Area Overlays

When drawings that contain areas are overlaid upon terrains, Manifold will "float" the areas above the terrain and, if desired, render walls for the areas to give the appearance of extruded shapes. Areas take their color from the area background color and walls take their color from the border foreground color specified for the overlaying drawing. When representing areas as overlays, area style and foreground color are not used.

$images\sc_terrain_area_overlay_01.gif$

Such overlays are particularly handy for the schematic representation of buildings and other structures in 3D views of terrains.

Area overlays are turned on using the controls in the Terrains - Overlays dialog when a terrain is open.

Example

$images\sc_terrain_area_overlay_02.gif$

Consider a map with a drawings layer that appears above a surface. The surface is the Montara Mountain sample surface that has been colored using the View - Display Options dialog using the Altitude and Bathymetry palette.

$images\sc_terrain_area_overlay_03.gif$

The drawing consists of areas only, and has only one column in the drawing's table, a Height column that gives the height of each area above the local ground level. Note that this value is not an absolute height above sea level, but rather is intended to represent a height above ground.

$images\sc_terrain_area_overlay_04.gif$

The drawing layer has been thematically formatted for area background color using the Height value so that each area is colored according to the height. The area border has also been colored by the same thematic format.

$images\sc_terrain_area_overlay_05.gif$

If we open up the terrain for the surface and choose Terrain - Overlay we can set up the Overlay dialog to show area overlays. We click on the Paint over terrain box and make sure the Walls and Vary height by column boxes are also checked. The latter box will vary the height at which areas are rendered over the terrain by the contents of a column named Height.

$images\sc_terrain_area_overlay_01.gif$

The result is the scene illustrated at the beginning of this topic. Areas have been rendered using the colors specified in the thematic format for area background color for each area. Walls have been rendered using the area border foreground color. Area walls and areas are rendered using the Terrain - Overlay dialog settings for Opacity for areas (100% in this case), as well as lighting specified for the terrain.

$images\sc_terrain_area_overlay_06.gif$

If we were to change the Opacity setting for areas in the Terrain - Overlay dialog to 50% we could see that the extruded 3D area shapes shown in the terrain become partially transparent.

$images\sc_area_overlay_buildings.gif$

Area overlays may be combined with other effects, such as overlay of images. The above scene was created by first overlaying a TerraServer image upon the terrain and then overlaying a drawing containing areas showing building outlines. The area objects have a Height field giving the height of each building. (This image was created at Cornell University and is used by permission.)

Tech Tip

In drawings and labels, setting the foreground color to be transparent color will hide the entire object or label. Terrains respect this convention: if a drawing object or label has foreground color set to transparent color it will not appear in a terrain overlay. Likewise, if an area border foreground color is set to transparent color the area walls will also be hidden.