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    <p>Yes, the faster way to do this is to use <code>Bitmap.LockBits</code> and use pointer arithmetic to retrieve the values instead of <code>GetPixel</code> and <code>SetPixel</code>. The downside, of course, is that you have to use unsafe code; if you make a mistake, you can cause some really bad crashes in your program. But if you keep it simple and self-contained, it should be fine (hey, if I can do, you can do it too).</p> <p>For example, you could do something like this (not tested, use at your own risk):</p> <pre><code>Bitmap mask = ...; Bitmap input = ...; Bitmap output = new Bitmap(input.Width, input.Height, PixelFormat.Format32bppArgb); var rect = new Rectangle(0, 0, input.Width, input.Height); var bitsMask = mask.LockBits(rect, ImageLockMode.ReadOnly, PixelFormat.Format32bppArgb); var bitsInput = input.LockBits(rect, ImageLockMode.ReadOnly, PixelFormat.Format32bppArgb); var bitsOutput = output.LockBits(rect, ImageLockMode.WriteOnly, PixelFormat.Format32bppArgb); unsafe { for (int y = 0; y &lt; input.Height; y++) { byte* ptrMask = (byte*) bitsMask.Scan0 + y * bitsMask.Stride; byte* ptrInput = (byte*) bitsInput.Scan0 + y * bitsInput.Stride; byte* ptrOutput = (byte*) bitsOutput.Scan0 + y * bitsOutput.Stride; for (int x = 0; x &lt; input.Width; x++) { ptrOutput[4 * x] = ptrInput[4 * x]; // blue ptrOutput[4 * x + 1] = ptrInput[4 * x + 1]; // green ptrOutput[4 * x + 2] = ptrInput[4 * x + 2]; // red ptrOutput[4 * x + 3] = ptrMask[4 * x]; // alpha } } } mask.UnlockBits(bitsMask); input.UnlockBits(bitsInput); output.UnlockBits(bitsOutput); output.Save(...); </code></pre> <p>This example derives the alpha channel in the output from the <strong>blue</strong> channel in the mask image. I’m sure you can change it to use the mask’s red or alpha channel if required.</p>
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    1. POAlpha masking in c# System.Drawing?
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    1. COI'd still have to iterate the pixels though, right?
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    2. COYes, but it’s very fast. Any other implementation that you could potentially use would equally have to iterate through the pixels.
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    3. COWell, it works, and it's definitely faster than my previous method, thank you!
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