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POHow can I make this faster? (C/C++) OpenCV
 

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  1. POHow can I make this faster? (C/C++) OpenCV
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    <p>I am processing frames in a video and displaying it live (real time). The algorithm is fast, but I am wondering if there's any optimizations that I can do that will make it even more seamless. I don't know what functions in my algorithm take up the most amount of time, my guess is the sqrt() function because apparently it does some look ups, but i am not sure.</p> <p>This is my algorithm:</p> <pre><code>IplImage *videoFrame = cvCreateImage(cvSize(bufferWidth, bufferHeight), IPL_DEPTH_8U, 4); videoFrame-&gt;imageData = (char*)bufferBaseAddress; int channels = videoFrame-&gt;nChannels; int widthStep = videoFrame-&gt;widthStep; int width = videoFrame-&gt;width; int height = videoFrame-&gt;height; for(int i=0;i&lt;height;i++){ uchar *col = ((uchar *)(videoFrame-&gt;imageData + i*widthStep)); for(int j=0;j&lt;width;j++){ double pRed = col[j*channels + 0]; double pGreen = col[j*channels + 1]; double pBlue = col[j*channels + 2]; double dRed = green.val[0] - pRed; double dGreen = green.val[1] - pGreen; double dBlue = green.val[2] - pBlue; double sDRed = dRed * dRed; double sDGreen = dGreen * dGreen; double sDBlue = dBlue * dBlue; double sum = sDRed + sDGreen + sDBlue; double euc = sqrt(sum); //NSLog(@"%f %f %f", pRed, pGreen, pBlue); if (euc &lt; threshold) { col[j*channels + 0] = white.val[0]; col[j*channels + 1] = white.val[1]; col[j*channels + 2] = white.val[2]; } } } </code></pre> <p>Thanks!</p> <p><strong>UPDATE</strong> Ok, so what this does is loop throughout every pixel in the image, and calculator the Euclidean distance between the color of the pixel and green color. So, overall this is a green screen algorithm. </p> <p>I did some benchmarks, and the fps without using this algorithm is 30.0fps. Using this algorithm, it falls down to about 8fps. But, the majority of the for drop comes from <code>col[j*channels + 0];</code> If the algorithm doesn't do anything else and use access the array elects, it drops down to about 10fps.</p> <p><strong>UPDATE 2</strong> Ok this is interesting, I was removing random lines from the stuff inside the double loop to see what causes the bigger overhead and this is what I found: Creating variables on the stack causes HUGE drop in FPS. Consider this example:</p> <pre><code>for(int i=0;i&lt;height;i++){ uchar *col = ((uchar *)(data + i*widthStep)); for(int j=0;j&lt;width;j++){ double pRed = col[j*channels + 0]; double pGreen = col[j*channels + 1]; double pBlue = col[j*channels + 2]; } } </code></pre> <p>This drops the fps to 11-ish.</p> <p>Now this on the other hand:</p> <pre><code>for(int i=0;i&lt;height;i++){ uchar *col = ((uchar *)(data + i*widthStep)); for(int j=0;j&lt;width;j++){ col[j*channels + 0]; col[j*channels + 1]; col[j*channels + 2]; } } </code></pre> <p>doesn't drop the FPS at all! The FPS stays at a pretty 30.0. Thought I should update this and let you guys know what's this is the real bottle neck, making variables not he stack. I wonder if I inline everything I might get a pure 30.0fps.</p> <p>Nvm...maybe the expressions that aren't assigned to a var aren't even evaluated.</p>
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      1. POHow can I make this faster? (C/C++) OpenCV
      1. USAbid Rahman K
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      1. POHow can I make this faster? (C/C++) OpenCV
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      1. POHow can I make this faster? (C/C++) OpenCV
      1. USKyle
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    1. COIs there a reason you're asking this before profiling on your own?
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      1. POHow can I make this faster? (C/C++) OpenCV
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    2. COSearch the web for "data driven design"; some of the articles tell how to optimize the processor's data and instruction caches.
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      1. POHow can I make this faster? (C/C++) OpenCV
      1. USThomas Matthews
    3. COIf I were doing this on a platform capable of it, and it made sense relative to the operations you perform before and after this operation, then I'd shift the computation to the GPU.
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