PDF font mapping error

W开发者_开发知识库hile rendering a PDF file generated by PDFCreator 0.9.x. I noticed it contains an error in the character mapping. Now, an error in a PDF file is nothing to be wondered about, Acrobat does wonders in rendering faulty PDF files hence a lot of PDF generators create PDFs that do not adhere fully to the PDF standard.

I trief to create a small example file: http://test.continuit.nl/temp/Document.pdf

The single page renders a single glyph (a capital A) using a Tj command (See stream 5 0 obj). The font selected (7 0 obj) contains a font with a single glyph embedded. So far so good. The char is referenced by char #1. Given the Encoding of the font it contains a Differences part: [ 1 /A ]. Thus char 1 -> character /A. Now in the embedded subset font there is a cmap that matches no glyph at character 65 (eg capital A) the cmap section of the font does define the character in exactly the order in the PDF file Font -> Encoding -> Differences array.

It looks like the character mapping / encoding is done twice. Only Files from PDFCreator 0.9.x seem to be affected.

My question is: Is this correct (or did I make a mistake and is the PDF correct) and what would you do to detect this situation in order to solve the rendering problem.

Note: I do need to be able to render these PDFs..

Solution

In the ISO32000 file there is a remark that symbolic TrueType fonts (flag bit 3 is on in the font descriptor) the encoding is not allowed and you should IGNORE it, using a simple 1on1 encoding always. SO all in all, if it is a symbolic font, I ignore the Encoding object altogether and this solves the problem.

---

The first point is that the file opens and renders correctly in Acrobat, so its almost certain that the file is correct. In fact it opens and renders correctly in a wide range of PDF consumers, so in fact it is correct.

The font in question is a TrueType font, so actually yes, there are two kinds of 'encoding'. First there is PDF/PostScript Encoding. This maps a character code into a glyph name. In your case it maps character code 1 to glyph name /A.

In a PostScript font we would then look up the name /A in the CharStrings dictionary, and that would give us the character description, which we would then execute. Things are different with a TrueType font though.

You can find this on page 430 of the 1.7 PDF Reference Manual, where it states that:

"A TrueType font program’s built-in encoding maps directly from character codes to glyph descriptions by means of an internal data structure called a “cmap” (not to be confused with the CMap described in Section 5.6.4, “CMaps”)."

I believe in your case that you simply need to use the character code (0x01) directly in the CMAP sub table. This will give you a GID of 36.