Fractals/png

PNG features: for png color depth can range from 1 to 64 bits per pixel All PNG images contain a single static image 1-, 2-, 4- and 8-bit palette support (like GIF) 1-, 2-, 4-, 8- and 16-bit grayscale support 8- and 16-bit-per-sample (that is, 24- and 48-bit) truecolor support full alpha transparency in 8- and 16-bit modes "palette-alpha" mode, effectively transforming normal RGB palette into RGBA gamma correction for cross-platform "brightness" control png file versions PNGN = PNG N bit_per_color ( = nr_of_channels * bits_per_channel) PNG8 has 2^8 = 256 shades of gray or 256 indexed-color with 8-bit samples, usually means opaque or with GIF-like, binary transparency, but sometimes includes translucencystackoverflow question : what-is-difference-between-png8-and-png24 ? PNG16 gray: Such images are typically used only in science, medicine, and the film industry It splits the color space evenly to shades of greyBit-Depth by Michael Reichmann December 2, 2011 Note that it may be converted down to 8-bit for display, when display devices don't support 16 bits. Any values greater than 256 will simply be clipped to 255.stackoverflow question: 16-bit-grayscale-png PNG24 has a color depth of 8 x 3 channels ( RGB) which is 24 bits per pixel color = total colours and 2^8 = 256 shades of gray = Truecolor PNG32 has a color depth of 8 x 4 channels ( RGBA) which is 32 bits per pixel color = colors = Truecolor and alpha PNG48 RGBA: has a color depth of 12 x 4 channels ( RGBA) which is 48 bits per pixel color = colors = Truecolor and alpha RGB: PNG64 has a color depth of 16 x 4 channels ( RGBA) which is 64 bits per pixel color = colors Key words sample = intersection of a channel and a pixel in an image. sample depth = number of bits used to represent a sample value. In an indexed-colour PNG image, samples are stored in the palette and thus the sample depth is always 8 by definition of the palette. In other types of PNG image it is the same as the bit depth An tagged image is an image that is explicitly assigned a color profile, as defined by the image format. This is usually done by including an International Color Consortium (ICC) profile [ICC].csswg : css-color-4 terminology File format A PNG file starts with an 8-byte signature ( file header) After the header, comes a series of chunks A chunk consists of four parts: length (4 bytes,https://books.google.com/books?id=lvMxDwAAQBAJ&pg=PT686 PoC or GTFO 9781593278984 Each chunk consists of four parts: Length, a Chunk Type, the Chunk Data, and a 32-bit CRC. The Length is a 32-bit unsigned integer indicating the size of only the Chunk Data field Laphroaig Manul 31 October 2017 big-endian), chunk type/name (4 byteshttps://books.google.com/books?id=lvMxDwAAQBAJ&pg=PT686 - Chunk Type is a 32-bit FourCC code such as IHDR, IDAT, or IEND. PoC or GTFO 9781593278984 Laphroaig Manul 31 October 2017) chunk data (length bytes) CRC (cyclic redundancy code/checksum; 4 bytes). The CRC is a network-byte-order CRC-32 computed over the chunk type and chunk data, but not the length. Length Chunk type Chunk data CRC 4 bytes 4 bytes Length bytes 4 bytes file header thumb|The PNG image 30px viewed with a hex editor application for Ubuntu. The first eight bytes of a PNG file is an 8-byte signatureW3C 2003 5.2 PNG signature ( a magic signature) with the following (decimal) values" DEC: 137 80 78 71 13 10 26 10 HEX: 89 50 4E 47 0D 0A 1A 0A ASC: NA P N G \r \n NA \n Values (hex)Purpose89Has the high bit set to detect transmission systems that do not support 8-bit data and to reduce the chance that a text file is mistakenly interpreted as a PNG, or vice versa.50 4E 47In ASCII, the letters PNG, allowing a person to identify the format easily if it is viewed in a text editor.0D 0AA DOS-style line ending (CRLF) to detect DOS-Unix line ending conversion of the data.1AA byte that stops display of the file under DOS when the command type has been used—the end-of-file character.0AA Unix-style line ending (LF) to detect Unix-DOS line ending conversion. Critical chunks IHDR must be the first chunk; it contains (in this order) the image's width (4 bytes) height (4 bytes) bit depth (1 byte) Bit depth is a single-byte integer giving the number of bits per sample or per palette index (not per pixel). Valid values are 1, 2, 4, 8, and 16, although not all values are allowed for all colour types. See the table belowPortable Network Graphics (PNG) Specification (Third Edition) W3C First Public Working Draft 25 October 2022 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).W3C 2003 11.2.2 IHDR Image header PLTE contains the palette: a list of colors. IDAT contains the