= !MapGuide RFC 69 - Rendering Layer Styles using Style Order   =

This page contains an change request (RFC) for the !MapGuide Open Source project.  
More !MapGuide RFCs can be found on the [wiki:MapGuideRfcs RFCs] page.


== Status == 
 
||RFC Template Version||(1.0)|| 
||Submission Date||Jun 26 15:40:00 2009 ||
||Last Modified||Zac Spitzer [[Timestamp]]||
||Author||Zac Spitzer||
||RFC Status||proposed||
||Implementation Status||||
||Proposed Milestone||2.3||
||Assigned PSC guide(s)||||
 
== Overview ==

Correct rendering of road networks requires ordering. Major roads should be rendered
over the top of minor roads.

== Motivation ==

Currently MapGuide Studio exposes the ability to order individual styles within a layer, [[BR]]
however, the server doesn't support ordered rendering. 

There are three possible known work arounds:[[BR]]

1. Break the road networks into separate layers.[[BR]]
2. Render the roads using composite type styles and specify the RenderingPass appropriately (see [http://trac.osgeo.org/mapguide/wiki/MapGuideRfc29 RFC 29] for details).[[BR]]
3. Set up the data source so that it returns readers with the features in the correct draw order.[[BR]]


||[[Image(mapguide studio support for style ordering.png)]]||[[BR]]

Adding support to respect this ordering is considered to resource expensive, as many
individual queries would be required.

||[[Image(sample map tile with unordered style rendering.png)]]||

== Proposed Solution ==

Add support to the rendering engine to render layer styles in ordered passes.

After some discussion, it appears that implementing ordering of results from FDO
could be an efficient way of approaching this. This avoids requiring MapGuide to
issue multiple FDO queries.

The question has been raised about the support for ordering within other 
providers such as SHP and FDO.

This would require a change to FDO [http://trac.osgeo.org/fdo/wiki/FDORfc40]

As this will add overhead, by taking the ordered approach, this could be turned
on by an optional field defining ORDER BY column(s) in the layer definition . 

== Implications ==

The current approach is to make this an optional enhancement, as to avoid impacting all layers.

== Test Plan ==

None.

== Funding/Resources ==

Help wanted

== Related Trac Tickets ==

[http://trac.osgeo.org/fdo/wiki/FDORfc40]
#618 Theming stylisation order isn't respected in a layer[[BR]]
[http://trac.osgeo.org/fdo/ticket/516  FDO Core enhancment: support standard ORDER BY expressions]

== Discussions ==

[http://n2.nabble.com/MapGuide-RFC-69---Rendering-Layer-Styles-using-Style-Order-td3158848.html][[BR]]
[http://n2.nabble.com/RE%3A-MapGuide-RFC-69---Rendering-Layer-Styles-td3159460.html]