Photoshop
Channels and Masks
Channels
In Photoshop, an image's colour information is stored in one or more channels. The number of colour channels is dependent on the image's colour mode. Alpha channels are another type of channel that can be created and used to load masks and selections into images. Each Photoshop file can contain up to 24 colour and alpha channels.
Colour Channels
When you open a new image file in RGB colour made, four colour information channels are created automatically. These can be viewed in the Channels panel, as shown here. There is a greyscale channel for each of the red, green, and blue colour components and there's one composite channel called RGB which is in colour. The composite channel is the one you see and edit in the image.
Images in other colour modes work in much the same way. A CMYK image has four greyscale channels for each of its four colour components plus a composite colour channel called CMYK. An image in Greyscale mode has only one channel that is called Grey. A Bitmap image has one channel called Bitmap.
Alpha Channels
Selections can be saved in alpha channels. An alpha channel is an 8-bit greyscale channel. A selection of the cat in Cat.psd saved as an alpha channel would appear like the example shown here. The areas within the selection border are stored as white values in the channel. The unselected areas are black. Partially selected (semi-transparent) pixels will appear as grey values.
A selection or mask can be loaded into an image based on the information stored in an alpha channel. Alpha channels can be edited, duplicated or deleted.
Channels and File Size
Each alpha channel in an image adds to its file size. This example shows the bottom of a document window that contains one layer and one alpha channel. The layer information alone is 754K. With the alpha channel added in, the file size became 960K. If the alpha channel is duplicated, the file size becomes 1.14M. Additional layers with pixel information will also add to the file size. So the number on the left indicates what the file size would be if the image was flattened into a single layer with no alpha channels.
Channels Summary
- Photoshop stores colour and selection information in greyscale channels.
- When working in RGB mode, there will be a Red channel, Green channel, Blue Channel and a composite channel called RGB in which image editing is usually done.
- Selections can be saved to alpha channels.
- An alpha channel is an 8-bit greyscale channel.
- In an alpha channel white areas represent fully selected areas, black represents unselected areas and grey represents a degree of partial selection or transparency.
- Alpha channels will add to the file size of a .psd file.
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