Color resolution and halftoning

Color resolution

When colors are represented by RGB, if each component (channel) is expressed by 8 bits = 256 colors, the difference in colors cannot be distinguished by human eyes. Therefore, at the personal computer level, a full-color image is usually represented by 24 bits (16,777,216 colors) or more, or by 8 bits for each channel of RGB. Most formats of image data generally support full-color images, but some formats such as GIF format can describe up to only 256 index colors. Conversely, some formats such as Photoshop PSD and TIFF support 16-bit / channel (16-bit = 64 K colors) mode, where the dynamic range is high enough for image processing such as brightness correction or adjustment of color balance.

* If you are adjusting the color of a photo with Photoshop, it is better to use 16-bit / channel mode while working. With 8 bits / channel mode, color difference may become noticeable when using a blurred selection range or when using a tone curve many times.
* In general, single-lens cameras can save images as RAW data, having a color resolution of 12 to 14 bits / channel, so various color adjustments are possible during the development process. Try to save important photos as RAW data. In addition, many single-lens cameras also support the 16-bit TIFF format, so you may want to save a photo as a TIFF file (though the file size might be large).

48 bits (16 bits × 3)  
280 trillion colors  
16 bit / channel
24 bits (8 bits×3)  
16M (16 million) colors  
8 bit / channel
16 bits
15 bits (5 bits×3)
64K colors
32K colors
high color
8 bits
256 colors
index color

On the other hand, when displaying an image on a display, the number of colors that can be displayed simultaneously may be limited depending on the graphic adapter. For example, when a full-color image is displayed in a mode with low color resolution, it will look like the figures below. A full-color mode is now a commonplace, but in the 1990s there were many computers that could display only 256 colors.

color resolution

Half toning

When displaying a full-color image in an environment where the number of available colors is limited, a technique in which colors are scattered and arranged so that the colors appear to be intermediate is widely used. Halftoning is indispensable when printing a full-color image with a printer having a limited number of inks. Halftoning is also used to represent a full-color image with a limited number of index colors.

Distributing colors is called dithering. Dithering includes the error diffusion and regular dithering (or pattern). For example, in Photoshop, when reducing a full-color image to the index color mode (256 colors or less), dithering method can be selected from "None", "Diffusion", and "Pattern".
In regular dithering, a pattern of 4x4 pixels as shown in the figure is usually used. The error diffusion is a method of arranging colors so as to minimize the error from the original color while distributing colors randomly, and is widely used in ink jet printers, etc. (see figure).

dither pattern
4×4 dither pattern

Dithering type 
  gray scale (with 2 colors)  
full-color (with 8 colors)
Regular dithering
regular dithering
regular dithering
Error diffusion

Error diffusion
Error diffusion

* In converting color profiles of an 8-bit/channel image in Photoshop with "Use Dither" option (in the color settings) ON, converted colors will be represented with dithering because the converted RGB values ​​will not be integers. If this image is processed to increase the contrast, the image might become somewhat rough. Dithering is not used when profile conversion is applied to a 16-bit/channel image.

T. Fujiwara