[ft] Fwd: FW: Fwd: FW: Handling an embedded CID font using Freetype

[tony smith]

Thanks for all the replies.
I'll have a look through the Adobe specs.

-----Original Message-----
From: Freetype [mailto:address@hidden
On Behalf Of Derek B. Noonburg
Sent: 22 March 2017 16:37
To: address@hidden
Subject: Re: [ft] Fwd: FW: Handling an embedded CID font using Freetype

As Chris said, the CFF font doesn't contain the encoding information, so
decompressing the font won't help you.

CID CFF fonts are indexed using "CIDs" (character IDs), which usually come
from a particular "character collection".  (This is all Adobe
terminology.)

In your case, the character collection, as specified in the CIDSystemInfo
dictionary, is Adobe-Japan1.  You can get the relevant specs here:

https://www.adobe.com/devnet/font.html

For PDF, you'll also need to look at the font's encoding, which may be
"Identity", meaning that the character codes are the CIDs; or it may be a
CMap, which maps (possibly variable length) character codes to CIDs.

The CMap can either be the name of a predefined CMap (those CMap files are
available at the adobe link above), or a CMap stream.  Either way, you'll
need to parse the CMap -- the CMap format spec is also on that web page.

To summarize:
* get the character code from the PDF content stream
* map the character code to CID, using a CMap
* use the CID to get a glyph from the font

If you want to map to Unicode (rather than drawing a character from the
font), replace that last step with: map the CID to Unicode, using the
character collection data.

- Derek


On 2017 Mar 22, tony smith wrote:
> Thanks for the quick reply.
>
> I've attached an example pdf file, which displays a single Chinese
> character.
> The font details and the string in the pdf data are both readable.
>
> The pdf string contains the single character <07F4>.
> Also the relevant font has the following entry
>   /CIDSystemInfo << /Registry (Adobe)/Ordering (Japan1)/Supplement 4
> >>
>
> Fontforge has a file which maps these CID values to the equivalent
> Unicode values.
> So in this case the Chinese character is \u9AD8 But Freetype has no
> way to workout what the Unicode mapping is for a CFF.
>
> So if my understanding is correct I need to uncompress the CFF file
> and workout the relevant mapping from the CMap information.
>
> Thanks for you time, Tony Smith.
>
>
> On 22/03/17 14:08, tony smith wrote:
>> Hello
>>
>> I'm using Freetype to display fonts embedded in pdf data.
>>
>> My problem seems to be with an embedded CCF file, where the CID
>> mappings is only defined in the pdf data.
>> I load a Compact File Format (CFF) stream into memory.
>> But when I try to select a character map, using FT_Select_CharMap, I
>> get error 0x26, Invalid_CharMap_Handle.
>> I've attached the CFF, font.cff, and a simple c++ program, cff.c,
>> which highlights the problem.
>>
>> From the pdf file I know that the font is a CID one.
>> So you can work out the mapping from the PDF strings to Unicode strings.
>> But this information doesn't seem to be available in the CFF stream.
>> So my question is how do I handle this situation?
>> I can't see any way in Freetype to add the missing character encoding.
>> Do I have to decompress the CFF stream, which should be a type1 font
>> and then explicitly add the missing character encoding information?
>
> CFF != compressed type 1. Although the imaging models, and many
> concepts are shared, they are not the same.
>
> CFF CIDFonts don't contain any encoding information, and there is no
> set glyph ordering, and there is no built-in character ID to glyph
> index mapping (like, for example, a TTF/OTF cmap table).
>
> To actually use a CIDFont you need to use the CMap (NOTE: not "cmap",
> but
> "CMap") specified by the PDF - that can either be custom, embedded in
> the PDF, or a named reference to a standard CMap (available from
> Adobe, but I can't remember right now where).
>
> Parsing the CMap table will give you a mapping from character code to
> CID, which you can then use to pull the glyph from the font. (In
> Postscript terms you "compose" the CIDFont with the CMap to create a
> usable instance of the CIDFont).
>
> CMaps are defined in a highly restricted variant of Postscript, so
> should be fairly easy to parse.
>
> Chris
>
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