Letter bounding box misaligned

[lihuanglx]

Thank you for maintaning this library! This is the most feature-rich C# PDF library I've found under a permissive license.

I got some letter.GlyphRectangle misaligned in a document. The screenshot and file are attached below.

From left to right, the symbols and their bounding boxes in the grid gradually become misaligned. The bounding boxes of title and the text at the top looks good though.

P2P-33713919.pdf

I also tested a Python library pdfplumber and it shows correct bounding boxes.

[lihuanglx]

@BobLd I also tried rendering the page with PdfPig.Rendering.Skia, and noticed that the symbol fonts are rendered incorrectly. This might provide a hint about the root cause. Do you want me to post an issue there?

[BobLd]

@lihuanglx please yes if you can, thanks a lot!

[lihuanglx]

PdfPig/src/UglyToad.PdfPig/PdfFonts/Composite/Type0Font.cs

Lines 131 to 137 in 22eab42

	var width = CidFont.GetWidthFromFont(characterIdentifier);
	var advanceWidth = GetFontMatrix().TransformX(width);
	// BobLD: Not sure why we don't need CidFont.GetFontMatrix(characterCode)
	// Might be related to https://github.com/veraPDF/veraPDF-library/issues/1010
	var result = new CharacterBoundingBox(boundingBox, advanceWidth);

@BobLd I found that the root cause of this issue is the advanceWidth calculation.

In my document, the current code makes GetFontMatrix return 1/1024 for the x-axis. When I change it to 1/1000, the output looks correct.

I also tested with the document from the veraPDF issue you mentioned in comments, namely felltypes-test.pdf. In that file, GetFontMatrix returns 1/1000, so it worked in your testing, I suppose.

I'm not an expert in this area, but I think your idea behind the current code might be correct. Is it just that people have been following a flawed convention?

Given the current situation, would it make sense to just divide by 1000 instead of using the matrix?

Appreciate your work on this!

[BobLd]

@lihuanglx that's very useful, thanks a lot! having a look to try to understand the root cause

[BobLd]

@lihuanglx this should now be fixed. Was more tricky than just using 1000, because it'd break other documents.

The current solution is hackish but seems to work. please flag if you have related issues

[BobLd]

@lihuanglx I finally found the actual problem, will be fixed with #1151

[lihuanglx]

Thank you very much! I'll try it.