Editing PNG (section)

==== Ancillary chunks ====
Other image attributes that can be stored in PNG files include [[gamma correction|gamma]] values, background color, and textual [[metadata]] information. PNG also supports [[color management]] through the inclusion of [[ICC profile|ICC color profiles]].{{Sfn|W3C|2003|loc=[https://www.w3.org/TR/PNG/#11iCCP 11.3.3.3 <code>iCCP</code> Embedded ICC profile]}}

* <code>bKGD</code> gives the default background color. It is intended for use when there is no better choice available, such as in standalone image viewers (but not web browsers; see below for more details).
* <code>cHRM</code> gives the [[chromaticity]] coordinates of the display [[Primary color|primaries]] and [[white point]].
* <code>cICP</code> specifies the color space, transfer function and matrix coefficients as defined in ITU-T [[H.273]].<ref>{{Cite web |date=2023-09-21 |title=PNG Specification (Third Edition), cICP Coding-independent code points for video signal type identification |url=https://www.w3.org/TR/png/#cICP-chunk |website=w3.org}}</ref> It is intended for use with [[High-dynamic-range television|HDR imagery]] without requiring a color profile.<ref>{{Cite web |date=2023-05-03 |title=Adding support for HDR imagery to the PNG format |url=https://github.com/w3c/ColorWeb-CG/blob/08d0981ae5163b76275d0777d0166eab85396371/hdr-in-png-requirements.md |publisher=W3C Color on the Web Community Group}}</ref>
* <code>dSIG</code> is for storing digital signatures.<ref>{{Cite web|last=Thomas Kopp|date=17 April 2008|title=PNG Digital Signatures: Extension Specification|url=http://libpng.download/documents/signatures/}}</ref>
* <code>eXIf</code> stores [[Exif]] metadata.<ref>{{Cite web|url=http://ftp-osl.osuosl.org/pub/libpng/documents/pngext-1.5.0.html|title=Extensions to the PNG 1.2 Specification, version 1.5.0|website=ftp-osl.osuosl.org}}</ref>
* <code>gAMA</code> specifies [[Gamma correction|gamma]]. The gAMA chunk contains only 4 bytes, and its value represents the gamma value multiplied by 100,000; for example, the gamma value 1/3.4 calculates to 29411.7647059 ((1/3.4)*(100,000)) and is converted to an integer (29412) for storage.{{Sfn|W3C|2003|loc=[https://www.w3.org/TR/PNG/#11gAMA 11.3.3.2 <code>gAMA</code> Image gamma]}}
* <code>hIST</code> can store the histogram, or total amount of each color in the image.
* <code>iCCP</code> is an [[ICC color profile]].
* <code>iTXt</code> contains a keyword and [[UTF-8]] text, with encodings for possible compression and translations marked with [[IETF language tag|language tag]]. The [[Extensible Metadata Platform]] (XMP) uses this chunk with a keyword 'XML:com.adobe.xmp'
* <code>pHYs</code> holds the intended pixel size (or pixel aspect ratio); the pHYs contains "Pixels per unit, X axis" (4 bytes), "Pixels per unit, Y axis" (4 bytes), and "Unit specifier" (1 byte) for a total of 9 bytes.{{Sfn|W3C|2003|loc=[https://www.w3.org/TR/PNG/#11pHYs 11.3.5.3 <code>pHYs</code> Physical pixel dimensions]}}
* <code>sBIT</code> (significant bits) indicates the color-accuracy of the source data; this chunk contains a total of between 1 and 5 bytes, depending on the color type.{{Sfn|W3C|2003|loc=[https://www.w3.org/TR/PNG/#11sBIT 11.3.3.4 <code>sBIT</code> Significant bits]}}<ref>{{cite web |url=https://www.w3.org/TR/PNG-Chunks.html |title=PNG (Portable Network Graphics) Specification \ Version 1.0 |website=w3.org |access-date=30 May 2022}} 4.2.6. sBIT Significant bits, 13 bytes total - color type 2 and 3 totaled 6 bytes</ref>{{Sfn|Roelofs|2003|loc=[http://libpng.org/pub/png/book/chapter11.html#png.ch11.div.7 Significant Bits (sBIT)]|postscript="Grayscale images are the simplest; sBIT then contains a single byte indicating the number of significant bits in the source data"}}
* <code>sPLT</code> suggests a palette to use if the full range of colors is unavailable.
* <code>sRGB</code> indicates that the standard [[sRGB color space]] is used; the sRGB chunk contains only 1 byte, which is used for "rendering intent" (4 values&mdash;0, 1, 2, and 3&mdash;are defined for rendering intent).<ref>{{Cite web|url=http://libpng.org/pub/png/spec/1.2/PNG-Chunks.html#C.sRGB|title = PNG Specification: Chunk Specifications}}</ref>
* <code>sTER</code> stereo-image indicator chunk for [[stereoscopic]] images.<ref>{{Cite web|title=PNG News from 2006|url=http://libpng.org/pub/png/png2006.html|publisher=Libpng.org}}</ref>
* <code>tEXt</code> can store text that can be represented in [[ISO/IEC 8859-1]], with one [[Attribute–value pair|key-value]] pair for each chunk. The "key" must be between one and 79 characters long. Separator is a null character. The "value" can be any length, including zero up to the maximum permissible chunk size minus the length of the keyword and separator. Neither "key" nor "value" can contain null character. Leading or trailing spaces are also disallowed.
* <code>tIME</code> stores the time that the image was last changed.
* <code>tRNS</code> contains transparency information. For indexed images, it stores alpha channel values for one or more palette entries. For truecolor and grayscale images, it stores a single pixel value that is to be regarded as fully transparent.
* <code>zTXt</code> contains compressed text (and a compression method marker) with the same limits as <code>tEXt</code>.

The lowercase first letter in these chunks indicates that they are not needed for the PNG specification. The lowercase last letter in some chunks indicates that they are safe to copy, even if the application concerned does not understand them.