Editing PNG (section)

==== Critical chunks ====
A decoder must be able to interpret critical chunks to read and render a PNG file.
* <code>IHDR</code> must be the first chunk; it contains (in this order) the image's
**width (4 bytes)
**height (4 bytes)
**bit depth (1 byte, values 1, 2, 4, 8, or 16)
**color type (1 byte, values 0, 2, 3, 4, or 6)
**compression method (1 byte, value 0)
**filter method (1 byte, value 0)
**interlace method (1 byte, values 0 "no interlace" or 1 "[[Adam7]] interlace") (13 data bytes total).<ref name="w3IHDR">{{Harvnb|W3C|2003|loc=[https://www.w3.org/TR/PNG/#11IHDR 11.2.2 <code>IHDR</code> Image header]}}</ref>
As stated in the [[World Wide Web Consortium]], bit depth is defined as "the number of bits per sample or per palette index (not per pixel)".<ref name="w3IHDR"/>
* <code>PLTE</code> contains the [[Palette (computing)|palette]]: a list of colors.
* <code>IDAT</code> contains the image, which may be split among multiple IDAT chunks. Such splitting slightly increases the file size, but makes it possible to generate a PNG in a streaming manner. The IDAT chunk contains the actual image data, which is the output stream of the compression algorithm.{{Sfn|W3C|2003|loc=[https://www.w3.org/TR/PNG/#11IDAT 11.2.4 <code>IDAT</code> Image data]}}
* <code>IEND</code> marks the image end; the data field of the IEND chunk has 0 bytes/is empty.{{Sfn|W3C|2003|loc=[https://www.w3.org/TR/PNG/#11IEND 11.2.5 <code>IEND</code> Image trailer]}}
The <code>PLTE</code> chunk is essential for color type 3 (indexed color). It is optional for color types two and six (truecolor and truecolor with alpha) and it must not appear for color types 0 and 4 (grayscale and grayscale with alpha).