PNG

Chunk layout

first 8 bytes of a PNG

    89 50 4e 47 0d 0a 1a 0a

1. 4 bytes for data size
2. 4 bytes for type code
3. data
4. 4 byte CRC

for type code the 5th bit of each byte -> upper/lower

1. Ancillary bit : 0 (uppercase) = critical, 1 (lowercase) = ancillary.
2. Private bit : 0 (uppercase) = public, 1 (lowercase) = private.
3. Reserved bit : Must be 0 (uppercase) in files conforming to this version of PNG.
4. Safe-to-copy bit : 0 (uppercase) = unsafe to copy, 1 (lowercase) = safe to copy. (1, 2, 3, 4)

CRC algorithm

Chunk Specifications

Critical chunks

IHDR Image header

The IHDR chunk must appear FIRST. It contains(bytes):

1. Width : 4
2. Height : 4
3. Bit depth : 1
4. Color type : 1
5. Compression method : 1
6. Filter method : 1
7. Interlace method : 1

PLTE Palette 调色板

IDAT Image data

IDAT

解码: 先LZ77 每一行第一位定义filter type,

LZ77 uncompress后总共有height*width*pixelSize + height bytes

执行filter Algorithm

An additional "filter-type" byte is added to the beginning of every scanline (see Filtering). The filter-type byte is not considered part of the image data, but it is included in the datastream sent to the compression step.

IEND Image trailer

end of image 00 00 00 00 49 45 4e 44 ae 42 60 82

Ancillary chunks

Transparency information

Color space information

gAMA Image gamma

cHRM Primary chromaticities

sRGB Standard RGB color space

iCCP Embedded ICC profile

Textual information

tEXt Textual data

zTXt Compressed textual data

iTXt International textual data

Miscellaneous information

bKGD Background color

pHYs Physical pixel dimensions

sBIT Significant bits

sPLT Suggested palette

hIST Palette histogram

tIME Image last-modification time

Filter Algorithms

Filtering algorithms are applied to bytes,

filter types

• 0 None
• 1 Sub
• 2 Up
• 3 Average
• 4 Paeth

Chunk Ordering Rules

for decoder suggest An un-known chunk type is not to be treated as an error unless it is a critical chunk.