Re: pngcairo: suggestion for additional option

[Ethan M. <merritt@u.washington.edu>]

On Wednesday 21 November 2007 09:13, Allin Cottrell wrote:
> I'll float this by the list first and submit a tracker item if 
> there's any support.
> 
> I would find it very useful to get a readback from gnuplot on the 
> internal dimensions of a PNG file -- that is, the pixel bounds of 
> the data area and also the data minima and maxima.  This is for 
> generating an interactive display (much like the mousable wxt 
> terminal display, but with additional features).

I don't think I quite understand what you are aiming for.

My first take was that you were suggesting that pngcairo is missing
the "crop" option of the libgd-based png terminal.
In general the gnuplot core routines will not know this information.
Consider the case of "plot with labels", where the label is rendered
(by cairo/pango in this case) as a scaled glyph.
What pixels are touched in drawing that glyph?  Gnuplot doesn't know.
It would have to be provided by the support library.
 
> The attached patch adds a boolean "boundsfile" option to pngcairo.  
...
> you get test.png as usual, but also a file named test.png.bounds
> with content
> 
> xleft=89 xright=620 ybot=37 ytop=460
> xmin=-10 xmax=10 ymin=-1 ymax=1

But then I backtracked and thought maybe you are asking for output of
the    [terminal coordinate] <--> [plot coordinate]
mapping that is used by the mousing code.  Is that the idea?
See for example the term->text() driver entry point for x11.trm
(X11_text).  That is exactly what it does.

I thought there was an ancient patchset that implemented this for
the libgd-based terminals by placing the information in a comment
record inside the image file proper. That might make more sense,
and as with the current x11 driver it would become a normal part of
the term->text() processing.  No special command-line options needed.
Unfortunately I can't seem to find that patchset on SourceForge any
more.  Also I don't know if cairo provides a mechanism to add 
arbitrary comment records to a PNG file.

Finally, let me note that one of my biggest long term wish-list items
is to have gnuplot emit similar information for use with SVG plots
embedded in a web page.  The idea is to implement full browser-side
mousing/zooming capabilities for each embedded plot using jscript or
the equivalent.  See for example the pages at:
	http://sitn.ne.ch/

-- 
Ethan A Merritt