This article describes PNG (Portable Network Graphics), an extensible file format for the lossless, portable and you can
convert graphics to html as well, well-compressed storage of raster images. PNG provides a patent-free replacement for GIF and can also replace many common uses of TIFF. Indexed-color, grayscale, and true color images are supported, plus an optional
PNG is designed to work well in online viewing applications, such as the World Wide Web, so it is fully stream able with a progressive display option. PNG is robust, providing both full file integrity checking and simple detection of common transmission errors. Also, PNG can store gamma and chromaticity data for improved color matching on heterogeneous platforms.
This International Standard specifies the PNG DataStream, and places some requirements on PNG encoders, which generate PNG data streams, PNG decoders, which interpret PNG data streams, and PNG editors, which transform one PNG DataStream into another. It does not specify the interface between an application and either a PNG encoder, decoder, or editor. The precise form in which an image is presented to an encoder or delivered by a decoder is not specified. Four kinds of image are distinguished.
The source image is the image presented to a PNG encoder.
The reference image, which only exists conceptually, is a rectangular array of rectangular pixels, all having the same width and height, and all containing the same number of unsigned integer samples, either three (red, green, blue) or four (red, green, blue, alpha). The array of all samples of a particular kind (red, green, blue, or alpha) is called a channel. Each channel has a sample depth in the range 1 to 16, which is the number of bits used by every sample in the channel. Different channels may have different sample depths. The red, green, and blue samples determine the intensities of the red, green, and blue components of the pixel's color; if they are all zero, the pixel is black, and if they all have their maximum values (2sampledepth-1), the pixel is white. The alpha sample determines a pixel's degree of opacity, where zero means fully transparent and the maximum value means fully opaque. In a three-channel reference image all pixels are fully opaque. (It is also possible for a four-channel reference image to have all pixels fully opaque; the difference is that the latter has a specific alpha sample depth, whereas the former does not.) Each horizontal row of pixels is called a scan line. Pixels are ordered from left to right within each scan line, and scan lines are ordered from top to bottom. A PNG encoder may transform the source image directly into a PNG image, but conceptually it first transforms the source image into a reference image, then transforms the reference image into a PNG image. Depending on the type of source image, the transformation from the source image to a reference image may require the loss of information. That transformation is beyond the scope of this International Standard. The reference image, however, can always be recovered exactly from a PNG data stream.
The PNG image is obtained from the reference image by a series of transformations: alpha separation, indexing, RGB merging, alpha compaction, and sample depth scaling. Five types of PNG image are defined. ions.) Although not all sample depths in the range 1 to 16 bits are explicitly supported in the PNG image, the number of significant bits in each channel of the reference image may be recorded. All channels in the PNG image have the same sample depth. A PNG encoder generates a PNG data stream from the PNG image. A PNG decoder takes the PNG data stream and recreates the PNG image.
The delivered image is constructed from the PNG image obtained by decoding a PNG data stream. No specific format is specified for the delivered image. A viewer presents an image to the user as close to the appearance of the original source image as it can achieve.
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