Question

Sometimes I have a true colored image, by using dithering algorithm, I can reduce the color to just 256. I want to know how the dithering algorithm achieve this.

I understand that dithering can reduce the error, but how can the algorithm decrease color depth, especially from true color to just 256 colors or even less.

Answer 1

Dithering simulates a higher color depth by “mixing” the colors in a defined palette to create the illusion of a color that isn’t really there. In reality, it’s doing the same thing that your computer monitor is already doing: taking a color, decomposing it into primary colors, and displaying those right next to each other. Your computer monitor does it with variable-intensity red, green, and blue, while dithering does it with a set of fixed-intensity colors. Since your eye has limited resolution, it sums the inputs, and you perceive the average color.

In the same way, a newspaper can print images in grayscale by dithering the black ink. They don’t need lots of intermediate gray colors to get a decent grayscale image; they simply use smaller or larger dots of black ink on the page.

When you dither an image, you lose information, but your eye perceives it in largely the same way. In this sense, it’s a little like JPEG or other lossy compression algorithms which discard information that your eye can’t see.