Colorize with a given color a texture

Question

I have a texture and I want to "colorize" it with a given color, lets say cyan (#00ffff) or purple (#800080). What I want to do, is get all the pixel values from the texture, and remove the color and keep the "brightness" and "saturation" and apply to the desired color. There is a tool in GIMP to do this called Colorize (Colors -> Colorize.. while editing), I made an example below.

This is will all be done in a shader (GLSL), although this is probably a general algorithm.

Answer 1

This live demo uses the algorithm described by @Dries in a fragment shader to colorize the incoming fragment color.

It basically can be summarised by:

vec4 getInputColor()
{
    TODO: sample from a texture / use a uniform var / varying ...
}

vec4 toGrayscale(in vec4 color)
{
  float average = (color.r + color.g + color.b) / 3.0;
  return vec4(average, average, average, 1.0);
}

vec4 colorize(in vec4 grayscale, in vec4 color)
{
    return (grayscale * color);
}

void main()
{
  // This is the color you want to apply
  // in the "colorize" step. Should ultimately be a uniform var.
  vec4 c = vec4(0.0, 1.0, 1.0, 1.0);

  // The input fragment color.
  // Can come from a texture, a varying or a constant.
  vec4 inputColor = getInputColor();

  // Convert to grayscale first:
  vec4 grayscale = toGrayscale(inputColor);

  // Then "colorize" by simply multiplying the grayscale
  // with the desired color.
  vec4 colorizedOutput = colorize(grayscale, c);

  // Done!
  gl_FragColor = colorizedOutput;
}

Answer 2

If you want it exactly like the thing above you'll first have to make your render grayscale.

You can do this in the pixel shader by taking a sample of the texture you were using (or only the color doesn't matter). Make a sum of the r, g and b value and divide that by 3. Return that value as the rgb values from the pixel shader.

Then do this (with or without the grayscale). Multiply the color of your texture (or the value of the grayscale) with the color you want.

Return that value in your pixel shader and that should be it.

Answer 3

If you just multiply a grayscale with a color, white will become that color instead of staying white. Instead, multiply only for gray values up to 0.5 and for brighter values, mix the color with white (the mix value will be the gray value normalized from [0.5..1.0] to [0.0..1.0]. So the mix amount would be (grayVal-0.5)*2.0