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POAlgorithm to scale a raw 24 bit pixelmap to greater than 50% in C
 

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  1. POAlgorithm to scale a raw 24 bit pixelmap to greater than 50% in C
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    <p>Reducing a raw pixelmap to a value of 50% is easy. I simply slide a 2x2 square across the map and average the RGB components of the 4 pixels as follows:</p> <pre><code> img = XGetImage(d_remote,RootWindow(d_remote,0),0,0,attr.width,attr.height,XAllPlanes(),ZPixmap); int i; int j; for(i=0;i&lt;attr.height;i=i+2){ for(j=0;j&lt;attr.width;j=j+2) { unsigned long p1 = XGetPixel(img, j, i); unsigned long p1R = p1 &amp; 0x00ff0000; unsigned long p1G = p1 &amp; 0x0000ff00; unsigned long p1B = p1 &amp; 0x000000ff; unsigned long p2 = XGetPixel(img, j+1, i); unsigned long p2R = p2 &amp; 0x00ff0000; unsigned long p2G = p2 &amp; 0x0000ff00; unsigned long p2B = p2 &amp; 0x000000ff; unsigned long p3 = XGetPixel(img, j, i+1); unsigned long p3R = p3 &amp; 0x00ff0000; unsigned long p3G = p3 &amp; 0x0000ff00; unsigned long p3B = p3 &amp; 0x000000ff; unsigned long p4 = XGetPixel(img, j+1, i+1); unsigned long p4R = p4 &amp; 0x00ff0000; unsigned long p4G = p4 &amp; 0x0000ff00; unsigned long p4B = p4 &amp; 0x000000ff; unsigned long averageR = (p1R+p2R+p3R+p4R)/4 &amp; 0x00ff0000; unsigned long averageG = (p1G+p2G+p3G+p4G)/4 &amp; 0x0000ff00; unsigned long averageB = (p1B+p2B+p3B+p4B)/4 &amp; 0x000000ff; int average = averageR | averageG | averageB; XPutPixel(newImg, j/2, i/2, average); } } </code></pre> <p>This would make a pixelmap that is 500x500 turn into one that is 250x250. This is a 50% reduction. What if I wanted to scale it by 20%. For example I would like my 500x500 image to turn into 400x400? The smallest square I can slide is a 2x2. I don't see how I can get a reduction that is not a perfect power of 2.</p> <h1>Solution:</h1> <p>How's this for effort?? I modified a script I found that does bi-linear interpolation to work on XImages. It should work for any generic pixelmap. I do find the code ugly though since I see images as 2d arrays. I don't see why all the image code is mapped to a 1d array. It's harder to visualize. This works for any resize.</p> <pre><code>void resize(XImage* input, XImage* output, int sourceWidth, int sourceHeight, int targetWidth, int targetHeight) { int a, b, c, d, x, y, index; float x_ratio = ((float)(sourceWidth - 1)) / targetWidth; float y_ratio = ((float)(sourceHeight - 1)) / targetHeight; float x_diff, y_diff, blue, red, green ; int offset = 0 ; int i=0; int j=0; int* inputData = (int*)input-&gt;data; int* outputData = (int*)output-&gt;data; for (i = 0; i &lt; targetHeight; i++) { for (j = 0; j &lt; targetWidth; j++) { x = (int)(x_ratio * j) ; y = (int)(y_ratio * i) ; x_diff = (x_ratio * j) - x ; y_diff = (y_ratio * i) - y ; index = (y * sourceWidth + x) ; a = inputData[index] ; b = inputData[index + 1] ; c = inputData[index + sourceWidth] ; d = inputData[index + sourceWidth + 1] ; // blue element blue = (a&amp;0xff)*(1-x_diff)*(1-y_diff) + (b&amp;0xff)*(x_diff)*(1-y_diff) + (c&amp;0xff)*(y_diff)*(1-x_diff) + (d&amp;0xff)*(x_diff*y_diff); // green element green = ((a&gt;&gt;8)&amp;0xff)*(1-x_diff)*(1-y_diff) + ((b&gt;&gt;8)&amp;0xff)*(x_diff)*(1-y_diff) + ((c&gt;&gt;8)&amp;0xff)*(y_diff)*(1-x_diff) + ((d&gt;&gt;8)&amp;0xff)*(x_diff*y_diff); // red element red = ((a&gt;&gt;16)&amp;0xff)*(1-x_diff)*(1-y_diff) + ((b&gt;&gt;16)&amp;0xff)*(x_diff)*(1-y_diff) + ((c&gt;&gt;16)&amp;0xff)*(y_diff)*(1-x_diff) + ((d&gt;&gt;16)&amp;0xff)*(x_diff*y_diff); outputData[offset++] = (int)red &lt;&lt; 16 | (int)green &lt;&lt; 8 | (int)blue; } } } </code></pre>
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    1. USjohn doe
    1. USjohn doe
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      1. POAlgorithm to scale a raw 24 bit pixelmap to greater than 50% in C
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    3. VO
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      1. POAlgorithm to scale a raw 24 bit pixelmap to greater than 50% in C
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    1. CO"Slide" a 1.25 x 1.25 square across. If the 1.25x1.25 square overlaps several squares, weigh the averages accordingly to pretend you are only looking at a 1.25x1.25 area.
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      1. POAlgorithm to scale a raw 24 bit pixelmap to greater than 50% in C
      1. USuser2461391
    2. COYes but my i and j can only increase by a whole number and I can only access a whole numbered pixel.
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      1. USjohn doe
    3. COThat's what the weighted averages are for - it let's you "access" a fraction of a pixel.
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      1. POAlgorithm to scale a raw 24 bit pixelmap to greater than 50% in C
      1. USDrew McGowen
 

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