[postgis-users] AsSVG() and precision control

[strk at refractions.net]

On Thu, Oct 14, 2004 at 10:06:23AM +0200, Klaus Foerster wrote:
> the current version of AsSVG() allows specification of the number of 
> significant digits as
> second argument. this is meaningful on a single value, but looses meaning 
> when coordinates
> are taken into group (e.g. different number of digits in x, y coordinates 
> or relative
> path-notation). i've written a patch for postgis_svg.c that uses the 
> following approach:
> 
>  if you omit the second argument, values are preserved as they are and 
>  formatted as %.*g,
>  if you use a second argument, values are formatted with desired precision 
>  using ".*f"
> 
> you can find a patch at http://svg.cc/pg/assvg/assvg-0.5.patch along with a 
> test-page,
> showing sample queries and their results at 
> http://svg.cc/pg/assvg/1_output.txt
> 
> having something like apply_grid() in postgis (1.0?) would make this 
> solution obsolete, but
> as long as it's not there, i think we should provide a way to specify 
> precision within AsSVG().
> any other thoughts on that subject from the rest of the group?
> 
> klaus

Your patch does not do what reported:

 if (precision >= 0) sprintf(result, "M %.*f %.*f h%.*f v%.*f h%.*....
 else sprintf(result, "M %.*g %.*g h%.*g v%.*g h%.*g  ...

Precision defaults to 15, so you need to give it an explicit negative
number for the %g to take effect. Default would be 0 padded numbers.

What I see is that the problem only arises for relative moves, where
number of significant digits wants to be different between absolute
and relative values.

The apply_grid() won't solve this as absolute numbers will fall on a grid,
still relative values would be hard to control in precision.

A proposal I can make is that:

	- On "absolute" are requested all values will have 'precision'
	  number of significant digits

	- On "relative" way initial (absolute) values will have 15
	  significant digits, and relative values will have 'precision'
	  number of significant digits.

Do you think this would solve the problem ?

--strk;

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