2.7.0 (2014-12-31)¶
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 ValueError
will be raised.
Individual text chunks are limited to
PIL.PngImagePlugin.MAX_TEXT_CHUNK
, set to 1MB by
default. The total decompressed size of all text chunks is limited to
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 resize()
and
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 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
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 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 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’)