Visualization Guide
Visualization Guide
Visualization Guide

Lighting considerations for photorealism

MicroStation's advanced photo-realistic rendering options — Ray Tracing, Radiosity, and Particle Tracing — let you produce lifelike images of your models from within MicroStation. When you are working with Radiosity solving or Particle Tracing, you still can use the standard light source cells to provide the illumination, but both processes use the Lumens value, multiplied by the Intensity value, to determine the brightness of the light source. Thus, you can set the correct Lumens value for the light source and then use the Intensity setting, like a dimmer switch, to quickly change its brightness, if required. For Ray Tracing without a particle traced or radiosity solution present, the Lumens setting is ignored unless Real World Lighting is enabled in the Ray Tracing dialog box. When ray tracing a Radiosity, or Particle Traced, solution, the Lumens value is considered.

Where correct lighting effects are required, it is recommended that you work with IES lighting data files. Additionally, when creating Particle Traced images that include Distant light sources or Solar lighting, you can use Sky Openings to “focus” the particles on to the required areas of the model.

Source lighting cells and IES files

For photo-realistic lighting in images MicroStation supports the use of IES lighting files for Ray Tracing (with Real World Lighting enabled), Radiosity solving and Particle Traced solutions. These are available for downloading, from various lighting manufacturers, via the world wide web. IES data files apply the correct values to the source lighting cells for Ray Tracing, Radiosity solving and Particle Traced images. As well, they define the photometric web, to correctly display the spread of the light from the light source, to produce more natural looking images.

How a particular IES light should be used is up to the manufacturer who defines the data. It is not possible always to tell, by looking at the IES file text, whether or not it is intended to be used as an area light or a point light. Often, however, there are some clues, as the manufacturer can (optionally) specify the dimensions of the luminaire. While these values are ignored by the rendering process, you can use them as a guide in using the IES file properly. That is, by applying it to a light source of the corresponding size, after viewing the IES information by turning on "Show IES Detailed Text" in the Define Light tool dialog box. For example, to use the IES file properly for an IES light that has a defined area, you could define an area light of the appropriate dimensions and then, in the Define Light tool dialog box, turn on “IES Data” for the light source. Bear in mind, however, that there are provisos to applying IES data to directional light sources such as Area Lights and Spot Lights. When an IES lighting file is specified for a light source, the IES values override the current values in the source lighting cell, except that any IES light that would be outside the cone angle of a spotlight, or behind the plane of an area light, is clipped off. If this “clipped” light is importantd, then It is better to use Point Light sources when working with IES data. This will ensure that the lighting is correctly matched to the data in the IES file.

Scene illuminated with 2 “standard” point light sources (left) and with IES lighting data applied to the point lights (right).

  

In the above particle traced images, the left image is illuminated with two standard point light sources. For the image on the right, IES lighting data was applied to the two point light sources. This changed the characteristics of the point light sources such that they shone upwards and out from the wall, producing a more natural looking lighting for the model. When IES data files are used, all settings for the light source, apart from its color, are read from the IES data, so that little or no further tweaking of the light source should be required.

IES data controls only the intensity and distribution of the light, not the color. Color is not included in the IES specification.

Using drag and drop with IES data files

If you “drag-and-drop” an IES data file into a MicroStation view, the Define Light tool starts, in Create mode, with IES Data enabled and the dragged IES file selected.

Accessing IES data files

Many manufacturers of lighting fixtures provide IES data for their products. Links to a number of these manufacturers are available from the Bentley Visualization Center web site http://www.bentley.com/vizcenter/links.htm.

Sky Openings

When using Distant light sources, or Solar lighting, the light rays are present throughout the DGN file, not just in the vicinity of the model. For efficiency, you can create one or more sky openings that restrict the calculations to the light that passes through the sky opening only.

You should use sky openings in particular where you have an interior space illuminated from “outside” by Distant light sources, and/or Solar lighting. For exterior scenes also, you can use sky openings to focus the processing on the region of the DGN where the model is located.

Sky openings and Particle Tracing

When one or more Sky Openings are present in a model, all particles from Distant or Solar light are restricted to pass through the sky openings and not be dispersed throughout the DGN file. This speeds up the rendering process by letting you use less particles than would be required were no sky openings present. Both images below are Particle Traced, with the same number of particles. On the left, the image does not have a Sky Opening, so many particles are “lost” to illuminate outside the building. When a Sky Opening is created (the same size as the window), all particles are shot through the Sky Opening and, in this case, through the window, producing a better image.

Light from Distant Light shining through a window, without Sky Opening(left) and with Sky Opening (right).

With Sky Opening active, all particles are shot through the window.

  

Sky Light and Distant/Solar lighting

In real-life, light coming through a window comes not just from the Sun, but from the entire surroundings. You can simulate this by turning on “Add Sky Light to all Solar and Distant Lights”. During processing, light is assumed to be emanating from throughout a hemisphere. When a sky opening is present, the calculations are restricted to only that part of the hemisphere seen through the sky opening.

The image below has been Particle Traced using the same number of particles as the two previous images (above). As you can see, it is much brighter because the light is not concentrated from only the direction of the Distant Light source (or Solar Light).

With Sky Light added, the image is brighter.

  

Controlling the display of “hot-spots”

Both the Radiosity and Particle Tracing processes work with an expanded color range, but not so basic ray tracing. When ray tracing a radiosity or particle traced solution, you can use the following key-ins to control the intensity of hot spots in overly bright areas.

Adjusting image brightness and contrast

Once you have rendered a view with Ray Trace (with Real World Lighting enabled), Radiosity or Particle Trace modes, you can use the brightness and contrast sliders in the Render tool dialog box to fine tune an image interactively. Move the sliders to the right to increase the brightness/contrast, or to the left to lessen them. With respect to Radiosity and Particle Trace rendering modes, using the sliders is similar to working with the Brightness Multiplier/Adapt to Brightness and Contrast settings, except that it is interactive and does not require an update of the view with the Display current solution (in any view) option in the Render tool dialog box. When you use the Slider controls, the Brightness Multiplier/Adapt to Brightness automatically and Display Contrast settings automatically are adjusted to match the sliders settings.