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9.3. Basic Graphics Concepts
An image is a rectangle of pixels that have various colors. Colors are identified by their position in the palette, an array of colors. Each entry in the palette has three separate color values—one for red, one for green, and one for blue. Each value ranges from 0 (this color not present) to 255 (this color at full intensity).
Image files are rarely a straightforward dump of the pixels and the palette. Instead, various file formats (GIF, JPEG, PNG, etc.) have been created that attempt to compress the data somewhat to make smaller files.
Different file formats handle image transparency , which controls whether and how the background shows through the image, in different ways. Some support an alpha channel, an extra value for every pixel reflecting the transparency at that point. Others simply designate one entry in the palette as indicating transparency.
Antialiasing is where pixels at the edge of a shape are moved or recolored to make a gradual transition between the shape and its background. This prevents the rough and jagged edges that can make for unappealing images. Some functions that draw on an image implement antialiasing.
With 256 possible values for each of red, green, and blue, there are 16,777,216 possible colors for every pixel. Some file formats limit the number of colors you can have in a palette (e.g., GIF supports no more than 256 colors); others let you have as many colors as you need. The latter are known as true color formats, because 24-bit color (8 bits for each of red, green, and blue) gives more hues than the human eye can distinguish.
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| 9.2. The GD Extension |  | 9.4. Creating and Drawing Images |
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