PNG: Chunk by Chunk

The PNG specification defines 18 chunk types. This document is intended to help users who are interested in a particular PNG chunk type. If you have a particular PNG chunk type in mind, you can look here to see what support PurePNG provides for it.

Critical Chunks

IHDR

Generated automatically by PurePNG. The IHDR chunk specifies image size, colour model, bit depth, and interlacing. All possible (valid) combinations can be produced with suitable arguments to the png.Writer class.

PLTE

Correctly handled when a PNG image is read. Can be generated for a colour type 3 image by using the palette argument to the png.Writer class. PNG images with colour types other than 3 can also have a PLTE chunk (a suggested palette); it is not currently possible to add a PLTE chunk for these images using PyPNG.

IDAT

Generated automatically from the pixel data presented to PurePNG. Multiple IDAT chunks (of bounded size) can be generated by using chunk_limit argument to the png.Writer class.

IEND

Generated automatically.

Ancillary Chunks

tRNS

Generated for most colour types when the transparent argument is supplied to the png.Writer to specify a transparent colour. For colour type 3, colour mapped images, a tRNS chunk will be generated automatically from the palette argument when a palette with alpha (opacity) values is supplied.

cHRM

When reading a PNG image the cHRM chunk is converted to a tuples white_point (2-tuple of floating point values) and rgb_points (3-tuple of 2-tuple of floating point) in the info dictionary. When writing, white_point and rgb_points arguments to the png.Writer class or calling apropriate set_ methods generate a cHRM chunk (only both, single will be ignored).

gAMA

When reading a PNG image the gAMA chunk is converted to a floating point gamma value; this value is returned in the info dictionary: info['gamma']. When writing, the gamma argument to the png.Writer class will generate a gAMA chunk.

iCCP

When reading a PNG image the iCCP chunk is saved as raw bytes and name. These data returned in the info dictionary: info['icc_profile'], info['icc_profile_name']. When writing, the icc_profile argument to the png.Writer class will generate a iCCP chunk, with name supplied in icc_profile_name argument or “ICC Profile” as default.

sBIT

When reading a PNG image the sBIT chunk will make PyPNG rescale the pixel values so that they all have the width implied by the sBIT chunk. It is possible for a PNG image to have an sBIT chunk that specifies 3 different values for the significant bits in each of the 3 colour channels. In this case PyPNG only uses the largest value. When writing a PNG image, an sBIT chunk will be generated if need according to the bitdepth argument specified. Values other than 1, 2, 4, 8, or 16 will generate an sBIT chunk, as will values less than 8 for images with more than one plane.

sRGB

When reading a PNG image the sRGB chunk is read to an integer value; this value is returned in the info dictionary: info['rendering_intent'] and can be compared to values like png.PERCEPTUAL. When writing, the rendering_intent argument to the png.Writer class will generate a sRGB chunk.

tEXt

When reading a PNG image the tEXt chunks are converted to a dictionary of keywords and unicode values in the info dictionary: info['text']. When writing, the text argument with same dict to the png.Writer class or arguments with registered keywords names will generate tEXt chunks.

zTXt

When reading a PNG image the zTXt chunks are converted to a dictionary of keywords and unicode values in the info dictionary: info['text']. It’s not possible to write zTXt chunsk for now, only tEXt will be written with text keyword.

iTXt

When reading append to text info same as tEXt or zTXt, translated keyword and language tags ignored.

Keywords within text that does not fit latin-1 will be saved as iTXt

bKGD

When a PNG image is read, a bKGD chunk will add the background key to the info dictionary. When writing a PNG image, a bKGD chunk will be generated when the background argument is used.

hIST

Ignored when reading. Not generated.

pHYs

When reading a PNG image the pHYs chunk is converted to form ((<pixel_per_unit_x>, <pixel_per_unit_y>), <unit_is_meter>) This tuple is returned in the info dictionary: info['resolution']. When writing, the resolution argument to the png.Writer class will generate a pHYs chunk. Argument could be tuple same as reading result, but also possible some usability modificatuion:

  • if both resolutions are same it could be written as single number instead of tuple: (<pixel_per_unit_x>, <unit_is_meter>)

  • all three parameters could be written in row: (<pixel_per_unit_x>, <pixel_per_unit_y>, <unit_is_meter>)

  • instead of <unit_is_meter> bool it’s possible to use some unit specification:
    1. omit this part if no unit specified ((<pixel_per_unit_x>, <pixel_per_unit_y>), )
    2. use text name of unit (300, ‘i’) ‘i’, ‘cm’ and ‘m’ supported for now.

sPLT

Ignored when reading. Not generated.

tIME

When reading generate last_mod_time tuple which is time.structtime compatible.

png.Writer have method png.Writer.set_modification_time() which could be used to specify tIME value or indicate that it should be calculated as file writing time.

PNG Extensions Chunks

See ftp://ftp.simplesystems.org/pub/png/documents/pngextensions.html

``oFFs ``

Ignored when reading. Not generated.

pCAL

Ignored when reading. Not generated.

sCAL

Ignored when reading. Not generated.

gIFg

Ignored when reading. Not generated.

gIFx

Ignored when reading. Not generated.

sTER

Ignored when reading. Not generated.

dSIG

Ignored when reading. Not generated.

fRAc

Ignored when reading. Not generated.

gIFt

Ignored when reading. Not generated.

Non-standard Chunks

Generally it is not possible to generate PNG images with any other chunk types. When reading a PNG image, processing it using the chunk interface, png.Reader.chunks, will allow any chunk to be processed (by user code).