I wish to scale an image to half its original size. The width and height of the image will both be even (although not necessarily equal).
I want the resize to be fairly high quality (so I don't just want to remove w/2 vertical strips, and h/2 horizontal strips). Then again, I don't want anything too expensive to perform - ideally it needs to resize potentially 800x600 within 200 ms.
Thanks in advance
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It depends on whether or not you are using "premultiplied" alpha.
I'm writing this in terms of
Non-premultiplied alpha: r=127, g=127, b=127, alpha=127 is half-transparent gray
premultiplied alpha: r=127, g=127, b=127, alpha=127 is half-transparent white.
Assuming you are using non-premultiplied alpha, you have to multiply each source component by the alpha value
dest[i][j].r = (src[2*i][2*j].r * src[2*i][2*j].alpha + src[2*i+1][2*j].r * src[2*i+1][2*j].alpha .......) / (4 * 255);
dest[i][j].alpha is just the average of the alphas of the 4 source pixels
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by: JR2003Posted on 2009-09-18 at 07:53:39ID: 25366607
You can take the average of each of the RGB values for each block of 4 cells (2 horizontal and 2 vertical).