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:
- omit this part if no unit specified ((<pixel_per_unit_x>, <pixel_per_unit_y>), )
- 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).