The alpha channel is a color component of RGBA color model. While R (red), G(green) and B(blue) components represent the color of a pixel, the alpha channel represents the degree of transparency (or opacity) of the color. It is used to determine how a pixel is rendered when blended with another.
When a color (source) is blended with another color (background), e.g., when an image is overlaid onto another image, the alpha value of the source color is used to determine the resulting color. If the alpha value is opaque, the source color overwrites the destination color; if transparent, the source color is invisible, allowing the background color to show through. If the value is in between, the resulting color has a varying degree of transparency/opacity, which creates a translucent effect.
The alpha channel is used primarily in alpha blending and alpha compositing.
see: https://www.techopedia.com/definition/1945/alpha-channel
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