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 PyPNG provides for it.
Critical Chunks¶
IHDR¶
Generated automatically by PyPNG. The IHDR chunk specifies image
size, colour model, bit depth, and interlacing.
PyPNG supports all valid combinations, except for interlacing.
The PNG spec has only 2 values for interlacing and has done for
years: no interlacing, and Adam7.
PyPNG supports either option when reading a PNG file, but
when writing only supports no interlacing.
Versions released prior to 2024 did support interlaced writing,
if you want to poke about in the archives.
PLTE¶
Correctly handled when a PNG image is read.
For writing, use the palette argument to the
png.Writer() class.
This can work with PNG colour type 3 (palette required), and
other colour types (palette optional).
IDAT¶
Generated automatically from the pixel data presented to PyPNG.
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¶
When writing,
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.
When reading will be processed into the alpha channel (if the appropriate method is used).
cHRM¶
Ignored when reading. Not generated.
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.
Note that there is no code to adjust the pixel values for gamma. Your code has to do that, if you want it.
iCCP¶
International Color Consortium Profile.
Ignored when reading. Not generated.
The supported prichunkpng command line tool can add an iCCP
chunk; and the not so supported iccp.py script can extract one
from a PNG file and display one in “readable” format.
The not so supported mkiccp.py script can make certain ICC
Profiles.
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¶
Ignored when reading. Not generated.
tEXt¶
Ignored when reading. Not generated.
zTXt¶
Ignored when reading. Not generated.
iTXt¶
Ignored when reading. Not generated.
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 a PNG image is read, a pHYs chunk will add the physical key to
the info dictionary.
When writing a PNG image, a pHYs chunk will
be generated if x_pixels_per_unit and y_pixels_per_unit
is not None.
The default is to not write this chunk.
sPLT¶
Ignored when reading. Not generated.
tIME¶
Ignored when reading. Not generated.
Non-standard Chunks¶
Generally it is not possible using PyPNG 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).