PNG (Portable Network Graphics) Specification, Version 0.90
Revision date: 3 November, 1995
Previous page
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.
Back to PNG table of contents
Next page