I occasionally encounter some special character while parsing PDF documents. They are actually two English letters, like 'fi', 'tt', or 'ti', but visually they look like conjuncted and they actually exist in PDF string as one character.
I checked the 'ToUnicode' for these characters, but I just found the 'ToUnicode' CMap table are disrupted, therefore I cannot find their unicode.
For example, <012E> Tj will print fi like attached picture. But in its corresponding Font's ToUnicode CMap: <012E> <0001>, which is meaningless.
Could anybody let me know their unicode code point? Possible to find it from the corresponding font program?
Thanks for any advice.
fi:
tt:
ti:
First of all, what you call letter conjunctions usually is known as ligatures. Thus, I will use that term here from now on.
Unicode discourages the use of specific code points for ligatures:
The existing ligatures exist basically for compatibility and round-tripping with non-Unicode character sets. Their use is discouraged. No more will be encoded in any circumstances.
Ligaturing is a behavior encoded in fonts: if a modern font is asked to display “h” followed by “r”, and the font has an “hr” ligature in it, it can display the ligature. Some fonts have no ligatures, while others (especially fonts for non-Latin scripts) have hundreds of ligatures. It does not make sense to assign Unicode code points to all these font-specific possibilities.
(Unicode FAQ on ligatures)
Thus, you should not use the existing ligature code points.
You appear to attempt to find the correct ToUnicode mapping for ligature glyphs. For this simply remember that the values of ToUnicode mappings do not need to be single code points but may be multiple ones:
n beginbfchar
srcCode dstString
endbfchar
where dstString may be a string of up to 512 bytes.
(ISO 32000-1, section 9.10.3 ToUnicode CMaps)
Concerning your example, therefore:
For example, <012E> Tj will print fi like attached picture. But in its corresponding Font's ToUnicode CMap: <012E> <0001>, which is meaningless.
Simply use
<012E> <00660069>
If you want to use ligature code points nonetheless, query the Wikipedia article on Orthographic Ligatures, it lists some ligature code points. In particular <FB01> for fi, so for your example:
<012E> <FB01>
But remember, their use is discouraged.
Related
I'm trying to read the official pdf specification "Document management — Portable document format — Part 1: PDF 1.7" (PDF32000_2008.pdf) as bytes and then interpret them according to that specification.
In Annex D, Character Sets and Encodings, there is a list of all named characters, like:
or
When I parse PDF32000_2008.pdf, there are also named characters like "f_f", "uni00D0" and "a204", which are missing in that specification.
My guess is that "f_f" is a symbol for two 'f' characters, which might get printed with a special glyph. There is a unicode "Latin Small Ligature Ff" for 'ff'.
For example, there is also "f_i" in that file, which I expect to mean 'fi', one glyph showing the 2 characters 'f' and 'i'. However, the pdf specification has 'fi' as named character "fi" and what is the point for having 2 named characters pointing to the same symbol ?
I can imagine that "uni00D0" means the unicode character 'Ð'. However, pdf defines it already as named character "Eth"
What could be "a204" ? Maybe Ansi 204 'Ì', which also has already a named character "Igrave" ?
Why do they use also "a62", which would be just a '<' ?
However, my main question is: Where can I find a specification for these additional named characters ?
Of course, Adobe Acrobat understands them, but also Gmail seems not to have a problem with them. So I guess, their meaning must be specified somewhere.
I have Unicode text (a sequence of Unicode codes) and a TTF font (bytes of a TTF file). I would like to write that text into a PDF file using that font.
I understand PDF quite well. I don't mind using two bytes per character. I would like to attach the TTF file as it is (charcode-to-glyf map should be used from a TTF file).
What font Subtype and Encoding value should I use? Is it possible to avoid having ToUnicode record?
I tried to use Subtype = "/TrueType", but it requires to specify FirstChar, LastChar and Widths (which are already inside TTF).
You cannot use Unicode with a Font, at all (except in the limited case of Latin, or nearly Latin, languages), because Fonts use an Encoding, and an Encoding is a single byte array. So you can't reference more than 256 characters from a Font, and a character code can't be more than a single byte.
The first problem with 'using Unicode' is that Unicode is not a simple 2-byte Encoding, its a multi-byte format, with variable lengths and sometimes a single glyph is represented by multiple Unicode code points.
So, in order to deal with this you need to use a CIDFont, not a Font. You cannot 'use the charcode-to-glyf map', by which I assume you mean the CMAP subtable in the TTF font. You must compose the CIDFont with a CMap in order to map the multiple bytes in the text string into the character codes for lookup in the CMap, which gives you the CID to reference the precise character program in the font.
