Speed up Image.blend()

[akx]

I was working on a project that uses Image.blend() and it showed up pretty red in a Viztracer timeline, so I decided to investigate a little.

Very small patch, but allows the C compiler to optimize the loops better:

• we know ysize and linesize can't change during iteration (but the compiler can't statically know that), so hoisting those to locals
• we know imOut is a fresh allocation and can't alias another pointer, so adding restrict to note that.
  • restrict should be available on MSVC in C99 mode without having to use the MS extension name __restrict; we'll see how it pans out in CI and adjust accordingly.

In addition, added a little fast-path if you pass in the same image object twice.

On my machine, this shows a 300% speed improvement (with blend_bench.py being a pytest-benchmark suite doing benchmark(Image.blend, im1, im2, 0.5) on two 2048x2048 RGB images).

(main) $ uv pip install -e . && uv run --no-sync pytest --benchmark-autosave blend_bench.py
[...]
Name (time in ms)        Min      Max    Mean  StdDev  Median     IQR  Outliers       OPS  Rounds  Iterations
-------------------------------------------------------------------------------------------------------------
test_blend_perf       9.1552  11.0795  9.7906  0.2351  9.7934  0.1972      17;5  102.1389      90           1

(faster-blend) $ uv pip install -e . && uv run --no-sync pytest --benchmark-autosave blend_bench.py
[...]
Name (time in ms)        Min     Max    Mean  StdDev  Median     IQR  Outliers       OPS  Rounds  Iterations
------------------------------------------------------------------------------------------------------------
test_blend_perf       2.1012  3.2935  2.2357  0.1044  2.2116  0.1040      43;7  447.2908     284           1
------------------------------------------------------------------------------------------------------------

(faster-blend) $ pytest-benchmark compare .benchmarks/Darwin-CPython-3.14-64bit/*.json --between=ops

----------------- benchmark: 1 tests, 2 sources ------------------
Name (time in ms)     0001_2d4bc04 OPS  0002_9da64fc OPS      ΔOPS
------------------------------------------------------------------
test_blend_perf               102.1389          447.2908   +337.9%
------------------------------------------------------------------

I also looked at using the integer math from AlphaComposite.c, but that only made things a little slower.

[hugovk]

Please can you share blend_bench.py or a similar benchmark?

[akx]

@hugovk Sure thing:

import pytest

from PIL import Image


@pytest.fixture(scope="module")
def images() -> tuple[Image.Image, Image.Image]:
    size = (2048, 2048)
    im1 = Image.new("RGB", size)
    im2 = Image.new("RGB", size)
    im1.frombytes(bytes(i % 256 for i in range(im1.width * im1.height * 3)))
    im2.frombytes(bytes((i * 7 + 31) % 256 for i in range(im2.width * im2.height * 3)))
    return im1, im2


def test_blend_perf(benchmark, images: tuple[Image.Image, Image.Image]) -> None:
    im1, im2 = images
    result = benchmark(Image.blend, im1, im2, 0.5)
    assert result.size == im1.size

[akx]

Btw, if this is merged, I can take a look at other similar loops and apply the same optimizations - looks like there could be lots of easy perf pickings 😊

[akx]

Is there something blocking merging this? 👀
As mentioned above, I do actually have a couple other similar fixes in the pipeline :)

[aclark4life]

Is there something blocking merging this? 👀 As mentioned above, I do actually have a couple other similar fixes in the pipeline :)

Typically @hugovk will merge no later than release time which is July 1

[hugovk]

It'd be good to get the benchmarks set up first in #9654 so we can see the improvement.

@akx Can you update that PR to setup the CI workflow, step 2 at #9654 (comment) ?

[hugovk]

Updated now #9654 is merged.

[codspeed-hq]

Merging this PR will improve performance by ×2.6

⚠️ Different runtime environments detected

Some benchmarks with significant performance changes were compared across different runtime environments,
which may affect the accuracy of the results.

Open the report in CodSpeed to investigate

⚡ 4 improved benchmarks
✅ 327 untouched benchmarks

Performance Changes

	Benchmark	BASE	HEAD	Efficiency
⚡	test_blend[1024x1024-LA]	28 ms	10.9 ms	×2.6
⚡	test_blend[1024x1024-RGBA]	28 ms	10.9 ms	×2.6
⚡	test_blend[1024x1024-RGB]	28 ms	10.9 ms	×2.6
⚡	test_blend[1024x1024-L]	7 ms	2.8 ms	×2.5

Tip

Curious why this is faster? Comment @codspeedbot explain why this is faster on this PR, or directly use the CodSpeed MCP with your agent.

---

_{Comparing akx:faster-blend (7a0534a) with main (80ac5e3)}

[hugovk]

Thanks!

[akx]

@hugovk If there's still time before the next release, I can create PRs of another batch of similar fixes tomorrow (Tuesday) AM? 🤞

[hugovk]

No promises but go for it!

Note that @radarhere is doing the next release and Wednesday happens earlier in Australia than Finland :)

And if they don't make this week's release, Q3 is just around the corner.