical <code>GetPixel</code> API thinking that it may be fast enough for random sampling. Sadly it is not. <code>GetPixel</code> takes the same amount of time that <code>GetFrontBufferData</code> takes. I guess it internally calls <code>GetFrontBufferData</code>.</p> <p>So for now I see two solutions: * Disable Aero and use <code>GetFrontBufferData</code> * Switch to windows 8 Both of them are not really good :(</p>
 

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