Ray Tracing

dEo	dΦs	,	(5.2)
	= ---------
	dAo

where dAo is an element of area on the output plane.

In reverse ray tracing, the “flux” of the ray emitted from the output plane is unknown, only the AΩ product, étendue, of the ray is known. If the ray hits the source, the flux of the ray at the output plane can be computed as

	Φo =	LsAoΩoτ	,	(5.3)
		--------------------N -

where Ωo is the solid angle of the ray as it is “emitted” from the output plane, i.e.

the solid angle subtended by the importance sampling target, and τ is the “transmittance” experienced by the ray along its path from the output plane to the source. Rays missing the source are not “seen” at the exit surface since the path would not have come from the source. For example, if a ray is emitted from an exit surface, reflects from a reflector with reflectance of 0.9 and passes through two glass surfaces each with transmittance 0.96 before striking a source surface, then the ray’s transmittance is 0.9 x 0.96 x 0.96 = 0.829.

Luminance/Radiance Ray Tracing Standard Expert

Luminance and Radiance Maps will be covered here as the third ray trace method since they are so closely linked to the ray trace. The options associated with the view of the data is covered in Chapter 6. See “Luminance/Radiance Maps” on page 6.14

You can obtain a Luminance or Radiance Map or rendered image (Photorealistic Rendering) of a TracePro model that is illuminated by the actual surface sources and uses the surface and material properties in the model. You can generate as many maps/renderings of a model as you like by defining a name for each one.

Luminance/Radiance maps work only with surface sources, not with grid or file sources.

Note: The wording of the Luminance/Radiance tab in the System Tree window is automatically set to either Luminance or Radiance by TracePro depending on analysis units of the model (see page 5.35). For convenience, Luminance,

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Tracing Rays

Radiance and Photorealistic Rendering will be referred to as Luminance for the remainder of this section.

FIGURE 5.18 - Luminance Tab in the System Tree with Luminance defined map displayed in the Model Window.

The plots are defined either from the Luminance tab in the System Tree by right clicking in the window or from the Define|Luminance/Radiance menu item. The Luminance tab is used to display all of the defined plots and to edit or add additional plots to the model, or to delete plots. The Luminance tab is shown in Figure 5.18.

Remember that the title of the dialog box will be Radiance if radiometric units are selected, or Luminance if photometric units are selected. You can choose the units by selecting Raytrace|Raytrace Options. The Options tab contains the selection for radiometric/photometric analysis units.

TracePro 5.0 User’s Manual

5.31

Ray Tracing

FIGURE 5.19 - Luminance dialog.

The rendering is accomplished by tracing rays in reverse, as is done in computer graphics programs. To set up a luminance map, specify the view point and the target point and ray tracing parameters by the following steps:

1.Open a model for which you wish to generate a map or rendering.

2.Orient the view using the View menu commands to an orientation from which you would like to generate the map/rendering.

3.Select Define|Luminance/Radiance from the TracePro main menu to open the Luminance/Radiance dialog box as shown in Figure 5.19.

4.Enter a name in the dialog box.

5.To import the eye point, target point, up vector, width and height from the current view of the Model Window, click the Model View button. Otherwise, enter the eye position, target position, up vector, width and height you wish to use.

a.The eye point is a Cartesian (x,y,z) point in space representing the point of view from which the scene will be generate, and can be thought of as a camera position.

b.The target point is a Cartesian (x,y,z) point in space representing the center of the rendering/map that will be generated and can be thought of as the position towards which the camera is pointing.

c.The up vector defines what direction is up in the rendering/map.

5.32	TracePro 5.0 User’s Manual