It may be possible to construct a single CMap which would cover every Unicode code point, but I have my doubts, it would certainly be a huge task. However certain CMaps already exist. Adobe publish a standard list on their web site which includes CMaps such as UniCNS-UCS2-H and UniCNS-UCS2-V or UniGB-UTF8-H etc.
You can probably use one of the standard CMaps.
Note that it doesn't matter that the FirstChar, LastChar etc are already stored in the TrueType font, you still need to specify them in the PDF Font object. That's because a PDF consumer might not be rendering the text at all, it could (for example) be extracting the text, in which case it doesn't need to interpret the font provided this information is available.
I have multiple pdf files without 'toUnicode' cmap table. Absence of cmap table restricts me from copying the text from pdf files.
As far as I know, there is a possibility to add 'toUnicode' mapping in pdf file, but in my case adding static values is not an option, different files have different glyph codes.
So the question is the following. Is there any possibility to restore 'toUnicode' cmap table, perhaps with the help of Ghostscript, or are there any options at all?
Thanks.
No, you cannot add ToUnicode CMaps to an existing PDF file using Ghostscript.
In the general case, you can't do it at all, except manually. As you note in the question, different files will be constructed to use different character code->Glyph mappings, which means that the character code to Unicode mapping will also be different.
Since the character code selection is often based on the order in which glyphs are used in a file (so the fist glyph is character code 1, the second is character code 2 etc) you can see that there is no prospect of identifying a 'one size fits all' solution.
You could use some kind of OCR to scan the rendered output, identify each glyph and find the Unicode code point for it. Then you could construct a CMap by identifying the character code for the glyph and mapping it to the Unicode value.
You could, then, add the ToUnicode CMap to the PDF file, and update the Font Descriptor with the object number of the ToUnicode CMap.
Ghostscript won't do any of that for you, and I haven't heard of any tool which will.
Situation: I've a PDF using version 1.6. In that PDF, there are several streams. There were compressed text (Flate) in that streams, so I decompressed these streams. After that, I extracted the Tj-parts of the corresponding, decompressed streams. I assumed that there would be readable text between the brackets before the Tj command, but the result was the following:
Actual Question: As I have no idea, what I've got thre, I would like to know what type of content it is. Furthermore: Is it possible to get a plain text out of these string or do I need further information to extract plain texts?
Further research: The PDFs, which I try to analyze where generated by iTextSharp (seems to be an C# Library for generating PDFs). Don't know whether it is a relevant information, but it might be that that Library uses a special way of encrypt it's text data or something...
I assumed that there would be readable text between the brackets before the Tj command
This assumption only holds for simple PDFs.
To quote from the PDF specification (ISO 32000-1):
A string operand of a text-showing operator shall be interpreted as a sequence of character codes identifying the glyphs to be painted.
With a simple font, each byte of the string shall be treated as a separate character code. The character code shall then be looked up in the font’s encoding to select the glyph, as described in 9.6.6, "Character Encoding".
With a composite font (PDF 1.2), multiple-byte codes may be used to select glyphs. In this instance, one or more consecutive bytes of the string shall be treated as a single character code. The code lengths and the mappings from codes to glyphs are defined in a data structure called a CMap, described in 9.7, "Composite Fonts".
(Section 9.4.3 - Text-Showing Operators - ISO 32000-1)
Thus,
I would like to know what type of content it is.
As quoted above, these "strings" consist of single-byte or multi-byte character codes. These codes depend on the current font's encoding. Each font object in a PDF can have a different encoding.
Those encodings may be some standard encoding (MacRomanEncoding, MacExpertEncoding, or WinAnsiEncoding) or some custom encoding. In particular in case of embedded font subsets you often find encodings where 1 is the code of the first glyph drawn on a page, 2 is the code for the second, different glyph, 3 for the third, different one, etc.
Furthermore: Is it possible to get a plain text out of these string or do I need further information to extract plain texts?
As the encoding of the string arguments of text showing instructions depends on the current font, you at least need to keep track of the current font name (Tf instruction) and look up encoding information (Encoding or ToUnicode map) from the current font object.
Section 9.10 - Extraction of Text Content - of ISO 32000-1 explains this in some more detail.
Furthermore, the order of the text showing instructions need not be the order of reading. The word "Hello" can e.g. be shown by first drawing the 'o', then going left, then the 'el', then again left, then the 'H', then going right, and finally the remaining 'l'. And two words need not be separated by a space glyph, there simply might be a text positioning instruction going right a bit.
Thus, in general you also have to keep track of the position of the separate strings drawn.
While 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.