Quick how to: Reduce number of colors programmatically

My colleague just asked me about how to reduce a number of colors in image programmatically. This is very simple task and contains of 43 steps:

First of all, you have to read a source image

using (var img = Image.FromFile(name)) {
var bmpEncoder = ImageCodecInfo.GetImageDecoders().FirstOrDefault(
    e => e.FormatID == ImageFormat.Bmp.Guid);

Then create your own encoder with certain color depth (32 bits in this case)

var myEncoder = System.Drawing.Imaging.Encoder.ColorDepth;
var myEncoderParameter = new EncoderParameter(myEncoder, 32L);
var myEncoderParameters = new EncoderParameters(1) {
    Param = new EncoderParameter[] { myEncoderParameter } };

Then save it

img.Save(name.Replace(”.png”, “.bmp”), bmpEncoder, myEncoderParameters);

It it enough? Not really, because if you’re going to loose colors (by reducing color depth), it makes sense to avoid letting default WIX decoder to do this, thus you have to find nearest base colors manually. How to do this? By using simple math

Color GetNearestBaseColor(Color exactColor) {
Color nearestColor = Colors.Black;
int cnt = baseColors.Count;
for (int i = 0; i < cnt; i++) {
int rRed = baseColors[i].R - exactColor.R;
int rGreen = baseColors[i].G - exactColor.G;
int rBlue = baseColors[i].B - exactColor.B;
int rDistance =
(rRed * rRed) +
(rGreen * rGreen) +
(rBlue * rBlue);
if (rDistance == 0.0) {
return baseColors[i];
} else if (rDistance < maxDistance) {
maxDistance = rDistance;
nearestColor = baseColors[i];
}
}
return nearestColor;
}

Now, you can either change colors on base image directly

unsafe {
uint* pBuffer = (uint*)hMap;
for (int iy = 0; iy < (int)ColorMapSource.PixelHeight; ++iy)
{
for (int ix = 0; ix < nWidth; ++ix)
{
Color nc = GetNearestBaseColor(pBuffer[0].FromOle());
pBuffer[0] &= (uint)((uint)nc.A << 24) | //A
(uint)(nc.R << 16 ) | //R
(uint)(nc.G << 8 ) | //G
(uint)(nc.B ); //B
++pBuffer;
}
pBuffer += nOffset;
}
}

Or, if you’re in WPF and .NET 3.5 create simple pixel shader effect to do it for you in hardware. Now, my colleague can do it himself in about 5 minutes . Have a nice day and be good people.

This article, along with any associated source code and files, is licensed under The Microsoft Public License (Ms-PL)