VRML files are created by texture-mapping visible images of the region onto a terrain grid which has been scaled by five times. Elevations in the MOLA strips, in those VRML scenes which have them, have also been scaled by five times. In general, the VRML scenes are 4° by 4° (236 km by 236 km). The Regional Apollinaris Patera, Arabia Terra, Hematite Deposit VRMLs are larger.
Java software was created to extract the required subimages from the original high-resolution (256 pixels/degree) images of the Mosaicked Digital Image Models (MDIMs), and to stitch subimages together for a given region. The stitching starts by finding which MDIM image contains a given set of landing site coordinates and working outward, extracting pixels from adjacent images as needed to composite the entire region-of-interest. The same process is used to create a composite terrain grid by extracting elevation maps from the database of Mars Digital Terrain Models (DTMs).
MOLAs and MOCs
High-resolution elevation data from the current Mars Orbiter Laser Altimeter (MOLA) is included in the form of MOLA profile strips, color-coded by altitude, if present in the region-of-interest. High-resolution (2 to 15 meters/pixel) images from the Mars Orbiter Camera (MOC) are embedded in the VRML terrain as well, and are available in the VRMLs labeled "MOC lo" and "MOC hi". The MOC images used here are 1/3 the resolution of the original MOC images, and even so, will be much more resource-consuming than the non-MOC VRMLs.
Lo-Res vs. Hi-Res
While the visible image used is the same, "lo-res" versions use elevation grids which have half the spatial resolution (in latitude and longitude) of the "hi-res" versions. (Hi-res VRMLS use every other pixel from the DTM, while lo-res VRMLS use every 4th one.) While the difference is not too obvious, it may be discernible, for instance, in areas of high relief. The hi-res versions will be truer to the original digital elevation models on which the VRMLs are based. Given that VRML browsers generally need a lot of memory to run, lower-resolution versions are provided for those users who may be "resource-challenged". Macintosh users should allocate at least 32 Mbytes to their browsers for the lo-res versions, and 56 Mbytes for the hi-res versions (the use virtual memory may be necessary).
Performance Issues
Please note that VRML technology is new, and your experience with it may be "less than satisfying". You may find, for instance, that loading two consecutive VRML files will cause your browser (and possibly your computer) to crash. In this instance, clicking on another (non-VRML) link between two VRML invocations may alleviate the problem. We have found that VRML browsers for UNIX workstations are the most stable in this regard, and those for the Macintosh are the least stable. This situation should improve over time, and we recommend that you upgrade both your Web browser (e.g. Netscape or Internet Explorer) and your VRML browser (e.g. Cosmo Player) when new upgrades are available.
Also, interaction with the VRML scene, particularly for the "hi-res" versions, may be very slow. We recommend that you make viewing changes slowly. If you've lost perspective on the scene (e.g. the terrain is out of your viewport), you can always pick the "View1" view from the viewpoint control at the left of the console to return to your initial viewpoint.
It is advisable to turn off the specular highlighting to reduce visible artifacts as well as speed up rendering time. In Cosmo Player, this parameter can be accessed via the Performance pane in the preferences of Cosmo Player.
Downloading VRML Browsers
VRML browser plugins are available for the following platforms using Netscape:
| Windows | CosmoPlayer (1) | CosmoPlayer (2) | CosmoPlayer (3) | Other VRML Browsers |
| Macintosh | CosmoPlayer (1) | CosmoPlayer (2) | CosmoPlayer (3) | Other VRML Browsers |
| IRIX (SGI) | CosmoPlayer | |||
| Solaris (Sun) | Presenter |