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An alpha channel passes on information about picture elements (pixels) in a color display on monitors. In addition to the red, green and blue (RGB) channels, one byte per pixel is required for the further display of colors. This allows 256 shades of gray to be reproduced, which determine the opacity or visibility of a pixel. An alpha channel file is a fourth channel that provides additional information about the transparency and opacity of the RGB color channels. While RGB images are entirely opaque, an alpha channel file can regulate the transparency of pixels. RGB images that have a fourth alpha channel are denoted by the abbreviation RGBA (Red, Green, Blue, and Alpha).
Applications for alpha channels are often found in direct digital printing. They enable even more varied effects in web design, which can only be realized to a limited extent when printing on colored textiles.
Alpha channels are a further development of the matte paintings that come from analogue film technology. These are elements that are attached to glass or background images as rear projection. Digitization resulted in digital matte paintings. Alpha channels gradually superseded their use.
Images usually consist of a pixel graphic of RGB colors, each with an 8-bit channel and together form a color depth of 256 bits. An additional bit per pixel is added using an alpha channel, which has the same color depth as the RGB color channel. It forms 256 levels in an 8-bit image, and its value is represented as a percentage, an integer or a real value. PNG and TIFF graphics formats support 8-bit alpha channels, while GIF file formats only support minimal one-bit alpha channels.
Alpha channels can be saved in three different ways. When a channel is saved as a grayscale file, it is referred to as a separate or external alpha channel file. If the image file is opened, the alpha channel file must also be imported in order to achieve complete and correctly displayed transparency of the file. If the alpha channel file is not saved externally and is already integrated into the image file with its own channel, then it is an integrated alpha channel file (“premultiplied alpha”). The straight alpha channel file (“Straight Alpha”) is a separate channel that contains all the transparency information. It is saved in addition to the three color channels. In contrast to the integrated alpha channel file, the information of the color channels is not directly influenced by the alpha channel file.
If several elements are put together to form an image file through compositing, this must have an alpha channel that is set in an image editing program. The image selection can be saved with background and format as an alpha channel file.
