Glossary of "Portable Network Graphics (PNG) Specification (Second Edition)"
Term entries in the "Portable Network Graphics (PNG) Specification (Second Edition)" glossary
W3C Glossaries
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- alpha
-
From Portable Network Graphics (PNG) Specification (Second Edition) (2003-11-10)
a value representing a
pixel's degree of opacity. The more
opaque a pixel, the more it hides the background against which
the image is presented. Zero alpha represents a completely
transparent pixel, maximum alpha represents a completely opaque
pixel.
an implicit representation of transparent
pixels. If every
pixel with a specific colour or
greyscale value is fully
transparent and all other pixels are fully opaque, the
alphachannel may be
represented implicitly.
separating an
alphachannel in which every
pixel is fully
opaque; all alpha values are the maximum value.
The fact that all pixels are fully opaque is represented implicitly.
- alpha compaction
-
From Portable Network Graphics (PNG) Specification (Second Edition) (2003-11-10)
an implicit representation of transparent
pixels
. If every
pixel with a specific colour or
greyscale
value is fully
transparent and all other pixels are fully opaque, the
alpha
channel
may be
represented implicitly.
- alpha separation
-
From Portable Network Graphics (PNG) Specification (Second Edition) (2003-11-10)
separating an
alpha
channel
in which every
pixel
is fully
opaque; all alpha values are the maximum value.
The fact that all pixels are fully opaque is represented implicitly.
- alpha table
-
From Portable Network Graphics (PNG) Specification (Second Edition) (2003-11-10)
- ancillary chunk
-
From Portable Network Graphics (PNG) Specification (Second Edition) (2003-11-10)
class of
chunk
that provides additional
information. A
PNG decoder
, without processing an
ancillary chunk, can still produce a meaningful image, though not
necessarily the best possible image.
- bit depth
-
From Portable Network Graphics (PNG) Specification (Second Edition) (2003-11-10)
- byte
-
From Portable Network Graphics (PNG) Specification (Second Edition) (2003-11-10)
8 bits; also called an octet. The highest bit (value 128) of
a byte is numbered bit 7; the lowest bit (value 1) is numbered
bit 0.
- byte order
-
From Portable Network Graphics (PNG) Specification (Second Edition) (2003-11-10)
- channel
-
From Portable Network Graphics (PNG) Specification (Second Edition) (2003-11-10)
array of all per-
pixel
information of a particular kind
within a
reference image
. There are five kinds of
information: red, green, blue,
greyscale
, and
alpha
. For example the alpha
channel is the array of alpha values within a reference
image.
- chromaticity (CIE)
-
From Portable Network Graphics (PNG) Specification (Second Edition) (2003-11-10)
pair of values
x,y
that precisely specify a colour,
except for the brightness information.
- chunk
-
From Portable Network Graphics (PNG) Specification (Second Edition) (2003-11-10)
section of a
PNG datastream
. Each chunk has a chunk
type. Most chunks also include data. The format and meaning of
the data within the chunk are determined by the chunk type.
Each chunk is either a
critical chunk
or an
ancillary chunk
.
- colour type
-
From Portable Network Graphics (PNG) Specification (Second Edition) (2003-11-10)
value denoting how colour and
alpha
are specified in the
PNG image
.
Colour types are sums of the following values: 1 (
palette
used), 2
(
truecolour
used), 4 (alpha used). The
permitted values of colour type are 0, 2, 3, 4, and 6.
- composite (verb)
-
From Portable Network Graphics (PNG) Specification (Second Edition) (2003-11-10)
to form an image by merging a foreground image and a
background image, using transparency information to determine
where and to what extent the background should be visible. The
foreground image is said to be "composited against" the
background.
- critical chunk
-
From Portable Network Graphics (PNG) Specification (Second Edition) (2003-11-10)
chunk
that
shall be understood and processed by the decoder in order to
produce a meaningful image from a
PNG datastream
.
- datastream
-
From Portable Network Graphics (PNG) Specification (Second Edition) (2003-11-10)
sequence of
bytes
. This term is used rather than
"file" to describe a byte sequence that may be only a portion of
a file. It is also used to emphasize that the sequence of bytes
might be generated and consumed "on the fly", never appearing in
a stored file at all.
- deflate
-
From Portable Network Graphics (PNG) Specification (Second Edition) (2003-11-10)
name of a particular compression algorithm. This algorithm is
used, in compression mode 0, in conforming
PNG
datastreams
. Deflate is a member of the
LZ77
family of
compression methods. It is defined in
[RFC-1951]
.
- delivered image
-
From Portable Network Graphics (PNG) Specification (Second Edition) (2003-11-10)
- filter
-
From Portable Network Graphics (PNG) Specification (Second Edition) (2003-11-10)
transformation applied to an array of
scanlines
with
the aim of improving their compressibility. PNG uses only
lossless (reversible) filter algorithms.
- frame buffer
-
From Portable Network Graphics (PNG) Specification (Second Edition) (2003-11-10)
the final digital storage area for the image shown by most
types of computer display. Software causes an image to appear on
screen by loading the image into the frame buffer.
- gamma
-
From Portable Network Graphics (PNG) Specification (Second Edition) (2003-11-10)
exponent that describes approximations to certain non-linear
transfer functions encountered in image capture and reproduction.
Within this International Standard, gamma is the exponent in the
transfer function from
display_output
to
image_sample
image_sample = display_outputgamma
where both
display_output
and
image_sample
are scaled to the range 0 to 1.
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