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Jan 26, 2020

README.md

Pillow-SIMD

Pillow-SIMD is "following" Pillow. Pillow-SIMD versions are 100% compatible drop-in replacements for Pillow of the same version. For example, Pillow-SIMD 3.2.0.post3 is a drop-in replacement for Pillow 3.2.0, and Pillow-SIMD 3.3.3.post0 — for Pillow 3.3.3.

For more information on the original Pillow, please refer to: read the documentation, check the changelog and find out how to contribute.

Why SIMD

There are multiple ways to tweak image processing performance. To name a few, such ways can be: utilizing better algorithms, optimizing existing implementations, using more processing power and/or resources. One of the great examples of using a more efficient algorithm is replacing a convolution-based Gaussian blur with a sequential-box one.

Such examples are rather rare, though. It is also known, that certain processes might be optimized by using parallel processing to run the respective routines. But a more practical key to optimizations might be making things work faster using the resources at hand. For instance, SIMD computing might be the case.

SIMD stands for "single instruction, multiple data" and its essence is in performing the same operation on multiple data points simultaneously by using multiple processing elements. Common CPU SIMD instruction sets are MMX, SSE-SSE4, AVX, AVX2, AVX512, NEON.

Currently, Pillow-SIMD can be compiled with SSE4 (default) or AVX2 support.

Status

Pillow-SIMD project is production-ready. The project is supported by Uploadcare, a SAAS for cloud-based image storing and processing.

Uploadcare

In fact, Uploadcare has been running Pillow-SIMD since 2015.

The following image operations are currently SIMD-accelerated:

  • Resize (convolution-based resampling): SSE4, AVX2
  • Gaussian and box blur: SSE4
  • Alpha composition: SSE4, AVX2
  • RGBA → RGBa (alpha premultiplication): SSE4, AVX2
  • RGBa → RGBA (division by alpha): SSE4, AVX2
  • RGB → L (grayscale): SSE4
  • 3x3 and 5x5 kernel filters: SSE4, AVX2
  • Split and get_channel: SSE4

Benchmarks

Tons of tests can be found on the Pillow Performance page. There are benchmarks against different versions of Pillow and Pillow-SIMD as well as ImageMagick, Skia, OpenCV and IPP.

The results show that for resizing Pillow is always faster than ImageMagick, Pillow-SIMD, in turn, is even faster than the original Pillow by the factor of 4-6. In general, Pillow-SIMD with AVX2 is always 16 to 40 times faster than ImageMagick and outperforms Skia, the high-speed graphics library used in Chromium.

Why Pillow itself is so fast

No cheats involved. We've used identical high-quality resize and blur methods for the benchmark. Outcomes produced by different libraries are in almost pixel-perfect agreement. The difference in measured rates is only provided with the performance of every involved algorithm.

Why Pillow-SIMD is even faster

Because of the SIMD computing, of course. But there's more to it: heavy loops unrolling, specific instructions, which aren't available for scalar data types.

Why do not contribute SIMD to the original Pillow

Well, it's not that simple. First of all, the original Pillow supports a large number of architectures, not just x86. But even for x86 platforms, Pillow is often distributed via precompiled binaries. In order for us to integrate SIMD into the precompiled binaries we'd need to execute runtime CPU capabilities checks. To compile the code this way we need to pass the -mavx2 option to the compiler. But with the option included, a compiler will inject AVX instructions even for SSE functions (i.e. interchange them) since every SSE instruction has its AVX equivalent. So there is no easy way to compile such library, especially with setuptools.

Installation

If there's a copy of the original Pillow installed, it has to be removed first with $ pip uninstall -y pillow. Please install prerequisites for your platform. The installation itself is simple just as running $ pip install pillow-simd, and if you're using SSE4-capable CPU everything should run smoothly. If you'd like to install the AVX2-enabled version, you need to pass the additional flag to a C compiler. The easiest way to do so is to define the CC variable during the compilation.

$ pip uninstall pillow
$ CC="cc -mavx2" pip install -U --force-reinstall pillow-simd

Contributing to Pillow-SIMD

Please be aware that Pillow-SIMD and Pillow are two separate projects. Please submit bugs and improvements not related to SIMD to the original Pillow. All bugfixes to the original Pillow will then be transferred to the next Pillow-SIMD version automatically.

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