Re: Type 42

[Sivan Toledo]

Hi David,

Thanks for the detailed reply. I'm afraid, however, that I wasn't clear
enough in my query.

I am not interested in rasterizing glyphs from a Type42 font, which
is what you explained how to do.

What I am basically interested in is a TTF -> Type42 converter. Actually
it's not a straight TTF->T42 file-to-file converter, because I want
the T42 font to include only a specified subset of the glyphs in the ttf
font.

The application is this: I want to provide a piece of software that
would (1) rasterize glyphs for on-screen disply in widgets, and
      (2) build a representation of the same fonts to send to the
          printer; this would be what we use the T42 file for.
For (1) we can either call FreeType directly or build a font-server
interface (or hack exising software like xfstt). For (2) we need
to build a Type42 representation of a subset of the glyphs in the
ttf file. We then include this T42 font in the postscript file
that we send to the printer, and which also contains text to
be rendered in the font, graphics, etc.

I apologize for being unclear before.

Sivan

> There are basically two ways to support Type42 fonts with FreeType2:
> 
> - one is to write a font driver that would be able to manage the postscript
> "wrapper", and use the TrueType driver to manage data within it.
> 
> - the other one is to support manage the wrapper yourself (this is important
> if you are a Postscript interpreter like Ghostscript) and provide a custom
> stream object in order to use the TrueType driver directly..
> 
> The first solution would need some work. It's do-able, but I'm really
> doubtful that it had any real interest, given that the only programs
> that need to support these fonts are Postscript interpreters (well, I
> may be wrong, so please let me know if this isn't true).
> 
> I'll know talk about the second method:
> 
> There are already hooks in the FreeType 2 TrueType driver to support
> Type42 fonts, but it's true that I haven't completed this yet. Could
> you tell me if the following scheme would allow you to support these
> fonts in your application (it'd need a few changes that I'm ready
> to commit if you agree with this):
> 
> - you would need to provide your custom implementation of
>   the "stream" object (see <freetype/ftsystem.h>) to access
>   the data inside the encoded font file
> 
> - you will need to provide an alternative to the function
>   TT_Goto_Table, defined in "src/sfnt/ttload.c" (line 102).
>   
> 
> basically, all tables are accessed through a line like:
> 
>   error = face->goto_table( face, table_tag, stream, &length );
> 
> "goto_table" is a field of TT_FaceRec which is set by default (in 
> TT_Init_Face)
> to TT_Goto_Table. However, you could provide your own implementation to
> seek to the data you need within your Type42 font. Its purpose is
> simple to "seek" the stream to the start of the table, and return its
> size in bytes in the "length" variable..
> 
> You would thus need something like the following to open a new font file:
> 
>   ... create your own stream object, named "my_stream"
>   {
>     FT_Open_Args   open;
>     FT_Parameter   parameter;
> 
>     /* this extra parameter is used to pass the address of a substitue for */
>     /* TT_Goto_Table                                                       */
>     parameter.tag  = FT_MAKE_TAG('g','o','t','o');
>     parameter.data = your_implementation_of_tt_goto_table;
> 
>     /* this indicates that we will open a new face with a custom stream, */
>     /* and that we'll pass one parameter to the font driver..            */
>     open.flags  = ft_open_stream | ft_open_params;
>     open.stream = my_stream;
>     open.num_params = 1;
>     open.params   = &parameter;
> 
>     error = FT_Open_Face( library, &open, 0, &face );
>   }
> 
> Note that the stream will be normally seeked to load the table directory
> though. We could also add an additional parameter to the font driver to
> specify the function to use to load the latter..
> 
> 
> So, would this fit your needs ?? It so, I only need to change TT_Init_Face
> in order to recognize the extra parameter..
> 
> Best regards,
> 
> - David
>