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Document Type: FAQ
Original Author: Bentley Technical Support Group
Legacy Document Number: 6212
Document Type: FAQ
Original Author: Bentley Technical Support Group
Legacy Document Number: 6212
A very good starting point is to download the V8 Particle Tracing Tutorial. This tutorial introduces the basics of particle tracing and follows on to give you pointers on what settings to use and how to improve your images.
Color blotches or spots in your image indicate that you need to use more particles. The unevenness is caused by not enough particles hitting the element to light it evenly.
No, you can use the Add more particles and redo mesh option. This can be accessed via the second icon (from left) in the Render tool dialog box, or from the Action option menu in the Particle Tracing dialog box.
Particle Tracing uses real world lighting values, and it calculates also the light reflected from walls and other objects just as happens in real life. After Particle Tracing, you can adjust the image with the Brightness Multiplier/Adapt to Brightness setting (followed by Display current solution), or interactively with the Brightness slider that is part of the Visualization Stream enhancements.
As in real-life, light from the Sun (Solar lighting) is so much brighter than normal interior lights that the effect from them is washed out. If you want mainly interior lighting, with some exterior, turn down the brightness of the Solar lighting (or any Distant Lights that you have on). Alternatively, use the Brightness Multiplier/Adapt to Brightness option to adjust the image (using Display Current Solution after changing the setting). Better still, if you have the Visualization Stream enhancements, use the Brightness slider to adjust the image interactively.
To get sharper shadows from Particle Tracing alone, you need to add more particles, or reduce the Smoothness setting. Reducing Smoothness, however, may introduce some noise (splotches) into the image, so you have to balance the two settings.
Another option is to turn on Ray Trace Direct Illumination. With this setting on, you get sharp shadows from any direct illumination, while still getting soft shadows from the indirect illumination (that is from the light reflected off other objects in the scene).
Particle Tracing can produce a complete solution. When Ray Traced, these produce images that include both diffuse and specular reflections, as well as caustics, such as light reflected by mirrors or focused through lenses. As well, Particle Tracing is better suited to larger designs because it writes its computations directly to disk.
Radiosity solutions, when Ray Traced, display only diffuse reflections and specular reflections/refraction, without caustics. As well, Radiosity works in RAM, rather than on disk. This restricts the size of models that can be used effectively with Radiosity.
Color displays on monitors can vary substantially from system to system. When we are dealing with photo-realistic images, this can create problems where an image looks life-like on one monitor, but unrealistic on another. Currently, there is no "standard" that makes all display devices display colors exactly the same. Because of this, we need some way to "calibrate" our monitors to produce some degree of uniformity, such as setting the colors to match a standard color card. Gamma Correction alone will not calibrate a monitor completely, as it applies to the intensity (brightness) of a pixel, but not its hue.
For the purposes of rendering and displaying images on screen, Gamma Correction does give us some degree of control to set up our systems to display images with a similar brightness on different monitors. If the images were to be displayed only on one system, then there would not be a problem. You could "tweak" the gamma (and other) settings on that system to get the desired result. Monitor display correction can be applied:
Typical settingsCRT type monitors require a gamma correction value in the range of 1.8 to 2.5, while for most LCD type monitors this can be left at the neutral value of 1.0.
Gamma settings and banded renderingWhen you create images, using banded rendering across two or more networked computers, the gamma correction factor is taken from the Save Image dialog box. The possibly varying gamma correction settings of the different machines, therefore, will have no effect on the saved image.
Viewing saved imagesWhen you view your saved images via the Display Image dialog box (Utilities > Image > Display), you see the image without MicroStation's view Gamma Correction. Any gamma correction that you specified at the time of saving will be part of the saved image, but the view gamma correction is ignored. Of course, if gamma correction has been set at the system level, this will have an effect on the displayed image.
