Alpha Channel

From Mographwiki.net

Definition

An alpha channel is a type of image data that defines the transparent areas of an image, and the degree to which they are transparent. They can be used to cut images into irregular shapes (generally known as 'masking'), and to evenly or unevenly decrease the overall opacity.

Explicit Alpha channels can be made using the pen tool, a brush to paint a mask, magic wand, selections based on luminance or chrominance in a variety of programs, one of the most commonly used being Adobe Photoshop. A practical application of chrominance and luminance based Alpha channel creation is known as Keying.

Alpha channels use greyscale values (in a 32 Bit image there are 256 discrete shades of grey available) to define transparency or opacity. Since there is no real transparency in computational graphics, the per-pixel brightness values of the Alpha channel define the amount of blending that the corresponding RGB pixels will undergo with any underlying images. There are no standardized rules governing how software applications handle this, but usually a white pixel in an Alpha channel is interpreted as fully opaque. This means the foreground fully blocks out the background pixels. Likewise, a black Alpha channel means that all background pixels are copied to the foreground (in technical terms: copy over). Shades of grey inbetween will be blended with corresponding percentages, so an Alpha channel value of 50% medium grey would mean that 50% of the foreground and 50% of the background pixel values are mixed.

Alpha Channels are sometimes refered to as Hicons, Luminance Mattes, Alpha Mattes or simply, Mattes.

When creating Alpha Channels for use in an Avid System, values are reversed; black is solid, white is transparent.

Tutorials

Create a Transparent Image with Photoshop

Examples