The algorithms

Valid colors

It is based on the HSV (hue, saturation, value) color's components. The hue value ranges from 0 to 360. The saturation and brightness range from 0 to 100 percent.

Every component is modified of more or less 10 percent.

Color computeHsvColor(Color initialColor, Color keptColor) {
  Color modifiedColor = initialColor;

  while (modifiedColor.getHue() < 360 && modifiedColor.getHue() > 0) {
     modifiedColor.setHue += 10%
     while (modifiedColor.getSaturation() < 100 && modifiedColor.getSaturation() > 0) {
             varySaturationAndTestColor(modifiedColor, +10%);
             varySaturationAndTestColor(modifiedColor, -10%);
        while (modifiedColor.getBrightness() < 100 && modifiedColor.getBrightness() > 0) {
                varyBrightnessAndTestColor(modifiedColor, +10%);
                varyBrightnessAndTestColor(modifiedColor, -10%);
        }
     }

     modifiedColor.setHue -= 10%
     while (modifiedColor.getSaturation() < 100 && modifiedColor.getSaturation() > 0) {
             varySaturationAndTestColor(modifiedColor, +10%);
             varySaturationAndTestColor(modifiedColor, -10%);
        while (modifiedColor.getBrightness() < 100 && modifiedColor.getBrightness() > 0) {
                varyBrightnessAndTestColor(modifiedColor, +10%);
                varyBrightnessAndTestColor(modifiedColor, -10%);
        }
     }
  }
  return (modifiedColor);
}

Color varySaturationAndTestColor(Color modifiedColor, int variation) {
  modifiedColor.setSaturation(variation);
  testContrast(modifiedColor, keptColor);
  return modifiedColor;
}

Color varyBrightnessAndTestColor(Color modifiedColor, int variation) {
  modifiedColor.setBrightness(variation);
  testContrast(modifiedColor, keptColor);
  return modifiedColor;
}

Each component is modified until a valid contrast is found.

For example : a color with this components : Hue (36), Saturation (32%), Brightness (45%), can be modify to obtain : Hue (72), Saturation (42%), Brightness (55%) or Hue (0), Saturation (22%), Brightness (35%), etc.

Then, we test the contrast between each modified color against the initial one. If the contrast is valid, the modified color is stored in the results table.

Valid colors and very close to initial color

It is based on the RGB (red, green, blue) color's components. Like the first algorithm, each component is modified until we find a valid contrast.

The red, green and blue values must be between 0 and 255.

For this algorithm, the principle is similar to the first one. After many tests, we have identified the following constraints :

• The component's step in one loop. We have decided to step the component values one by one.
• The max shift for one component. We have decided to limit the maximum shift :
  • to 60 when the gap between initial contrast ratio and the choosen contrast ratio is between 0 and 1
  • to 80 when the gap between initial contrast ratio and the choosen contrast ratio is between 1 and 2
  • to 110 when the gap between initial contrast ratio and the choosen contrast ratio is between 2 and 3
• The hue bounder, set to 5%. Its aim is to limit the hue. If the initial color (to modify) is blue, we don't want to suggest a green, so we stay close to the initial hue.
• The maximum coefficient. Its target is to limit the contrast ratio. For instance, if the selected ratio is 4.5, we want color that are 4.5 + this maximum coefficient. The value we chose is 0.001 (thus a color must have a ratio between 4.5 and 4.501, which is very very closed, but also what we want).

For example : an initial color with this components : Red (35), Green (74), Blue (87) is limited between : Red (0-95), Green (14-134), Blue (27-147). The initial color's hue is 194. The hue bounder will limit the result's hue between 146 and 212 (194 +- 5%). In this example, the initial color is the foreground and its background is black, the contrast ratio is 2.1886. If you want a ratio 3, the maximum shift is 60 (3 - 2.1886 < 1), the maximum coefficient limit the result between 3 and 3.001.