Alpha masking in c# System.Drawing?

alpha, c#, masking

Solution

Yes, the faster way to do this is to use `Bitmap.LockBits` and use pointer arithmetic to retrieve the values instead of `GetPixel` and `SetPixel`. 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).

For example, you could do something like this (not tested, use at your own risk):

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 < 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 < 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(...);

This example derives the alpha channel in the output from the blue channel in the mask image. I’m sure you can change it to use the mask’s red or alpha channel if required.

Problem

I'm trying to draw an image, with a source `Bitmap` and an alpha mask `Bitmap`, using the `System.Drawing.Graphics` object. At the moment I loop X and Y and use `GetPixel` and `SetPixel` to write the source color and mask alpha to a third `Bitmap`, and then render that. However this is very inefficient and I am wondering if there is an faster way to achieve this? The effect I'm after looks like this: The grid pattern represents transparency; you probably knew that.

Original source