[lazerg]

sharp always passes tune to heifsave, and the default is auto. The libheif aom plugin accepts auto, but the x265 plugin only knows psnr, ssim, grain and fastdecode, so it rejects it and every HEVC encode with default options fails with heif: Invalid parameter value (5.2006).

This leaves tune unset when it is auto. libvips then skips heif_encoder_set_parameter_string, so each encoder uses its own default. For aom that default is already auto, so AV1 output doesn't change.

I reproduced it on macOS with sharp built against a global libvips 8.18.7 and Homebrew libheif 1.23.6 (x265 4.3). heif({ compression: 'hevc' }) fails before this change and works after, via both toBuffer and toFile. AV1 output is byte-identical. I haven't added a unit test, since the prebuilt libvips has no HEVC encoder for CI to exercise. The existing unit tests all pass.

Fixes #4621

[lovell]

Thanks for taking the time to open a PR, I've taken an alternative approach to do this in JavaScript - see #4621 (comment)