Virtual Islands II

Walkthrough

Start by scanning a copy of a student's map as per the OS article. The map should be clear and legible, with plenty of contours drawn in a high contrast colour, and saved in bmp format.

Open the Heightfield Editor. Unless you have a square map, it will be necessary to create a new grid and adjust it to A4 size by clicking File / New and adjusting the grid points on the x axis to 210 and the y axis to 295. You should also set the grid point spacing so that the total size of the grid agrees with you map.

You can use more points than this, indeed for professional purposes a greater density would be recommended, however for classroom use around 50,000 points gives very fast responses with acceptable detail.

The map now approximates A4 dimensions. To set the background to the scanned image open the 'Tracing Parameters' section then simply click the set Background button and browse for the scanned image. Make sure you have the 'show background' checkbox ticked.

A useful technique here is to open the 'Height Display' section and set the minimum and maximum settings to values appropriate for your map, as this will then give you maximum differentiation between traced contours. We also prefer to set the colour scheme to RGB for tracing as this uses a greater colour range.

There are a number of tools that can be used to generate the 3D topography, perhaps the most useful is the Trace Contour button. It is probably best to enlarge the map using the zoom tool before embarking. Select the Contour Trace button (coloured blue) and carefully click on a contour. With luck, the tool will pick out the entire contour in dark green, though if it is reluctant, try increasing the colour tolerance slider.

Set the height of the contour in the pop-up box that follows. Initially this procedure can be fiddly, but it rapidly becomes easier with judicious use of the colour tolerance slider. When all the contours have been identified, it might be beneficial to add some selected spot heights with the appropriate tool before interpolating the rest of the height points on the map.

To use the Interpolation tool, click the interpolate button on the toolbar and draw a rectangle over the area to be interpolated. Make sure that Nearest Neighbour interpolation is selected and the minimum height is set to zero. Other parameters may be left as the defaults, in particular ensure that extrapolate is unchecked.

Click OK and the program will interpolate all available points, with our example the map will look similar to the next image.

You can see that interpolation has filled in heights for all areas inside the edge of the island, i.e. at the zero height contour. The blank areas of the map are undefined. This is because we did not check the extrapolate box and interpolation has only filled in points which are between defined points.

If we had checked the extrapolation box then all heights in the rectangle we chose would have been set.

Extrapolation is useful in some circumstances, particularly with maps just containing land, but must be used with care, particularly if interpolating several small areas.

As we have an island in this case we can use the Trend Analysis interpolation method to fill in out missing heights quickly. Use the interpolation tool again but this time select Trend Analysis when the dialog is displayed.

Interpolation after using nearest neighbour.

Trend Analysis always acts on all points, so with a minimum height set the rest of the landscape will be set to zero height. Trend analysis interpolation is also useful for filling in final groups of points after using Nearest Neighbour to fill in a landscape in small blocks. Trend Analysis should generally be avoided however for interpolating large complex areas as it tends to produce a plateau-like heightfield.

The final stage defining the landscape is to add the sea around the islands. If you set the height display to RGB this tends to be easier to see if you switch back to the default colours, so after doing that select the 'Flood with water' tool and click on a sea area. GenesisIV sets all flat areas at zero height to water. You can see from this image that the sea has encroached slightly over our zero contour, our interpolation function may have decided from the contours that these areas appeared flat within the margin of error.

Once complete close the Heightfield Editor and return to the GenesisIV main screen.

The photorealistic renderer opens and renders the landscape using the same settings as the main screen. Click 'Full Photo Render' to draw the landscape using the photorealistic render.

Settings for the render are accessible from the various buttons shown in the Photorendering Options box. This image was created using the Azure Blue atmosphere presets.

The next stage could be to tweak the environment settings, add vegetation or other layers to the island, or drape the original map over the contours.