Re: [Indic-computing-devel] Re: NCST Indix Examined

[Arun S. <ar...@sh...>]

On Tue, Feb 19, 2002 at 10:10:41AM -0800, Arun Sharma wrote:
> The existing algorithm in Xaw:
> 
> // startx = x[1]
> // pixel_width =  x[2] - x[1]
> FindPosition(textpos, startx, pixel_width)
> 	nchars = 0
> 	curpos = startx 
> 	while 1:
> 		// Uses XFontStruct for efficiency
> 		width = compute the width of the next char in the text buf
> 		nchars++;
> 		curpos += width
> 		if (curpos >= startx + pixel_width)
> 			break;
> 
> 	// everything starting from textpos to textpos + nchars is "selected"

I've studied how gtk implements this algorithm. The algorithm can be
found at:

gtk/gtktext.c - find_mouse_cursor_at_line()

The algorithm is very similar, proceeding one character at a time (in
other words, broken for (context sensitive) Indic fonts).

It has a 256 byte cache which caches the widths of all the characters
in the range 0-256 (gtk/gtktext.c - find_char_width()) per font. (Hint:
Ammo for those of you looking for "latin1 bias" :)

For everything > 256, it delegates it to gdk, which delegates it to
XTextExtents(). XTextExtents is a network efficient function, in that it
is capable of responding to requests locally (unlike XQueryTextExtents, 
which has to consult the X server).

The way IndiX has implemented it, XTextExtents is mapped to
XQueryTextExtents - sacrificing network efficiency. In other words, when you
drag your mouse, gtk will execute the above loop and a X protocol
request is made for every character to compute its width.

A new protocol request - such as XComputeWidth (Is XComputeNChars a
better name ?) will batch these XQueryTextExtent requests into a single
request. Keyur, I didn't understand your comment on why this
functionality doesn't belong to the X server. Or did I misread your
statement ?

Another data point: Using character codes is NOT unprecedented in
XFree86. I just finished reading the i18n specs.

http://www.x-docs.org/i18n/Framework.pdf

and looked at the implementation:

xc/lib/X11/omDefault.c - _Xutf8DefaultDrawString

The input is clearly a UTF-8 string. It's calling _XmbDefaultDrawString
which is calling XDrawString, which is doing a PolyText request in the X
protocol. In other words, a character code is being sent over the X
protocol.

On a related note, can anyone on the list enlighten me on the difference
between utf8 and mbs (multi byte string) ? I thought UTF-8 was a mbs
too.

	-Arun