For further researchA very good website to investigate more fully the effects of gamma correction, how to calibrate monitors, and other related topics is http://selectservices.bentley.com/technotes/faqs/www.aim-dtp.net.
Typically, the default settings are a very good starting point. Often, the only setting that needs to be changed is that for "Use...Million Particles". Even so, it is a good idea to start with the default of 1 million to have a preview look at the image, before adding more particles (and taking more time). Using the Add more particles and redo mesh tool, you can add more particles rather than starting again from scratch.
To set your Particle Tracing settings to the defaults open the Particle Tracing dialog box, select Interface > Reset Default Settings.
It is unlikely that the default setting of 1 million particles will be enough for a finished presentation image, but it does let you see, in a relatively short time, whether there are any glaring mistakes. These may be materials missing, or lights not correctly set up. If everything is satisfactory, you then can use the Add more particles and remesh option to keep adding particles until you have the quality of image that you require. Typically, this may be 10-50 million for small or simple models through to 200-500+ million particles for bigger, or more complex models.
Where an "overall" view is required, often you can use a smaller number of particles compared to when you want the ability to zoom in on particular areas of the model. The following image has been Particle Traced with 1 million particles. You can see "splotches" on the walls and other imperfections, due to not enough particles being used.
When the value is increased to 50 million particles (see following image), there is an improvement, with the walls looking much smoother. Notice also that the sofas in the foreground don't have the "rainbow" effect that was evident in the first image. When you get this effect in your images it is a sure sign that you need to increase the number of particles used.
The relationship between Particles and Hit Points
While one of the most important settings for Particle Tracing is the number of particles used, in fact it is the number of hit points (from these particles) that determines the quality of the resulting image. One particle can generate numerous hit points as it bounces around the model. Particles that do "hit" the model, are absorbed, reflected, refracted/transmitted by the material, and thereby "bounce" around the model. Particles that do not hit the model have no bearing on the result. Unfortunately, there is no magic number of hit points that automatically gives realistic results. It depends on many factors, but especially the complexity of your design.
Take, for example, the situation where you found that a particular number of hit points on a design of an office building, was exactly right to get a realistic solution. If you then added a second copy of the same building, the chances are that you would need about twice as many hit points to get a similar quality image of both buildings.
Typically, to get the best results you need hundreds of millions of hit points for a moderate to large design. In the example images discussed above, 1 million particles produced more than 3 million hits (remember the light particles "bounce" around the model. Similarly, the 50 million particle solution produced more than 150 million hits (again 3:1 hit points to particles).
Fortunately, the particle tracer is reasonably linear and predictable in performance. For example, if you double the number of particles, it takes about twice as long to process (from start to finish).
When running PT, the system decomposes the model into triangles (in memory), onto which particle hit points are recorded. The time needed to mesh each of these triangles is directly related to the number of hit points on it. So, the more hit points it has, the longer it takes to mesh.
There are several areas that should be watched in order to speed up the meshing:
MaterialsDo not make materials overly reflective. In nature, very few materials reflect more than 70 or 80% the light they receive. This applies to textures as well as base colors.
When creating a material definition, the Specular setting is used for both specular reflection (in conjunction with the Reflect setting) and for specular highlights (in conjunction with the Finish setting). Even a small amount is enough to slow down the calculations.
The number of light sources and number of area light samples also can affect Specular material calculations. Similarly, turning on Anti-aliasing will further complicate the processing, because it takes even more samples per pixel. As a rule, avoid superfluous use of specular materials. You can do this by:
Mesh DetailIf sharp shadows are not important, you can lower the Mesh Detail setting from its default of 2.0, down to 1.5, or even lower. This will give better performance with only some loss of detail. Conversely, it should never be necessary to raise Mesh Detail any higher than 2.0, because it really slows things down and usually there is no noticeable improvement (Mesh Detail can be any value from 1.0 to 5.0, but values above 2.0 may need to be keyed in, depending on the version of the software - visualization stream enhancements).
