EarthMap1

Keywords: space situational awareness

Summary

This demo shows how to create an object that represents a miniature Earth for visible region simulations only.

Details

The scenario uses an reflectance property that allows RGB images to be interpreted as RGB reflectances. This can be useful for working with 3D models generated by the computer graphics community.

Important Files

Scene Geometry

The scene geometry uses the built-in plane and sphere objects. The sphere object is used to create the Earth, however the radius is only 50 meters in this example (the user can scale this sphere to be full-sized if they want). All the geometry is defined in the demo.glist file.

Materials

The key configuration in this simulation is the material description for the earth object. The reflectance for this object is computed from the 16,200 x 8,100 pixel RGB map of the planet Earth, which was obtained from here: http://www.shadedrelief.com/natural3/pages/textures.html

A copy of the JPEG image file is in the maps folder.

The sphere is assigned material ID #2, which has the following material configuration:

MATERIAL_ENTRY {
    ID = 2
    NAME = Earth Map
    DOUBLE_SIDED = TRUE

    SURFACE_PROPERTIES {
        REFLECTANCE_PROP_NAME = RgbImage
        REFLECTANCE_PROP {
            IMAGE_FILENAME = earth_rgb_16k.jpg
            SCALE = 0.00392157
            UV_PROJECTOR {
                ORIGIN = IMAGE
                FLIPX = FALSE
                FLIPY = FALSE
                EXTENDX = MIRROR
                EXTENDY = MIRROR
            }
        }
    }

    RAD_SOLVER_NAME = Simple
    RAD_SOLVER {
        QUALITY = Low
    }
}

The MappedRgb reflectance model interpretes the RGB pixel values as reflectances for the red, green and blue wavelengths (0.65, 0.55 and 0.45 microns, respectively). The SCALE variable scales the RGB image digital counts from 8-bit values (ranging 0 - 255) into hemispherical reflectances. In this case, the value of 0.00392157 would scale a digital count of 255 to a DHR of 1.0. Note, that this is probably a conservative scaling.

The second major aspect of the material configuration is that it employs the Simple radiometry solver. We have set the quality to Low because this is a simple demo and there are no major background objects for the earth sphere to interact with.

Note	Both the MappedRgb and Simple radiometry solver must be manually configured by hand editing the material file.

Setup

To run the simulation, perform the following steps:

Run the DIRSIG demo.sim file
Load the resulting demo.img file in the image viewer.

Results

The output of simulation is shown below in the built-in image viewer using the Bandwise Min/Max Scaling method.

Figure 1. Output image.