So would this fix the weird uneven baseline that seems to happen in some (all?) mathtext test images?

Not right now, but I have a plan...

Basically the origin of the uneven baselines, I think, is that draw_glyph_to_bitmap specifies thetop right corner of the glyph, but that can be a fractional value (FreeType gives glyph sizes in (1/64)px) and then there is some roundoff when actually copying the glyph outline to the FT2Image which leads to the baseline being unaligned.

I think best would be to fix the mathtext logic to position glyphs by baseline/right position and adjust draw_glyph_to_bitmap accordingly; alternatively we can "reverse-engineer" the values that need to be sent to draw_glyph_to_bitmap to fix that roundoff.

I was just digging through this code in an attempt to understand#15313. If you are thinking about it all and understand it, can you add a few comments for posterity?

Yes, I need to write up something...

This also mostly fixes#18899, but the example at#18899 actually shows that the glyphs are also cut in the horizontal direction:

The bottom left corner of thex and the right edges of the digits (using mathtext) are clearly cut with this PR :/

I think another problem is that draw_text_to_bitmap doesn't support subpixel positioning, there seems to be some story related tosub_offset at the C level and old commits from@mdboom mentioning links with the wiggly baseline problem...

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Should likely be kept and put in in some form once the baseline image machinery is revamped.