Typically, in Particle Tracing, faceted geometry occurs on curved surfaces due to insufficient particles being shot and recording hit points on these surfaces.
As with other rendering modes, when a view is Particle Traced, the geometry first is decomposed into a polygon mesh. The Stroke Tolerance setting (Rendering Settings dialog box) controls the size of the polygons forming this mesh. Decreasing the Stroke Tolerance will decrease the size of these polygons and increase processing time, but it will not increase the number of hit points on the surfaces.
Reducing the Smoothness setting will help to reduce the amount of faceting, but with the possible increase of "noise" (splotches) in the image. For the best result, with Particle Tracing, use more particles to reduce faceting.
Stroke Tolerance is set from the Rendering Setting dialog box (Settings > Rendering > General) or via the General tab of the Rendering Setup dialog box (Settings > Rendering > Setup). Mesh Detail is set via the Meshing Settings section of the Particle Tracing dialog box (Advanced Settings).
Stroke ToleranceDetermines how curved surfaces are broken up into a triangulated polygon mesh. It defines how far from the original surface any one polygon can be. This determines the geometric shape of a surface in the rendered view. Smaller settings produce surface meshes much closer to the original surface, at the expense of longer processing time.
By default, Stroke Tolerance is set as a value in pixels, which means that the size of the geometry as seen in the view being rendered determines the accuracy of the surface representation. If you later zoom in on a part of the view, and redisplay the Particle Trace solution, some curved surfaces and edges may appear faceted. This is because the rendering database was created at the accuracy of the zoomed out view. This same phenomenon can occur when you are rendering a camera view, because the camera is turned off (in memory) prior to processing, which may cause the geometry to appear much smaller in the view.
OverrideIs relevant only for Ray Tracing (with Render All Objects enabled), Radiosity, or Particle Tracing. It lets you override the Stroke Tolerance setting in pixels and specify the maximum deviation from the original surface in Master Units. This ensures that all surfaces are treated the same, no matter what the magnification of the view. This is the safest option when using camera views, or when creating animation files where various magnifications will be used with the existing solution. You can set the Override to a figure that ensures the surfaces display correctly at all required magnifications. Typically, the closer to the curved surfaces that you expect to go, the lower the Override setting should be, bearing in mind that lower settings produce longer processing times.
Mesh DetailControls the fineness of the Particle Tracing mesh on the surface. It doesn't change the shape of the surface. Typically, you should leave the Mesh Detail setting at its default value of 2.0. If you want to lose some image quality in return for faster performance, you could try lowering it to 1.5 or even 1.0.
There are times when you want to have your model illuminated "generally", with very soft shadows (as though there was a lighting dome over it). One way of doing this is to use Solar Lighting, with the Altitude Angle set to 90 degrees, in conjunction with added sky light. By adjusting the settings for Cloudiness and Air Quality, you can further "tweak" the image to produce various effects, as required.
In the following sequence of images, using such a lighting setup, the default Sky samples settings were used for Particle Tracing - 145, and Ray Tracing - 32. In each case, the Particle Traced image is shown on the left, with the Ray Traced image on the right.
Sky Openings are created in a similar fashion to an Area Light source. That is, you create a polygon, and then use the Define Light tool to convert this element to a (directional) Sky Opening. Like light sources, you can turn Sky Openings on or off as required.
In Particle Traced images, Sky openings are a means of "focusing" particles shot from Solar or Distant light sources on to a particular part of a model. They ensure that particles are not "wasted" on parts of the model not required for the image being produced.
In operation, each sky opening (that is turned on) is assigned a number of particles to shoot, based on its projected area as seen from the light source. The particles then are shot from random locations on each opening (in the appropriate directions). Sky Openings themselves don't receive or reflect any particles.
