PNG Graphic Formats
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PNG
Portable Network Graphics (PNG) is a bitmapped image format
that employs lossless data compression. PNG was created to
improve upon and replace GIF (Graphics Interchange Format)
as an image-file format not requiring a patent license.
PNG images can either use palette-indexed color or be made
up of one or more channels (numerical values directly representing
quantities about the pixels). When there is more than one
channel in an image all channels have the same number of bits
allocated per pixel (known as the bit depth of the channel).
Although the PNG specification always talks about the bit
depth of channels, most software and users generally talk
about the total number of bits per pixel (sometimes also referred
to as bit depth or color depth). Since multiple channels can
affect a single pixel, the number of bits per pixel is often
higher than the number of bits per channel.
PNG offers a variety of transparency options. With truecolor
and greyscale images either a single pixel value can be declared
as transparent or an alpha channel can be added. For paletted
images, alpha values can be added to palette entries. The
number of such values stored may be less than the total number
of palette entries, in which case the remaining entries are
considered fully opaque.
PNG uses a non-patented lossless data compression method
known as DEFLATE, which is the same algorithm used in the
zlib compression library. This method is combined with prediction,
where for each image line, a filter method is chosen that
predicts the color of each pixel based on the colors of previous
pixels and subtracts the predicted color of the pixel from
the actual color.
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