Pillow/docs/releasenotes/2.7.0.rst

2.7.0
=====

Sane Plugin
-----------

The Sane plugin has now been split into its own repo:
https://github.com/python-pillow/Sane .


Png text chunk size limits
--------------------------

To prevent potential denial of service attacks using compressed text
chunks, there are now limits to the decompressed size of text chunks
decoded from PNG images. If the limits are exceeded when opening a PNG
image a :py:exc:`ValueError` will be raised.

Individual text chunks are limited to
:py:attr:`PIL.PngImagePlugin.MAX_TEXT_CHUNK`, set to 1MB by
default. The total decompressed size of all text chunks is limited to
:py:attr:`PIL.PngImagePlugin.MAX_TEXT_MEMORY`, which defaults to
64MB. These values can be changed prior to opening PNG images if you
know that there are large text blocks that are desired.

Image resizing filters
----------------------

Image resizing methods :py:meth:`~PIL.Image.Image.resize` and
:py:meth:`~PIL.Image.Image.thumbnail` take a ``resample`` argument, which tells
which filter should be used for resampling. Possible values are:
``NEAREST``, ``BILINEAR``, ``BICUBIC`` and ``ANTIALIAS``. Almost all of them
were changed in this version.

Bicubic and bilinear downscaling
^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^

From the beginning ``BILINEAR`` and ``BICUBIC`` filters were based on affine
transformations and used a fixed number of pixels from the source image for
every destination pixel (2x2 pixels for ``BILINEAR`` and 4x4 for ``BICUBIC``).
This gave an unsatisfactory result for downscaling. At the same time, a high
quality convolutions-based algorithm with flexible kernel was used for
``ANTIALIAS`` filter.

Starting from Pillow 2.7.0, a high quality convolutions-based algorithm is used
for all of these three filters.

If you have previously used any tricks to maintain quality when downscaling with
``BILINEAR`` and ``BICUBIC`` filters (for example, reducing within several
steps), they are unnecessary now.

Antialias renamed to Lanczos
^^^^^^^^^^^^^^^^^^^^^^^^^^^^

A new ``LANCZOS`` constant was added instead of ``ANTIALIAS``.

When ``ANTIALIAS`` was initially added, it was the only high-quality filter
based on convolutions. It's name was supposed to reflect this. Starting from
Pillow 2.7.0 all resize method are based on convolutions. All of them are
antialias from now on. And the real name of the ``ANTIALIAS`` filter is Lanczos
filter.

The ``ANTIALIAS`` constant is left for backward compatibility and is an alias
for ``LANCZOS``.

Lanczos upscaling quality
^^^^^^^^^^^^^^^^^^^^^^^^^

The image upscaling quality with ``LANCZOS`` filter was almost the same as
``BILINEAR`` due to a bug. This has been fixed.

Bicubic upscaling quality
^^^^^^^^^^^^^^^^^^^^^^^^^

The ``BICUBIC`` filter for affine transformations produced sharp, slightly
pixelated image for upscaling. Bicubic for convolutions is more soft.

Resize performance
^^^^^^^^^^^^^^^^^^

In most cases, convolution is more a expensive algorithm for downscaling
because it takes into account all the pixels of source image. Therefore
``BILINEAR`` and ``BICUBIC`` filters' performance can be lower than before.
On the other hand the quality of ``BILINEAR`` and ``BICUBIC`` was close to
``NEAREST``. So if such quality is suitable for your tasks you can switch to
``NEAREST`` filter for downscaling, which will give a huge improvement in
performance.

At the same time performance of convolution resampling for downscaling has been
improved by around a factor of two compared to the previous version.
The upscaling performance of the ``LANCZOS`` filter has remained the same. For
``BILINEAR`` filter it has improved by 1.5 times and for ``BICUBIC`` by four
times.

Default filter for thumbnails
^^^^^^^^^^^^^^^^^^^^^^^^^^^^^

In Pillow 2.5 the default filter for :py:meth:`~PIL.Image.Image.thumbnail` was
changed from ``NEAREST`` to ``ANTIALIAS``. Antialias was chosen because all the
other filters gave poor quality for reduction. Starting from Pillow 2.7.0,
``ANTIALIAS`` has been replaced with ``BICUBIC``, because it's faster and
``ANTIALIAS`` doesn't give any advantages after downscaling with libjpeg, which
uses supersampling internally, not convolutions.

Image transposition
-------------------

A new method ``TRANSPOSE`` has been added for the
:py:meth:`~PIL.Image.Image.transpose` operation in addition to
``FLIP_LEFT_RIGHT``, ``FLIP_TOP_BOTTOM``, ``ROTATE_90``, ``ROTATE_180``,
``ROTATE_270``. ``TRANSPOSE`` is an algebra transpose, with an image reflected
across its main diagonal.

The speed of ``ROTATE_90``, ``ROTATE_270`` and ``TRANSPOSE`` has been significantly
improved for large images which don't fit in the processor cache.

Gaussian blur and unsharp mask
------------------------------

The :py:meth:`~PIL.ImageFilter.GaussianBlur` implementation has been replaced
with a sequential application of box filters. The new implementation is based on
"Theoretical foundations of Gaussian convolution by extended box filtering" from
the Mathematical Image Analysis Group. As :py:meth:`~PIL.ImageFilter.UnsharpMask`
implementations use Gaussian blur internally, all changes from this chapter
are also applicable to it.

Blur radius
^^^^^^^^^^^

There was an error in the previous version of Pillow, where blur radius (the
standard deviation of Gaussian) actually meant blur diameter. For example, to
blur an image with actual radius 5 you were forced to use value 10. This has
been fixed. Now the meaning of the radius is the same as in other software.

If you used a Gaussian blur with some radius value, you need to divide this
value by two.

Blur performance
^^^^^^^^^^^^^^^^

Box filter computation time is constant relative to the radius and depends
on source image size only. Because the new Gaussian blur implementation
is based on box filter, its computation time also doesn't depend on the blur
radius.

For example, previously, if the execution time for a given test image was 1
second for radius 1, 3.6 seconds for radius 10 and 17 seconds for 50, now blur
with any radius on same image is executed for 0.2 seconds.

Blur quality
^^^^^^^^^^^^

The previous implementation takes into account only source pixels within
2 * standard deviation radius for every destination pixel. This was not enough,
so the quality was worse compared to other Gaussian blur software.

The new implementation does not have this drawback.

TIFF Parameter Changes
----------------------

Several kwarg parameters for saving TIFF images were previously
specified as strings with included spaces (e.g. 'x resolution'). This
was difficult to use as kwargs without constructing and passing a
dictionary. These parameters now use the underscore character instead
of space. (e.g. 'x_resolution')