Faster contrast algorithm for a bitmap

Question

I have a tool with trackbar slider controls used to adjust an image\'s brightness, contrast, gamma, etc.

I am trying to get real-time updates to my image while the user

Answer 1

Depending on the machine you're running this on, your technique could be quite slow. If you're using an ARM system without an FPU, each of those operations would take quite a while. Since you're applying the same operation to every byte, a faster technique would be to create a 256-entry lookup table for the contrast level and then translate each image byte through the table. Your loop would then look like:

byte contrast_lookup[256];
double newValue = 0;
double c = (100.0 + contrast) / 100.0;

c *= c;

for (int i = 0; i < 256; i++)
{
    newValue = (double)i;
    newValue /= 255.0;
    newValue -= 0.5;
    newValue *= c;
    newValue += 0.5;
    newValue *= 255;

    if (newValue < 0)
        newValue = 0;
    if (newValue > 255)
        newValue = 255;
    contrast_lookup[i] = (byte)newValue;
}

for (int i = 0; i < sourcePixels.Length; i++)
{
    destPixels[i] = contrast_lookup[sourcePixels[i]];
}