Question

How do I achieve this kind of color replacement programmatically?

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So this is the function I have used to replace a pixel:

Color.FromArgb(
    oldColorInThisPixel.R + (byte)((1 - oldColorInThisPixel.R / 255.0) * colorToReplaceWith.R),
    oldColorInThisPixel.G + (byte)((1 - oldColorInThisPixel.G / 255.0) * colorToReplaceWith.G),
    oldColorInThisPixel.B + (byte)((1 - oldColorInThisPixel.B / 255.0) * colorToReplaceWith.B)
    )

Thank you, CodeInChaos!

Answer 1

The formula for calculating the new pixel is:

newColor.R = OldColor;
newColor.G = OldColor;
newColor.B = 255;

Generalizing to arbitrary colors:

I assume you want to map white to white and black to that color. So the formula is newColor = TargetColor + (White - TargetColor) * Input

newColor.R = OldColor + (1 - oldColor / 255.0) * TargetColor.R;
newColor.G = OldColor + (1 - oldColor / 255.0) * TargetColor.G;
newColor.B = OldColor + (1 - oldColor / 255.0) * TargetColor.B;

And then just iterate over the pixels of the image(byte array) and write them to a new RGB array. There are many threads on how to copy an image into a byte array and manipulate it.