PNG (Portable Network Graphics) Specification, Version 0.92

Revision date: 26 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.
Ancillary chunk
A chunk which provides additional information. A decoder can still produce a meaningful image, though not necessarily the best possible image, without processing the chunk.
Byte
Eight bits; also called an octet.
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.
Chromaticity
A pair of values x,y that precisely specify the hue, though not the absolute brightness, of a perceived color.
CRC
Cyclic Redundancy Check. A CRC is a type of check value 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.
CRT
Cathode Ray Tube: a common type of computer display hardware.
Critical chunk
A chunk which must be understood and processed by the decoder in order to produce a meaningful image from a PNG file.
Datastream
A sequence of bytes. This term is used rather than "file" to describe a byte sequence that is only a portion of a file. We also use it when we wish to emphasize that a PNG image might be generated and consumed "on the fly", never appearing in a stored file at all.
Deflate
The name of the compression algorithm used in standard PNG files, as well as in zip, gzip, and other compression programs. Deflate is a member of the LZ77 family of compression methods.
Filter
A transformation applied to the image data in hopes of improving its compressibility. PNG uses only lossless (reversible) filtering algorithms.
Frame buffer
The final digital storage area for the image being shown by a computer display. Software causes an image to appear onscreen by loading it into the frame buffer.
Gamma
A parameter that describes the shape of the transfer function for one or more stages in an imaging pipeline. The transfer function is given by the expression
  output = input ^ gamma
where both input and output are normalized to a zero to one range.
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.
LSB
Least Significant Byte of a multi-byte value.
LUT
Look Up Table. In general, a table used to transform data. In frame buffer hardware, a LUT may be used to map indexed-color pixels into a selected set of truecolor values, or to perform gamma correction. In software, a LUT can be used as a fast way of implementing any one-variable mathematical function.
MSB
Most Significant Byte of a multi-byte value.
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 stored for 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.
White point
The chromaticity of a computer display's nominal white value.
zlib
A particular format for data that has been compressed using deflate-style compression. Also the name of a library implementing this method. PNG implementations need not use the zlib library, but they must conform to its format for compressed data.
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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