Interior viewsYou should use sky openings to force any Solar and Distant light particles to be shot through openings into the interior scene. You can use more than one sky opening. For example, you can place sky openings in front of each doorway and/or each window, to concentrate the particles through these openings. Without the sky openings, many particles from the Solar and Distant lighting would be "lost" illuminating the walls outside the room or building. Where the external Solar or Distant light is shining through small openings then this is even more important and can make an enormous difference to the number of hit points recorded in the required part of the model.
In the following image, 300 million particles were shot, but no sky openings were used.
We can see that light is coming in through the windows, but there is not enough to illuminate the interior properly. In fact the 300 million particles produced only 132 million hits. In the following image, showing an exterior view of the model, we can see that many of these particles that produced hits were wasted on illuminating the exterior walls.
By adding sky openings in front of each window, we see the result (following image), which is produced from the same quantity of particles, 300 million.
This time, the 300 million particles produced 427 million hit points. The sky openings forced all the particles to pass through the windows to the interior. Looking at an exterior view, we see that none were wasted on the exterior walls.
Exterior viewsWith exterior views, again you can use sky openings to maximize the number of hit points used in the calculations. A good general technique to use is to place sky openings in the form of a box, surrounding (just clear of) the model, or the part of the model required for the image. If you are particle tracing just part of the model, you should take into account any possible effects from nearby objects. That is, the sky openings should envelope all objects that may impact on the image.
Remember, also, that Solar and Distant lighting only shines "down", so there is no need to place a sky opening underneath the model. In the following image, no Sky Openings were used. From a setting of 50 million particles, only 42 million hit points were recorded.
When Sky Openings are placed around and above the model as shown in heavy line weight black rectangles in the next image, the number of hit points from the same 100 million particles increases from 42 to 111 million.
Looking at the resulting image, at this magnification it looks similar to the first image (note the darkened band at the top and top right, at the limit of the sky openings). At this magnification, there is not much to show for the extra hit points.
When you zoom in closer, to a darker area in particular, the difference is easily noticeable, and shows the advantage of the extra hit points. Without Sky Openings, a close up shows that many more hit points are required for the columns, arches, and the foliage in the foreground.
Although the image produced with Sky Openings active also needs more particles (hit points), it is more advanced than the first image. In this second image, you can see that the arches, columns, and foreground foliage already are better defined.
When you turn on Add Sky Light to all Solar and Distant Lights, in the Global Lighting dialog box, the Sky Samples setting determine how many "patches" the sky hemisphere is divided into to simulate the sky. They apply only to Solar and Distant light sources.
For simple scenes, using Particle Tracing, the default setting of 145 sky patches is sufficient, but there are cases where you may need to increase this number to get better accuracy.
One such example would be a room with very small windows. In that case, you may have to increase the setting to make sure that light from at least one sky patch gets through the window.
Particle tracing, without Ray Trace Direct Illumination, is most efficient at handling large numbers of sky samples without losing performances (the same is true for large numbers of area light samples).
In general, how many sky samples you need depends on the rendering mode, each of which has a default setting on the Sky Samples option menu.
If you are using Particle Tracing without Ray Trace Direct Illumination, then the default of 145 samples should be sufficient, and efficient in terms of performance, though the number of samples really should not affect performance in this case.
If, however, you have Ray Trace Final Display enabled, and your materials have a specular component, this could slow down the ray tracing portion of the process as checks are made for specular highlights.
Area lights simulate diffuse lighting like that produced by fluorescent tubes, for example. The Samples setting is used by the Ray Tracing process (only) to "soften" the shadows and give a more natural look. Increasing the Samples setting, while softening the shadows, also increases processing time. The following images show the effect of various values of Samples from a single rectangular Area Light source. In each case the images were Particle Traced with Ray Trace Direct Illumination turned on.
Looking at the three images, notice that setting Samples too low produces individual shadows (at this magnification). With the maximum setting, this is not noticeable, but the processing time is much longer. If using this feature, it is a good idea to render a simple example to see what setting is most suitable for your model and view magnification.
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