[matmen]

Bilinear and area resizing now use 8-bit SWAR in both passes: the intermediate image is rounded to 8 bits, so every multiplication handles two channels. This is the new default, and ResizeOptions.fast: false keeps the 3.2.1 resampling.

Opaque images

The second pass now also multiplies two channels at once (the first pass already did in 3.2.1).

Resampling a 1920×1080 photo:

	no SWAR	1 pass (3.2.1)	2 passes (fast, default)
halving, bilinear	27.4 ms	14.8 ms	12.0 ms
halving, area	15.6 ms	8.9 ms	7.6 ms
doubling, bilinear	106.4 ms	79.9 ms	61.1 ms

Compared with fast: false across 1313 corpus images and 9 sizes, every result is within 1 per channel. 10–25% of the values differ by 1, versus about 1% between fast: false and an exact float reference.

Transparent images

Before, transparent images always took the precise premultiplied path. With fast, they are now premultiplied to 8 bits, the way browsers and Skia do it:

• Both passes multiply red+blue and green+alpha as 16-bit lanes.
• A reciprocal table replaces the division when unpremultiplying.

resize() of the same photo with alpha:

	3.2.1	fast
halving	42.6 ms	24.1 ms
doubling	260.3 ms	118.5 ms

Compared with fast: false across 178 transparent corpus images and 14 size/mode combinations:

• What shows once composited (channel × alpha) differs by at most 2.4 steps.
• Alpha differs by at most 2.
• Only 1–4% of the values differ at all.
• Colors of nearly transparent pixels keep less precision: up to 12 steps at alpha 32. At that alpha, 8-bit premultiplication has only about 32 levels per channel. blur() has done the same since 3.2.1.

Limits

• When a pass needs more than 4 source pixels per output pixel (shrinking below about a third), the rounding of the 8-bit weights adds up. Those resizes keep the precise path.
• Bicubic resizing is unchanged: its negative weights don't work with packed lanes.

Changes

• src/util/resample.ts:
  • resampleOpaqueFast and resampleTransparentFast.
  • Tap counts are padded to 2 or 4, so area resizing to a third runs the unrolled loops as well.
• ResizeOptions.fast is passed through scale, resize, contain, fit, cover and Animation.resize.
• tests/resize.js checks, on a photo crop and the same crop with an alpha gradient:
  • fast results stay within the limits above;
  • fast results do differ at half size;
  • bicubic results are identical.
• CHANGELOG: 3.3.0 (new option).

🤖 Generated with Claude Code

https://claude.ai/code/session_01YJgRK5qpwJEbHPuFv6kgAQ