image, which may be split among multiple IDAT chunks. Such splitting increases filesize slightly, 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.W3C 2003 11.2.4 IDAT Image data IEND marks the image end; the data field of the IEND chunk has 0 bytes/is empty.W3C 2003 11.2.5 IEND Image trailer + PNG color types Color type Name Binary Masks decimal value binary value   A C P 0 Grayscale 000000002 2 Truecolor 0010 color20102 3 Indexed 0011 color, palette30112 4 Grayscale and alpha 0100 alpha41002 6 Truecolor and alpha 0 1 1 0 alpha, color61102 + Allowed combinations of colour type and bit depth Color type Channels Allowed bits per channel 1 2 4 8 16 Indexed 1 1 2 4 8 Grayscale 1 1 2 4 8 16 Grayscale and alpha 2 16 32 Truecolor 3 24 48 Truecolor and alpha 4 32 64 Ancillary chunks bKGD 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). cHRM gives the chromaticity coordinates of the display primaries and white point. dSIG is for storing digital signatures.Thomas Kopp 17 April 2008 PNG Digital Signatures: Extension Specification http://libpng.download/documents/signatures/ eXIf stores Exif metadata.http://ftp-osl.osuosl.org/pub/libpng/documents/pngext-1.5.0.html Extensions to the PNG 1.2 Specification, version 1.5.0 ftp-osl.osuosl.org gAMA specifies 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.W3C 2003 11.3.3.2 gAMA Image gamma hIST can store the histogram, or total amount of each color in the image. iCCP is an ICC color profilew3.org png specyfication iCCP iTXt contains a keyword and UTF-8 text, with encodings for possible compression and translations marked with language tag. The Extensible Metadata Platform (XMP) uses this chunk with a keyword 'XML:com.adobe.xmp' pHYs 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.W3C 2003 11.3.5.3 pHYs Physical pixel dimensions sBIT (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.W3C 2003 11.3.3.4 sBIT Significant bitshttps://www.w3.org/TR/PNG-Chunks.html PNG (Portable Network Graphics) Specification \ Version 1.0 w3.org 30 May 2022 4.2.6. sBIT Significant bits, 13 bytes total - color type 2 and 3 totaled 6 bytesRoelofs 2003 Significant Bits (sBIT) "Grayscale images are the simplest; sBIT then contains a single byte indicating the number of significant bits in the source data" sPLT suggests a palette to use if the full range of colors is unavailable. sRGB indicates that the standard sRGB color space is used; the sRGB chunk contains only 1 byte, which is used for "rendering intent" (4 values—0, 1, 2, and 3—are defined for rendering intent).http://www.libpng.org/pub/png/spec/1.2/PNG-Chunks.html#C.sRGB PNG Specification: Chunk Specifications sTER stereo-image indicator chunk for stereoscopic images.PNG News from 2006 http://www.libpng.org/pub/png/png2006.html Libpng.org tEXt can store text that can be represented in ISO/IEC 8859-1, with one key-value pair for each chunk. The "key" must be between 1 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. tIME stores the time that the image was last changed. tRNS 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. zTXt contains compressed text (and a compression method marker) with the same limits as tEXt. Rendering intent values (Value Name Description) 0 = Perceptual for images preferring good adaptation to the output device gamut at the expense of colorimetric accuracy, such as photographs. 1 = Relative colorimetric for images requiring colour appearance matching (relative to the output device white point), such as logos. 2 = Saturation for images preferring preservation of saturation at the expense of hue and lightness, such as charts and graphs. 3 = Absolute colorimetric for images requiring preservation of absolute colorimetry, such as previews of images destined for a different output device (proofs). Examples + Structure of a very simple PNG file 89 50 4E 47 0D 0A 1A 0APNG signature IHDRImage header IDATImage data IENDImage end + Contents of a minimal PNG file representing one red pixelHexAs characters89 50 4E 47 0D 0A 1A 0A 00 00 00 0D 49 48 44 52 00 00 00 01 00 00 00 01 08 02 00 00 00 90 77 53 DE 00 00 00 10 49 44 41 54 08 D7 63 F8 CF C0 00 00 03 01 01 00 18 DD 8D B0 00 00 00 00 49 45 4E 44 AE 42 60 82.PNG........IHDR ..............wS .....IDAT..c.... .............IEN D.B`. +IHDR ChunkOffset into chunkHex ValueDecimal ValueTextMeaning00x0D13IHDR chunk has 13 bytes of content40x49484452IHDRIdentifies a Header chunk80x011Image is 1 pixel wide120x011Image is 1 pixel high160x0888 bits per pixel (per channel)170x022Color type 2 (RGB/truecolor)180x000Compression method 0 (only accepted value)190x000Filter method 0 (only accepted value)200x000Not interlaced210x907753DECRC