[HarfBuzz] Eager CS undergrad thirsty for knowledge about the inner bowels of typeface rendering. Specifically for supporting vertical ligature caret definitions.

[Robin Skahjem-Eriksen]

Thanks for link. I shall make sure to read up on that.
I am have slowly started the process of writing some software for Tibetan writing. I thought about how to handle the complex caret behaviour, when people on IRC stated that I should look into getting that done deeper in the text rendering stack.

Do you know if Graphite currently support caret placements for ligatures which have been transmuted from a stack of glyphs?

-Robin


On Wed, Jan 29, 2014 at 02:37:18PM -0500, Ed Trager wrote:
> Hi, Robin,
> 
> You might also want to read up about Graphite:
> 
> http://scripts.sil.org/cms/scripts/page.php?site_id=projects&item_id=graphite_home
> 
> I believe that Graphite has more sophisticated support (compared to
> OpenType technology) for caret placement for complex scripts such as
> Tibetan.  On the flip side, in comparision to OpenType, Graphite is not
> widely used.
> 
> - Ed
> 
> 
> On Mon, Jan 27, 2014 at 7:32 PM, Robin Skahjem-Eriksen <
> robinds at student.matnat.uio.no> wrote:
> 
> > I have started to investigate into the matter and I find that there is
> > very sparse information readily available. Microsoft had a very interesting
> > document, which I have been trying to understand:
> > http://www.microsoft.com/typography/OpenTypeDev/tibetan/intro.htm
> > In the section "Examples of Tibetan" (bottom of document), the first
> > example shows how a sequence of eight code points are strung together to
> > form a Tibetan "syllable". It's not really a syllable. It's called a
> > tsheg-bar in Tibetan and a Tibetan word can consist of multiple of these.
> > Anyways, all the code points from the second one up to and including the
> > fourth one are as far as I can tell formed into a ligature. I have opened
> > the MS Himalaya font in Fontforge and seen that this ligature is defined as
> > "tibSa_Ga_Rata_Shapkyu" in a location outside of the Unicode address space.
> >
> > Now, if a user is to work with Tibetan text like any other user of a roman
> > script language, the user of Tibetan script would be very disappointed. The
> > reason for this is that it is impossible to place the caret and select
> > individual characters in this ligature. As of now, you can only select the
> > entire stack as a whole selection. This is partly because the glyphs have
> > been transmuted into a ligature, but perhaps also because there seems to be
> > no definitions of vertical caret ligatures anywhere.
> >
> > How would one go about defining such an important feature? Should this be
> > implemented in the font? Should it be implemented in the software that
> > handles the type face? Or perhaps both? While digging through the MS
> > Himalaya font, I found that there is a value for a Ligature Caret Count.
> > What is this value supposed to be used for? For the ligatures that are
> > supposed to represent stacks of multiple glyphs, the Ligature Caret Count
> > had values up to 4, which I hope can mean that the font itself contains the
> > information I am looking for. Is my assumption correct?
> >
> > Also. I am highly willing to learn more about the inner bowels of typeface
> > rendering. I have taken a course on Computer design and understand how
> > everything are bits and how Asssembly language and C handles this. I also
> > understand the general idea about Unicode and how this is defined on a low
> > level. I've also understood that fonts are basically Bézier curves which
> > are rasterized to the screen buffer. There is still a lot of this process
> > which I still find very murky, so if anybody knows any in depth reading
> > material, I would be very happy to start reading those. I have read State
> > of Text Rendering by Behdad Esfahbod, which was a great overview of the
> > text rendering stack. But I would really like to get more in depth
> > understanding of each layer in the stack.
> >
> > I would really like to also learn more about Harfbuzz and how to work on
> > it. I would really love to spend some time working on it, if I am at a
> > level where my code submission would be acceptable to the standards of this
> > project. Is there any documentation for Harfbuzz? I've taken a quick glance
> > at the source code and run some scripts and make commands, but I honestly
> > don't know what's going on. Why are the C files named .cc and some of the
> > header files named .hh? I can recognize some font lingo and have a slight
> > understanding of what might be going on, but it would be really helpful to
> > have something like this for an Openfont file: http://imgur.com/a/JEObT#0.
> > I must also say that I have no idea of what harfbuzz is supposed to do and
> > how I test or use it once it's compiled. I've tried running some shell
> > scripts and bin files that were compiled, but I really have no clue.
> >
> > Is there somewhere I can learn how to get a handle on understanding the
> > technicalities of the Harfbuzz project and learn what I need to start
> > contributing?
> >
> > Sincerely,
> > Robin Skahjem-Eriksen
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