Performance problem with large number of ColorMaps with ImageAttributes.SetRemapTable

Question

I'm dynamically mapping colours in an image that has alpha transparency in it to another (smaller set) of colours.

I've got code like:

var mappings = new List<ColorMap>();
foreach(var color in mapcolours)
{
    // Add mappings for all alpha values of the mask colour
    for(int i = 0; i < 256; i++)
    {
        var cm = new ColorMap();
        cm.OldColor = Color.FromArgb(i, mapcolour);
        cm.NewColor = Color.FromArgb(i, GetDestinationColour(mapcolour));
        mappings.Add(cm);
    }
}

var ia = new ImageAttributes();
ia.SetRemapTable(mappings.ToArray(), ColorAdjustType.Bitmap);
g.DrawImage(image_mask, destrect, 0, 0, image_mask.Width, image_mask.Height, GraphicsUnit.Pixel, ia);

But unfortunately (and I think its because I'm mapping all 255 alpha values of each colour) this eats up a LOT of CPU which has shown up during some profiling.

Is there a more efficient way to accomplish this? I can't stop mapping all 255 alpha values as otherwise the result looks terrible.

Answer 1

You should accquire a pointer to the raw image data with Bitmap.LockBits. Then you just loop over each pixel and look up it's color in a Dictionary. It will be super-fast.