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Lesson 11. Alpha Channels and Masks > Defining the Alpha Channel

Defining the Alpha Channel

To understand the alpha channel, you need to understand how bitmapped graphics are colored. A complete 24-bit bitmap image has three full channels of color: an 8-bit red component, an 8-bit green component, and an 8-bit blue component. These three components define the RGB (red-green-blue) values of an image. Each pixel in the image includes a value for each of the three RGB color channels. Each RGB value can range from 0 to 255. When all three of a pixel's channels are set at 255, the pixel's color is white, because red, green, and blue are all at full strength. If only one channel is 255—blue, for example—and the rest are 0, the color is blue for that pixel because only the blue channel includes a value. Since each of the red, green, and blue components can be assigned values between 0 and 255, a wide range of colors is available—literally millions of colors.

Tip

Don't try this at home with your old cans of paint. Colors of paint and colors of light are determined in different ways. Mixing dark red, green, and blue paints will make black, not white. This is because the light from paint is a reflective light, which means that the light you see is reflected by the object that has been painted. The paint is actually absorbing the red, green, and blue, leaving black. A red paint, then, is actually absorbing all the colors except red, so that you see the reflected red. This is also known as the subtractive color method. On your computer or television screen, the light is shining at you, not reflecting. Imagine a friend standing half a mile away pointing red, green, and blue flashlights at you; you would see a white light (in theory, at least). You may have seen a demonstration of a spinning disc with red, green, and blue colors painted on it. As the disc spins, the colors blend and make white. This demonstrates the additive color method. Computers and televisions use the RGB additive method.



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