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. 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 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
¶
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
¶
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.
iCCP
¶
Ignored when reading. Not generated.
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
(default: None
).
sPLT
¶
Ignored when reading. Not generated.
tIME
¶
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).