PNG (Portable Network Graphics) Specification, Version 0.90

Revision date: 3 November, 1995

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11. Glossary

This section provides definitions of some terms used in this specification.
Alpha
A value representing the degree of transparency of a pixel. The more transparent a pixel, the less it hides the background against which the image is presented. Actually, in PNG alpha is the degree of opacity: zero alpha represents a completely transparent pixel, maximum alpha represents a completely opaque pixel. But most people refer to alpha as providing transparency information, not opacity information, and we continue that custom here.
Channel
The set of all samples of the same kind within an image; for example, all the blue samples in a truecolor image. (The term "component" is also used, but not in this specification.) A sample is the intersection of a channel and a pixel.
Chunk
A section of a PNG file. Each chunk has a type indicated by its chunk type name. Most types of chunks also include some data. The format and meaning of the data within the chunk is determined by the type name.
CRC
Cyclic Redundancy Check. A CRC is a type of checksum designed to catch most transmission errors. A decoder calculates the CRC for the received data and compares it to the CRC that the encoder calculated, which is appended to the data. A mismatch indicates that the data was corrupted in transit.
Filter
A transformation applied to the image data in hopes of improving its compressibility. PNG uses only lossless (reversible) filtering algorithms.
Gamma
A parameter describing the correspondence between image sample values and actual light intensity, according to the equation
  sample value = intensity ^ gamma
where sample values and intensities are measured on a scale from 0 to 1. More generally, gamma is used to describe the behavior of a device that converts light intensities to or from numeric values; most real cameras and displays respond according to a power curve with some specific gamma value. In this case we generalize the equation to
  output = input ^ gamma
To obtain proper display of an image, it must be converted to have a gamma which is the inverse of the gamma of the device the image will be displayed on. This gives a linear correlation between original light intensity and final display output.
Grayscale
An image representation in which each pixel is represented by a single sample value representing overall luminance (on a scale from black to white). PNG also permits an alpha sample to be stored for each pixel of a grayscale image.
Indexed color
An image representation in which each pixel is represented by a single sample that is an index into a palette or lookup table. The selected palette entry defines the actual color of the pixel.
Palette
The set of colors available in an indexed-color image. In PNG, a palette is an array of colors defined by red, green, and blue samples. (Alpha values can also be defined for palette entries, via the tRNS chunk.)
Pixel
A pixel is the information representing a single grid point in the image; the complete image is a two-dimensional array of pixels.
Sample
A sample is a single number in the image data; for example, the red value of a pixel. A pixel is composed of one or more samples. We use "sample" for both color values and the palette index values of an indexed-color image.
Scanline
One horizontal row of pixels within an image.
Truecolor
An image representation in which pixel colors are defined by storing three samples for each pixel, representing red, green, and blue intensities respectively. PNG also permits an alpha sample to be stored for each pixel of a truecolor image.
x^y
Exponentiation; x raised to the power y. C programmers should be careful not to misread this notation as exclusive-or. Note that in gamma correction calculations, zero raised to any power is valid and should give a zero result.

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