Reduce server load, response time and transfer size, when using AJAX selections

[ksgeograf]

Currently the mapguide server operation GETDYNAMICOVERLAYIMAGE is used to render the map, when a selection is made.

This requires that the server re-renders the image and transfers the whole image to the client.

It would be a lot more efficient if the server only produced a transparent overlay image and returned that. My testing shows that it is possible to produce the desired image by a simple process:

1. Edit the MapDefinition xml, and set the alpha value of the background color to 0 (transparent).
2. Show the map, having only one layer visible and selectable.
3. Select the feature in the visible layer.
4. Toggle the layer invisible in the legend.

The returned png is transparent and contains a nicely alpha blended selection overlay.

A sample measuring indicates that the normal selection procedure requires 350kb datatransfer and 1.2 seconds delay. The overlay-only version requires 7kb data and 300 ms.

This can be implemented, either by a new operation : "GETDYNAMICOVERLAYSELECTION" or by an aditional parameter to GETDYNAMICOVERLAYIMAGE : "SELECTIONONLY".

Eiter way, it should do the same as the current GETDYNAMICOVERLAYIMAGE but:

1. Ensure the background is transparent
2. Not render any layers, only the final selection part

[jbirch]

This enhancement is also reflected in

​http://trac.osgeo.org/mapguide/wiki/MapGuideRfc38

[jbirch]

I believe that #343 has superceded this ticket