.. $URL$ .. $Rev$ 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 :meth:`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 :meth:`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 :meth:`png.Writer` class. ``IEND`` ^^^^^^^^ Generated automatically. Ancillary Chunks ---------------- ``tRNS`` ^^^^^^^^ Generated for most colour types when the ``transparent`` argument is supplied to the :meth:`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 :meth:`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`` ^^^^^^^^ Ignored when reading. Not generated. ``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).