CombineAlpha.cs

public struct RGBA
{
	public byte R, G, B, A;
}

class MyClass
{
	static public RGBA[] ColorPixels = new RGBA[800 * 480];
	static public byte[] Lookup = new byte[256];

	public void CombineAlpha(RenderTarget2D RenderTarget, Texture2D TextureColor, byte[] AlphaChannel, float Step)
	{
		int Width = TextureColor.Width, Height = TextureColor.Height;
		var WidthHeight = Width * Height;

		int Offset = (int)(((Step * 2) - 1) * 255);
		for (int n = 0; n < 256; n++)
		{
			var Alpha = n;
			Alpha += Offset;
			if (Alpha < 0) Alpha = 0;
			if (Alpha > 255) Alpha = 255;
			Lookup[n] = (byte)Alpha;
		}
		if (LastTextureColor != TextureColor)
		{
			TextureColor.GetData(0, new Rectangle(0, 0, Width, Height), ColorPixels, 0, WidthHeight);
			LastTextureColor = TextureColor;
		}
		for (int n = 0; n < WidthHeight; n++)
		{
			ColorPixels[n].A = Lookup[AlphaChannel[n]];
		}
		RenderTarget.SetData(0, new Rectangle(0, 0, Width, Height), ColorPixels, 0, WidthHeight);
	}
}