Which of the Image processing methods are lossless?

support
1

When converting from png to webp?

hugo.toml

[imaging]
  quality = 100
2

For webp, 100 isn’t lossless, I’m afraid. I just checked the code, and the value 0 passed to the upstream encoder would have given a lossless result, but that’s not possible to set in Hugo’s API (the valid values are 1-100).