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docs/reference/block_allocator.rst
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docs/reference/block_allocator.rst
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Block Allocator
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===============
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Previous Design
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---------------
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Historically there have been two image allocators in Pillow:
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``ImagingAllocateBlock`` and ``ImagingAllocateArray``. The first works
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for images smaller than 16MB of data and allocates one large chunk of
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memory of ``im->linesize * im->ysize`` bytes. The second works for
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large images and make one allocation for each scan line of size
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``im->linesize`` bytes. This makes for a very sharp transition
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between one allocation and potentially thousands of small allocations,
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leading to unpredictable performance penalties around the transition.
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New Design
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----------
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``ImagingAllocateArray`` now allocates space for images as a chain of
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blocks with a maximum size of 16MB. If there is a memory allocation
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error, it falls back to allocating a 4KB block, or at least one scan
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line. This is now the default for all internal allocations.
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``ImagingAllocateBlock`` is now only used for those cases when we are
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specifically requesting a single segment of memory for sharing with
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other code.
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Memory Pools
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------------
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There is now a memory pool to contain a supply of recently freed
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blocks, which can then be reused without going back to the OS for a
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fresh allocation. This caching of free blocks is currently disabled by
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default, but can be enabled and tweaked using three environment
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variables:
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* ``PILLOW_ALIGNMENT``, in bytes. Specifies the alignment of memory
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allocations. Valid values are powers of 2 between 1 and
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128, inclusive. Defaults to 1.
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* ``PILLOW_BLOCK_SIZE``, in bytes, K, or M. Specifies the maximum
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block size for ``ImagingAllocateArray``. Valid values are
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integers, with an optional `k` or `m` suffix. Defaults to 16M.
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* ``PILLOW_BLOCKS_MAX`` Specifies the number of freed blocks to
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retain to fill future memory requests. Any freed blocks over this
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threshold will be returned to the OS immediately. Defaults to 0.
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@ -6,3 +6,5 @@ Internal Reference Docs
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open_files
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limits
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block_allocator
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@ -87,16 +87,32 @@ operation. The original :py:class:`PIL.ImageFilter.Filter` class
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remains for image filters that can process only single band images, or
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require splitting of channels prior to filtering.
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Other Changes
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=============
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Loading 16-bit TIFF Images
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==========================
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^^^^^^^^^^^^^^^^^^^^^^^^^^
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Pillow now can read 16-bit multichannel TIFF files including files
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with alpha transparency. The image data is truncated to 8-bit
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precision.
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Pillow now can read 16-bit signed integer single channel TIFF
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files. The image data is promoted to 32-bit for storage and
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processing.
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SGI Images
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^^^^^^^^^^
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Pillow can now read and write uncompressed 16-bit multichannel SGI
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images to and from RGB and RGBA formats. The image data is truncated
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to 8-bit precision.
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Pillow can now read RLE encoded SGI images in both 8 and 16-bit
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precision.
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Performance
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===========
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^^^^^^^^^^^
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This release contains several performance improvements:
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@ -109,3 +125,14 @@ This release contains several performance improvements:
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friendly algorithm.
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* ImageFilters based on Kernel convolution are significantly faster
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due to the new MultibandFilter feature.
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* All memory allocation for images is now done in blocks, rather than
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falling back to an allocation for each scan line for images larger
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than the block size.
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CMYK Conversion
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^^^^^^^^^^^^^^^
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The basic CMYK->RGB conversion has been tweaked to match the formula
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from Google Chrome. This produces an image that is generally lighter
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than the previous formula, and more in line with what color managed
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applications produce.
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