format,png,file-format , What do these bytes do?


What do these bytes do?

Question:

Tag: format,png,file-format

This is the hexdump of a black 1x1 PNG made in Gimp and exported with minimal information:

89 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 0C 49 44 41 54 08 D7 63 60 60 60 00
00 00 04 00 01 27 34 27 0A 00 00 00 00 49 45 4E
44 AE 42 60 82

Now after reading the specification I am quite sure what most of them mean, except for bytes 30-34 between the IHDR and IDAT chunk: 90 77 53 DE

Can someone enlighten me?


Answer:

Those numbers are the CRC checksum for the previous chunk. See in the official specification: 5 Datastream structure for a general overview, and in particular 5.3 Chunk layout.

A CRC is calculated for, and appended to each separate chunk:

A four-byte CRC (Cyclic Redundancy Code) calculated on the preceding bytes in the chunk, including the chunk type field and chunk data fields, but not including the length field. The CRC can be used to check for corruption of the data. The CRC is always present, even for chunks containing no data.

Here is your 1x1 pixel image, annotated byte for byte. Right after the data of each of the chunks IHDR, IDAT, and IEND is a CRC for the preceding data.

File: test.png
89 50 4E 47 0D 0A 1A 0A
  Header 0x89 "PNG" CR LF ^Z LF checks out okay
===========
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
  block:  "IHDR", 13 bytes [49484452]
  Width:              1
  Height:             1
  Bit depth:          8
  Color type:         2 = Color
  (Bits per pixel: 8)
  (Bytes per pixel: 3)
  Compression method: 0
  Filter method:      0
  Interlace method:   0 (none)
  CRC: 907753DE
===========
00 00 00 0C
49 44 41 54
08 D7 63 60 60 60 00 00 00 04 00 01
27 34 27 0A
  block:  "IDAT", 12 bytes [49444154]
  expanded result: 4 (as expected)
  (Row   0 Filter:0)
  decompresses into
  00 00 00 00
  CRC: 2734270A
===========
00 00 00 00
49 45 4E 44
AE 42 60 82
  block:  "IEND", 0 bytes [49454E44]
  CRC: AE426082

The IDAT data decompresses into four 0's: the first one is the row filter (0, meaning 'none') and the next 3 bytes are Red, Green, Blue values for the one single pixel.


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