The previous post listed an algorithm for converting TrueType glyph advances into PDF glyph widths. It worked, but was weird, kludgy, complicated and all around bad. It was also completely wrong. As in "not even remotely in the direction of the correct solution" but it happened to work by accident with the examples I had. Trying it with some more fonts yields the now familiar error.
What makes debugging this issue harder is that Freetype exposes the same information multiple times. There are three or four different places where glyph advancement can be read and multiple multipliers that could potentially be used. No combination of these provides a value that would be even relatively close to the correct one.
So let's work backwards. What is the multiplier that we need to get? A single example says that a glyph with font advancement 1229 should have a PDF width value of 600. Therefore 1229/600 = 2.048333, and we should divide by roughly half.
After some debugging one can find a struct entry that is set to 1000 for fonts that work and 2048 for those that don't: face->units_per_EM. This scaling value is arbitrary but most (but not all) Adobe Type 1 fonts use 1000 while most (but not all) TrueType fonts use 2048. PDF uses 1000, probably to mach Type 1 conventions. This is even documented in the PDF specification 1.7, section 9.2.4, page 241: "For all font types except Type 3, the units of glyph space are one-thousandth of a unit of text space". This is incredibly easy to miss when reading the 700+ pages of text in the specification. I know because I did it. Several times.
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