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Topic: Re: Tc & Tw correspondance..
Conf: (P-PDF) Developers, Msg: 58612
From: LeonardR
Date: 5/29/2002 05:33 PM

At 1:12 PM -0500 2/23/01, p-pdf-developer Listmanager wrote:
>I would like to understand the link between the Tc & tw operator
>with the value contain in an array of string like this :
>
>11.0391 0 0 11.0391 90.0332 688.6886 Tm
>-0.0015 Tc
>0.7671 Tw
>[(1.)-6.3(1.)-6.3(1)11.2( Logi)-6.3(cal)-6.3(
>)760.8(st)-5.5(ruct)-5.5(ure )]TJ
>
I'm probably going to miss something since I'm doing this w/o
looking at my sources, but the following should be helpful (I hope).

First, as Aandi points out, Tc sets up the current spacing
between characters while Tw sets up the spacing between words. These
are used by the text placement "engine" to determine where to place
each character in a string - whether it's a simple ()Tj or a complex
[]TJ.

The general case of rendering text looks something like this:

MoveTo( Text Start Location )
for each character in the string
ComputeWidth( c ) // font size * scaling * charWidth
+ charSpace
if IsWordBreak( c ) then width += wordSpace
Move Relative( width )
DrawChar( c )

Given that core rendering routine, think of the handling of a
[]TJ as a preprocessor that breaks the TJ array into commands (the
numbers) and then strings to process. When the preprocessor finds
an number, it adjusts the current pen position by that value adjusted
by the current font size ( -x * 0.001 * fontSize ). When it finds a
string, it follows the above algorithm (in fact, in probably simply
passes the string to the Tj handler ;).

One thing that isn't specifically mentioned above is that
when computing coordinates, you have to also take into account the
current Tm and Ctm.

Hope that helps!

Leonard


--
----------------------------------------------------------------------------
Leonard Rosenthol
Sr. Software Engineer (215) 922-3509 (voice)
Digital Applications (215) 440-0504 (fax)

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