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# Gaussian Blur

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Hy. I need some info how to do gaussian blur. Thanks.

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In what context? If you want to do Gaussian blur in realtime it''s going to be rather tricky. If you''re looking to do a preprocess on textures, then... essentially, Gaussian blur is just a filtering operation on textures; it replaces each pixel with a weighted average of all the pixels within some distance of it (from a mathematical perspective the distance should be infinite but for practical considerations everyone clamps it to some range). The weight function is just a ''bell curve'' -- each pixel is weighted by e^-(d^2/d0^2), where d is the distance between the two pixels and d0 is a reference distance that says how wide your filter is (how much blur you get). Here''s some code off the top of my head, demonstrating how you''d do it on a greyscale image (i.e., one float per pixel, as opposed to 3 or 4 bytes):

  void GaussianBlur(float *RawBits, float *BlurredBits, float FilterWidth, int ImageWidth, int ImageHeight) { int Clamp; // How far out we sample int XLoop, YLoop, XInner, YInner; float Dist, Weight; float DataSum, WeightSum; Clamp = (int) (3*FilterWidth); // This is a good heuristic for (YLoop = 0; YLoop < ImageHeight; YLoop++) { for (XLoop = 0; XLoop < ImageWidth; XLoop++) { DataSum = WeightSum = 0.0f; for (YInner = -Clamp; YInner <= Clamp; YInner++) { for (XInner = -Clamp; XInner <= Clamp; XInner++) { X = XLoop+XInner; Y = YLoop+YInner; // the pixel we sample if ((X >= 0) && (X < ImageWidth) && (Y >= 0) && (Y < ImageHeight)) { // This pixel falls within the bounds of the image; // add it to the weighted average Dist = (float) (XInner*XInner+YInner*YInner); Weight = fexp(Dist/(FilterWidth*FilterWidth)); DataSum += Weight*RawBits[Y*ImageWidth+X]; WeightSum += Weight; } } } if (WeightSum > 0.0f) { BlurredBits[YLoop*ImageWidth+XLoop] = DataSum/WeightSum; } else { BlurredBits[YLoop*ImageWidth+XLoop] = 0; } } }}

Note that I _don''t_ recommend ever using the above code as is. It''s completely unoptimized, the most egregious sin probably being the fact that weights should be precomputed so you don''t have to make a couple dozen calls to fexp() per pixel. There are a number of other smaller optimizations that could stand to be done, too; hopefully this will at least give you the basic gist of things, though.

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Thanks man, I''ll give it a try.
Actualy I need it to do a mip-map images, but I think this could be usefull.

Drki.

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Actually, for MIPmaps you probably _don''t_ want Gaussian blur; all you really want is to scale down your initial texture image by averaging it; that is, each texel in the subsampled (half-size) image is simply the average of the four pixels in the original image that it covers. This is basically because trilinear sampling is a (rough) approximation to summed-area tables which traditionally use a box filter (all ones in the area that it covers), and so a box (i.e., averaging) filter is the appropriate one to use when subsampling.

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I need to do this in my own application as well. Does anybody know a fast way to do this?

I was thinking of using GDI''s GetPixel for averaging, but that is really slow.

Should I instead lock the surface, and then get the pixelformat, and then do math to figure the byte in the surface? I''ve never directly manipulated a surface before (without using GDI).

Does anybody have a MIP map blur function already made that is fast?

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