I'm not sure if I'm doing it wrong, but I used this tool on a font with an earlier version and I could get the xheight and capHeight from it, but now I cant seem too. The code hasn't been updated only the tool used,

Just mentioning in case, might be me doing something wrong though, will keep looking, just wanted to mention :)

The underlying font library sets x-height based on x and capital height based on H (source).

If your font does not contain these symbols, the two will not be set.

I have lowercase x and uppercase H. The output file is 4kb, the older version was 117kb. Maybe I enabled/disabled something I shouldn't have.

It seems its because of the glyph colour. My image consists of only white glyphs and 'auto' for the glyph colour worked in the previous version for me, but not now. I changed it to white and it just works now :)

Ah, a good mystery - don’t think I’ve touched that since adding the feature.