REC-png.html
PNG (Portable Network Graphics) Specification
Version 1.0
W3C Recommendation 01-October-1996
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- a^b
- Exponentiation; a raised to the power b.
C programmers should be careful not to misread this notation as
exclusive-or. Note that in gamma-related calculations, zero raised
to any power is valid and must give a zero result.
- Alpha
- A value representing a pixel's degree of transparency. The
more transparent a pixel, the less it hides the background against
which the image is presented. In PNG, alpha is really 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 that provides additional information. A decoder can
still produce a meaningful image, though not necessarily the best
possible image, without processing the chunk.
- Bit depth
- The number of bits per palette index (in indexed-color PNGs) or
per sample (in other color types). This is the same value that appears
in IHDR.
- 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.
- Chromaticity
- A pair of values x,y that precisely specify the hue,
though not the absolute brightness, of a perceived color.
- 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 are determined by
the type name.
- Composite
- As a verb, to form an image by merging a foreground image and
a background image, using transparency information to determine where
the background should be visible. The foreground image is said to be
"composited against" the background.
- 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.
- Critical chunk
- A chunk that must be understood and processed by the decoder
in order to produce a meaningful image from a PNG file.
- CRT
- Cathode Ray Tube: a common type of computer display hardware.
- 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 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, pkzip, and other compression programs.
Deflate is a member of the LZ77 family of compression methods.
- Filter
- A transformation applied to image data in hopes of improving
its compressibility. PNG uses only lossless (reversible) filter
algorithms.
- Frame buffer
- The final digital storage area for the image shown by
a computer display. Software causes an image to appear onscreen
by loading it into the frame buffer.
- Gamma
-
The brightness of mid-level tones in an image. More precisely,
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 scaled to the range 0 to 1.
- 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.
- Lossless compression
- Any method of data compression that guarantees the original data
can be reconstructed exactly, bit-for-bit.
- Lossy compression
- Any method of data compression that reconstructs the original
data approximately, rather than exactly.
- LSB
- Least Significant Byte of a multi-byte value.
- Luminance
- Perceived brightness, or grayscale level, of a color.
Luminance and chromaticity together fully define a perceived color.
- LUT
- Look Up Table.
In general, a table used to transform data. In frame buffer hardware,
a LUT can 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
- The information stored for a single grid point in
the image. The complete image is a rectangular array of pixels.
- PNG editor
- A program that modifies a PNG file and preserves ancillary
information, including chunks that it does not recognize. Such a
program must obey the rules given in
Chunk Ordering Rules.
- Sample
- A single number in the image data; for example, the
red value of a pixel. A pixel is composed of one or more samples.
When discussing physical data layout (in particular, in
Image layout), we use "sample" to mean
a number stored in the image array. It would be more precise but
much less readable to say "sample or palette index" in that context.
Elsewhere in the specification, "sample" means a color value or alpha value.
In the indexed-color case, these are palette entries not palette indexes.
- Sample depth
- The precision, in bits, of color values and alpha values. In
indexed-color PNGs the sample depth is always 8 by definition of
the PLTE chunk. In other color types it is the same as
the bit depth.
- 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.
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