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LaesieWorks

Image Compression

In book printing, a photo with no color but many shades of gray, can be printed with just black ink on white paper. The halftone screen transformed the levels of gray into a uniform grid of dots. These dots have different sizes, and give, from enough distance, the illusion of grays.

Dither is when a grayscale image is not transformed into dots of different sizes, but "frequency modulated", meaning that only the smallest possible dots are used. The more dots, the darker the area. This gives a better quality print relative to the good old halftone screen, but it is (was) harder to create and print.

On a modern display we have a grid of digital pixels, and millions of colors possible per pixels (note: every pixel on the display is actually a group of 3 pixels: R, G and B). But for data compression, it's important to bring the amount of colors down to an acceptable minimum. One good way of doing that, is to use dither. Dither can create the illusion of tones. A mixture of black and white pixels gives the illusion of grays. But just black and white pixels doesn't give a high quality image, mainly because the pixels are quite big. Lucky it's possible to dither a photo with more than just black and white. Using for example: black, dark gray, light gray, and white, the result is much better. The more tones, the better. But also: the more tones, the more information, the bigger the file.

How many shades of gray are enough to get a high image quality, hard to distinguish from the origin with 256 shades of gray?
I think 32. (efficient to choose a power of 2)

32 tones are enough when using gray, where the eyes are very sensitive for. For most colors, the eyes are less sensitive. For the yellow to white spectrum, 8 tones are enough.

The gray spectrum on the far left has only 32 tones in it, doesn't look smooth at all.

The right gray spectrum, has only 32 gray tones also, but now it seems smooth because it is dithered.

32 tones