public static void SeparateAlphaChannel(byte[] png, out byte[] rgb, out byte[] alpha)
{
MemoryStream s = new MemoryStream(png);
Bitmap bmp = new Bitmap(s);
MemoryStream rgbStream = new MemoryStream(bmp.Width * bmp.Height * 3);
MemoryStream alphaStream = new MemoryStream(bmp.Width * bmp.Height);
bool hasAlpha = false;
switch (bmp.PixelFormat)
{
case PixelFormat.Format24bppRgb:
// 8 bits per channel, no alpha
SeparateAlphaChannel_LockBits(bmp, rgbStream, alphaStream);
hasAlpha = false;
break;
case PixelFormat.Canonical:
case PixelFormat.Format32bppArgb:
// 8 bits per channel, with alpha
SeparateAlphaChannel_LockBits(bmp, rgbStream, alphaStream);
hasAlpha = true;
break;
case PixelFormat.Format32bppRgb:
// 8 bits per channel, with an unused alpha byte
SeparateAlphaChannel_LockBits(bmp, rgbStream, alphaStream);
hasAlpha = false;
break;
default:
// Do it the slow way
SeparateAlphaChannel_GetPixel(bmp, rgbStream, alphaStream);
hasAlpha = true;
break;
}
rgbStream.Seek(0, SeekOrigin.Begin);
alphaStream.Seek(0, SeekOrigin.Begin);
rgb = Compression.Compress(rgbStream).GetBuffer();
alpha = hasAlpha ? Compression.Compress(alphaStream).GetBuffer() : null;
// A quick performance test with a release build gave these results.
// Generally the LockBits method out-performed the GetPixel method
// by a factor of three or four.
//
// Image size Iterations LockBits GetPixel
// 50 x 66 1 0.001 0.002
// 50 x 66 10 0.006 0.020
// 50 x 66 100 0.067 0.215
// 50 x 66 1000 0.645 2.151
// 100 x 132 1 0.002 0.007
// 100 x 132 10 0.022 0.080
// 100 x 132 100 0.219 0.831
// 100 x 132 1000 2.193 8.370
// 150 x 198 1 0.005 0.017
// 150 x 198 10 0.051 0.214
// 150 x 198 100 0.487 1.826
// 150 x 198 1000 4.909 18.398
//
// int[] iterations = { 1, 10, 100, 1000 };
// foreach(int iteration in iterations)
// {
// System.Diagnostics.Stopwatch timer = new System.Diagnostics.Stopwatch();
// timer.Start();
// for(int x = 0; x < iteration; ++x)
// {
// byte[] r;
// byte[] a;
// Image.SeparateAlphaChannel(bits, out r, out a);
// }
// timer.Stop();
// string msg = $"{size.Width} x {size.Height} run {iteration} times: {timer.Elapsed:mmss\\.fff}\r\n";
// File.AppendAllText(@"c:\users\ciaran\desktop\getpixel.txt", msg);
// System.Diagnostics.Debugger.Log(0, null, msg);
// }
//
// In these tests I commented out compression calls so that I just measured
// the channel separation stuff.
}