Pre-RFC, pulling rendering/labeling parameters from attributes...

[Steve Lime]

Hi all: Before running off and doing an RFC I wanted to gather some feedback. Currently we do
support pulling things like angle or size from an attribute. Works fine but is clunky in many ways
since you see things like angleitem and angleitemindex in the structures. Adding more support
for will just add more needless complexity (I could see folks wanting to set colors, start angles
and other stuff).

Anyway what might be nice would be to write fragments like:

  STYLE
    SIZE 'sizeitem'
    ANGLE 'angleitem'
    COLOR 0 0 255
  END

We could then use an hashTableObj called items to track the various values to pull from attributes.
At parse time the hash would contain the item names. msWhichItems could turn those into array
indicies (can the hashTableObj handle arbitrary types?) for speed. Then the rendering code could
do a quick lookup to decide whether to pull from an attribute or not.

It would be very easy to add support for new parameters then, just tweak the high-level rendering
code (before handoff to native drivers). In MapScript we'd need a method to set a style or label
items (e.g. $style->setItem('angle', 'myAngleItem');) or just use the hashTableObj interface.

The bad? Syntax change (I'd leave labelitem and classitem alone) so this would be a good 5.0 feature.
There might be a perforance hit with the high number of hash lookups.

Worth thinking about?

Steve