Channels and filters

In a colour system every colour splits into components: red, green and blue in RGB; cyan, magenta, yellow and black in CMYK. Each component of a picture is stored in its own channel as a grayscale image: an RGB picture has three colour channels and a CMYK picture has four; the Channels palette (Window → Channels) also shows their combined view at the top. Clicking the eye next to a channel shows or hides it; in Windows Ctrl+3, Ctrl+4 and Ctrl+5 show the red, green and blue channels and Ctrl+2 shows the combined picture. A red apple looks light in the red channel and dark in the blue channel, because it has little blue light. To see CMYK, choose Image → Mode → CMYK Color. Besides colour channels there is an alpha channel: it stores a selection (white – selected, black – not) or transparency; Select → Save Selection writes a selection into a channel. A filter is processing of a picture by a predefined algorithm. For example, Gaussian Blur blurs the picture by replacing each pixel with a weighted average of its neighbours (nearer neighbours count more); Invert subtracts each channel value from 255. Filters are grouped in the Filter menu (Blur, Sharpen, Noise, Stylize, Distort and others); Ctrl+F applies the last filter again, and if the result is unwanted, Ctrl+Z or the History palette helps. A filter applied to a selection changes only part of the picture. When you work with AI filters and programs that alter a person's face, do not hide that the picture was changed, and do not alter someone's picture without permission.