RE: Metapost - text alignment

["Young, Robert" <Robert Young dsto defence gov au>]

> > > It would be an excellent idea to implement draw_text_line.
> > > You should then be able to drop draw_text entirely and have 
> > > draw_string call draw_text_line like in diagdkrenderer.
> >
> > OK, I've taken more of a look. From my reading of DiaRenderer.c, 
> > draw_text calls draw_text_line for each line. This is where the 
> > metapost output becomes a problem - if I am rendering $A_{BC}$ in 
> > metapost, centre aligned, then draw_text subtracts half the length
> > (text_line_get_alignment_adjustment) and tells 
> render_metapost.c (via 
> > either draw_text_line or draw_string) that it's left aligned at the 
> > aligned_pos - then metapost renders the text in the wrong position 
> > because the length correction was not really correct.
> >
> > Am I reading the code correctly? Should I implement draw_text in 
> > render_metapost.c so that I get to control the text 
> positioning in the 
> > metapost output? I'm in the "let the export renderer control 
> the text 
> > positioning" camp, if that makes more sense? I note that svg export 
> > also effectively removes the alignment on export.
>
> Unless DIA goes so far as to have some tight integration with 
> a TeX renderer, it will never be able to properly calculate 
> the size of $A_{BC}$, which makes proper centering impossible.
>
> MetaPost has two commands for positioning text.
>
> label.xxx   or   draw
>
> The label command will do the left right positioning 
> correctly, but it is based on the bounding box, not the 
> baseline, so it will not get the vertical positioning correct.
>
> The draw command is based on the left hand end of the baseline.
> Vertical positioning will be correct, but unless you can 
> calculate text width, you won't get horizontal positioning right.
>
> I am of the opinion that draw is the better way to go.  Most 
> text will not have special characters or formulas, so I think 
> it is most important to get the common case right (rather than 
> none of them).  If a user is using lots of special LaTeX 
> formatting, then manually adjusting the horizontal positioning 
> isn't that big of a deal.

Rob,

I see your point, especially about the vertical positioning.

However, I'd suggest that at a minimum you would want to know where the
Metapost output will align to in the horizontal dimension?

For $A_{BC}$, if you wanted it to be aligned to the centre of a box, or
a line on a diagram, you want to know how MetaPost will position it.

My experience of Metapost is short. All the diagrams I have done in
Dia->Metapost have had text which is labeling lines, boxes, etc, and it
seemed to work OK in 0.94 and 0.95-1 (ignoring all the font scaling
problems). I could anchor the text, and know that it would then all
appear around the anchor, or to the left or right, depending on how the
alignment was set.

My thoughts for implementation were to obtain the bounding box from Dia
and then pass this to the Metapost label command. My reading of the code
is that for each string the bounding box is available, as are the size
of the ascent and descent. My understanding was that dia used the
baseline to position the text, and so obtaining the descent for each
string would allow me to calculate where to put the bounding box for
MetaPost. This would make the vertical positioning correct for standard
text.

I've read back throught mail list archive, and read your bug report on
the subject. I will test with a UML class diagram.

Thanks,
Rob.