[mapserver-dev] SYMBOLTRANFORM for special symbols rendering

[Alan Boudreault]

Hi devs,

A while ago, it has been discussed about a new way to render a symbol, 
which in this case is an arrow. The need is to be able to draw the body 
arrow (a line) scaled depending of a value (ie, the magnitude in vector 
field) but to not scale the arrow head.

Mike, Daniel and Thomas discussed this problem in Denver during FOSS4G 
and they came up with the concept of ARROWBODY and ARROWHEAD. Those 
concepts  are essentially to determine what need to be scaled. Following 
a discussion this morning.. here's the approach we would like to use:

* We define two symbols: the arrow body and the arrow head.
* The arrow head symbol needs to have the ANCHORPOINT set to the head.
* We define 2 styles: one for the body and the other for the head.
* The two style would use a new option: SYMBOLTRANSFORM, which is to 
draw the symbol correctly (scalable of not).

Mapfile example:

SYMBOL
   NAME "arrowbody"
   TYPE vector
   FILLED true
   POINTS
      0 0
      1 0
   END
END

SYMBOL
   NAME "arrowhead"
   TYPE vector
   FILLED true
   ANCHORPOINT 0.25 0
   POINTS
      0 0.25
      0.25 0
      0 -0.25
   END
END


# in the layer definition....

STYLE
    SYMBOLTRANSFORM arrowbody
    WITDH 1
    SIZE [uv_length]
    ANGLE [uv_angle]
  END
  STYLE
    SYMBOLTRANSFORM arrowhead
    SYMBOL "arrowhead"
    ANGLE [uv_angle]
    OFFSET [uv_length] 0
    SIZE 4
  END

In the arrowhead, we *NEED* to pass the length of the arrowbody (via 
OFFSET), since it will be used to determine where the arrow head should 
be positioned. the SYMBOLTRANSFORM arrowhead option would tell the 
symbol renderer TO NOT scaled the symbol in length. In this example, we 
use the terminology arrowbody/arrowhead.... but we are going to use 
generic terms to use that feature in other cases.

I'm glad to hear your comments and suggestions about that approach!

Regards,
Alan