Re: [Indic-computing-devel] NCST IndiX examined

[Keyur S. <key...@ya...>]

Dear Joseph,

--- Joseph Koshy <jk...@Fr...> wrote:
> 
> ks> Err! Please carefully see the source code of
> xc/lib/X11/QuTextExt.c

> I don't see anything specific to Unicode or UCS-2 in this
> file.

When Xlib converts 8-bit string into 16-bit string, it
first send MSB first. This is same as Little-Endian UCS-2. 

> There is only one [QueryTextExtents] protocol request in
> the X11
> protocol.  This request is used for both the
> `XQueryTextExtents16()'
> and `XQueryTextExtents()'.  It expects 2 byte glyph
> indices.

These are not glyph indices. These are character codes
which are passed in the request. The X server passes it to
the appropriate font library which then maps these
character codes to the glyph codes and do the further
processing.

  "The client simply indicates values used to index the
   glyph array."

In this sentence 'values used to index the glyph array'
means 'character codes' which are used to index the glyph
array using some mapping table (e.g., cmap table in
TrueType font) in the font.

> 
> For "linear" (single byte) glyph indices, the X library
> makes the MSB
> of each 2 byte glyph index to be zero (i.e. linear
> encodings are
> treated as row 0 of a 2-D glyph matrix).  All this is
> explained in the
> X Protocol specification.  [See Page 37,
> QueryTextExtents]

Please read the first sentence in
   X Protocol Specification, Glossary, pp 37

"This request returns the logical extents of the 
specified string of characters in the specified font".
          ^^^^^^^^^^^^^^^^^^^^

Let me explain this through an example. Client passes a
string of characters, e.g., "Hello World", in
XQueryTextExtent. Xlib will convert it into 16-bit string
before sending it to Xserver in 'QueryTextExtents' request.
At this place no conversion from these character codes to
glyph codes is done. At the server side, proper font
renderer (font library) is chosen (see xc/lib/font). This
font library then gets glyph ids and other glyph
information (glyph metrics etc.) from this character string
using a mapping table stored in the font. Font library then
passes this information back to the XServer which then
processes the request further and finally either send
reply/error/event or fulfill the request (as in case of
XDrawString).

In the mapping table of the font, character code is not
necessarily same as glyph code (glyph id). For example,
character 'A' which has character code 65 may be at glyph
position 10 and thus having glyph code 10. In the font
table, there is a mapping from character code 65 to glyph
code 10.

If you are still not happy with my explanation, then put a
'printf' sentence in the function XQueryTextExtents and see
the values passed in the request. :)

> ks> In my view, it is compatible with the X Window System
> protocol.
> 
> You seem to have ignored the part of the X protocol
> specification
> (that I had quoted in my review) that explicitly states
> that the X
> protocol DOES NOT deal with character codes and that the
> clients just
> use indices into the glyph array.

As I have explained earlier, you have misinterpreted the
sentence. How can client have knowledge about the glyph
indices? Client always pass character string in Xlib
routine.

> 
> If you want to see the effect of your changes on protocol
> compliance,
> you could:
> 
>     (a) run the X protocol test suite.  In particular,
> 

Unfortunately, I don't have test suite installed on my
system. It is not there in xc/test :( I am also not able to
locate it on XFree86 site. Will you please tell me where
can I get it from?

> You are probably the best placed in our group of
> developers to talk
> about the technology behind Indic script rendering.  I 
> am looking forward to learning from your experience.  Do 
> you have tutorial or writeup on Indic rendering that you 
> could share with this group?

Sure. Working as a group, we shall definately arrive at
some solution. I'll be happy to share my experience with
this group. I would also like to comment on various design
issues that you explained in one of your earlier mails. 
There are some documents on Indic rendering (not written by
me). I'll send you pointers. I'll also give the document
written by us. We also have developed a series of printing
tools that can produce high quality PS file using outlines.
It uses OpenType font and supports UTF-8, ISCII, and UCS-2
(Little-Endian and Big-Endian) encodings. I am looking
forward for your feedback on these tools. I'll also
register all our projects on sourgeforge.

Regards,
Keyur


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