Use of stbtt_ScaleForPixelHeight Incorrect in typographer_context_stb.cc #146080
Description
Steps to reproduce
- Run a test case which uses the
stbbased glyph atlas implementation. - Examine the contents of the Glyph Atlas at runtime using
renderdocor other
Expected results
The size of the glyphs in the atlas should be correct for a given typeface size in Points.
That is, for a typeface of size 12pt, the difference between the highest ascender and lowest descender should be 16 pixels (12pt == 16px @ 160 dpi).
Actual results
Currently, the resultant glyphs are (for a 12pt font) about 12px` high. This is despite correct scaling calculations.
See the screenshots below.
Code sample
Code sample
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Screenshots / Video demonstration
Here's an analysis of the issue.
Baseline behavior for the Noto Sans typeface (normal, weight 400) can be gathered here.
This looks like:
Note that the vertical bar character (linux "pipe" symbol) covers the range from the highest ascender to the lowest descender. Thus it is 16px high for a 12pt font.
Looking at the current glyph atlas for an example using the same typeface at the same size (using renderdoc):
The vertical bar is 12 pixels high. This is plainly incorrect.
Debugging a little, we find the scaling calculations are correct. We scale the desired 12pt to 16px at runtime.
I've debugged this further and find that the function call stbtt_ScaleForPixelHeight is returning incorrect scale values for correct inputs.
Using the function stbtt_ScaleForMappingEmToPixels with identical inputs results in correctly sized glyphs.
I'll try to present a PR to change this function call ASAP.
CC @bdero and @chinmaygarde
Logs
Logs
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