of chunk's type and content (but not length) +IDAT ChunkOffset into chunkHex ValueMeaning00x10IDAT chunk has 16 bytes of content40x49444154Identifies a Data chunk80x08DEFLATE compression method using a 256-byte windowrfc1950 https://datatracker.ietf.org/doc/html/rfc1950#page-4 2021-08-18 datatracker.ietf.org May 1996 Deutsch L. Peter Gailly Jean-Loup90xD7ZLIB FCHECK value, no dictionary used, maximum compression algorithm100x63F8CFC00000A compressed DEFLATE block using the static Huffman code that decodes to 0x00 0xFF 0x00 0x00rfc1951 https://datatracker.ietf.org/doc/html/rfc1951#page-3 2021-08-18 datatracker.ietf.org May 1996 Deutsch L. Peter160x03010100The ZLIB check value: the Adler32 checksum of the uncompressed data200x18DD8DB0CRC of chunk's type and content (but not length) Displayed in the fashion of hex editors, with on the left side byte values shown in hex format, and on the right side their equivalent characters from :w:ISO-8859-1 with unrecognized and control characters replaced with periods. Additionally the PNG signature and individual chunks are marked with colors. Note they are easy to identify because of their human readable type names (in this example PNG, IHDR, IDAT, and IEND). PNG Signature(8 bytes): 89 50 4E 47 0D 0A 1A 0A =======Chunk start======= IHDR Chunk: IHDR chunk length(4 bytes): 00 00 00 0D IHDR chunk type(Identifies chunk type to be IHDR): 49 48 44 52 Image width in pixels(variable 4): xx xx xx xx Image height in pixels(variable 4): xx xx xx xx Flags in the chunk(variable 5 bytes): xx xx xx xx xx CRC checksum(variable 4 bytes): xx xx xx xx =======Chunk end======= read png Read png with od program od -c test.png | head -4 Example output: 0000000 211 P N G \r \n 032 \n \0 \0 \0 004 C g B I 0000020 P \0 002 + 325 263 177 \0 \0 \0 \r I H D R 0000040 \0 \0 \0 ` \0 \0 \0 ` \b 006 \0 \0 \0 342 230 w 0000060 8 \0 \0 \0 c H R M \0 \0 z % \0 \0 200 Programs: pngcheck a command-line program by Greg Roelofs pngcheck -c *.png pngcheck -cvt *.png code ImageMagic creating png in polish wikibooks rpng is a simple and easy-to-use C library to manage png chunks by raysan5 octave Octave code that creates array ( memory image) and saves it to the fileSigel disc in Matlab by Chris King # octave m-file based on the m-file by Chris King # http://www.dhushara.com/DarkHeart/Viewers/source/siegel.m # an MyImage here is a matrix for 24 bit (3 byte) colors # load packages pkg load image; # imwrite pkg load miscellaneous; # waitbar # constan values nx = 480; ny = 480; MyImage = zeros(ny,nx,3); # 2D matrix filled with zeros magc=0.65; dSide=1/magc; Zxmin = -dSide; Zxmax = dSide; Zymin = -dSide; Zymax = dSide; stepy = (Zymax - Zymin)/(ny - 1); stepx = (Zxmax - Zxmin)/(nx - 1); # computations waitbar(0,'Please wait...'); # info for iy = 1:ny Zy = Zymax - iy*stepy; # invert y axis for ix= 1:nx Zx = Zxmin + ix*stepx; if(Zy>0 && Zx>0) # first quadrant should be in upper right position MyImage(iy,ix,2)=255-MyImage(iy,ix,2); endif; end # for ix waitbar(iy/ny); end # for iy # image(MyImage); # display image imwrite(MyImage,'s.png' ); # save image to the file collection of images PngSuite created by Willem van Schaik The "official" test-suite for PNG Rich Franzen's PNG Gallery 16-bit integer ProPhotoRGB png : saturated-colors.png Creating png files Image Magic Checking png files file p.png ICC embaded profile PNG-ICC-tests results Editing png files hex editors: editing of pixel data in a hex editor is pretty much impossible because of of the compression algorithmHacking Image Files – With a Hex Editor psychocod3r April 15, 2019 one can edit header, use an online crc calculator to calculate the checksum for the png use TS/JSMathilde E. Sep 8, 2020 : Understanding and decoding PNGs With an example in TypeScript pngcheck Hexdump hexdump --canonical p.png To show the output of the PNG header as described by the official libpng specHow Hexdump works. Hexdump helps you investigate the contents of binary files. Learn how hexdump works. By Seth Kenlon (Team, Red Hat) August 12, 2019 to process the PNG file in 8-byte chunks the PNG spec is documented in decimal, which is represented by %d according to the hexdump documentation make the output perfect by adding a blank space after each integer hexdump -n8 -e'8/1 "%d ""\n"' p.png ICC profile tasksPNG-ICC-tests Extract ICC profile from the file To remove an ICC profile To embed an ICC prolile Extract ICC profile from the file: exiftool -icc_profile -b -w icc somefile.png To remove an ICC profile pngcrush -rem iCCP foo.png result.png See also png-embedded-malicious-payload-hidden-in-a-png-file by THIAGO MARQUES png compatybility pngquant References