The line integral convolution takes 21 bilinear samples and 20 tangent lookups per
output pixel, and every one of those computed its coordinate with `f32::floor` or
`f32::round`. On the baseline x86-64 target both lower to library calls (`jmp floorf`
and `jmp roundf` in `rustc --emit asm`), so the filter spent most of its time in 82
call-and-return pairs per pixel.
`sample()` now truncates the coordinate it already clamped to the window, and the
tangent and bristle lookups use `round_i32`/`round_index`, which produce the same
integer for every position a tile can produce.
The output is bit-identical: six 600x400 fixtures (default, short, long, unlit, RGB
without alpha, CMYK) compare sample for sample with `cmp` and a per-channel maximum
delta of 0.0. 24 MP Oil Paint is 1.88x faster on the minimum of five alternating pairs,
measured on a contended 4-core machine; docs/oil-paint-performance.md has the numbers,
that caveat and the alternatives that were measured and rejected.
ebosido444-ai
7ed4b9e0ef