Resizing Luma+alpha image on ARM (Apple) causes stripe artifacts

[MichaelGrupp]

Hi,

thanks for this nice library. I noticed an issue with LA8 grayscale images that have an alpha channel on ARM, specifically Apple Silicon (I used M1 & M4). With the NEON extensions that are enabled by default, the resized LA8 image has stripes. Disabling NEON removes those stripes. The issue also doesn't seem to affect RGB, RGBA8 or L8; these can be resized with NEON in my tests. Haven't tested LA16.

Here is an example using dummy images created with Gimp: one LA8 image and an RGBA8 image.

This can be reproduced using the resizer example from the main branch (a472768):

cargo run -p resizer <image> -h 50% -w 50%

Original	Resized with NEON

I didn't dig deeper than that, for my personal project it was sufficient to convert to RGBA8 to "fix" the issue. I also don't know if this only an issue on Apple Silicon or also on other ARM platforms.

[awxkee]

unsafe fn multiply_alpha_row(src_row: &[U8x2], dst_row: &mut [U8x2]) {
    let src_chunks = src_row.chunks_exact(32);
    let src_remainder = src_chunks.remainder();
    let mut dst_chunks = dst_row.chunks_exact_mut(32);

to

unsafe fn multiply_alpha_row(src_row: &[U8x2], dst_row: &mut [U8x2]) {
    let src_chunks = src_row.chunks_exact(16);
    let src_remainder = src_chunks.remainder();
    let mut dst_chunks = dst_row.chunks_exact_mut(16);

if that makes sense.

P.S. NEON is mandatory extension on Aarch64, #[target_feature(enable = "neon")] is enforced on all code which target is Aarch64.

[Cykooz]

Thank you for report.
I won't be able to fix this problem for the next two weeks because I'm on holiday. But after that I'll take care of it.