Advanced Illumination Examples

Realsoft Graphics Oy		Advanced Illumination Examples
	Chapter 7. Rendering

Illumination Maps

Big part of real-world illumination is created by surfaces that reflect illumination from actual light sources. White walls of a room spread the light from the lamp in the ceiling to shadowed places such as behind the furniture.

The UVImage tool can evaluate and store this kind of indirect lighting to so called illumination maps. The following simple example shows how to use this feature.

1. Model a simple scene shown in the image on the right: a rectangular floor surface, one wall on the left and a table standing on the floor. Alternatively, you can load the sample scene 'tutorprojects/rendering/table'. The table consists of a cube and four cylinders. Place one point light source on top and to the right of the table. Open the property window of the light source and adjust its Intensity to level 1.0. You may also open the view property window and set Camera tab's Flashlight to black color (0,0,0). Then the only light source of the scene is the point light and shadows will be quite dramatic.

A table illuminated by a single light source

2. Next we generate the illumination maps. Go to the Materials tab of the toolbar and activate the UVImage tool. The default resolution is fine, but change the Type selector to Indirect lighting. If you have time to wait a bit more accurate computation (and better quality), increase the Samples value from 1 to 3 - 5.

UVImage options for indirect illumination

3. Select all objects for which you want to generate maps - in this case the wall, the floor and the table. Select also the point light source. This is important; surfaces have always two sides and the program must know for which side the illumination should be evaluated. The selected light source indicates it. Click Accept.

Wait until the progress indicator shows that the tool has finished. Now look at the select window: the object hierarchy has changed slightly because the tool has inserted new illumination maps to the hierarchy. The material library contains a set of new materials as well.

Object selection for illumination evaluation

4. Hit the render button to ray trace an image of the scene. The space under the table, which was previously totally dark, now has some subtle illumination. The indirect illumination is usually weak compared to direct lighting. If you wish to enhance it afterwards, you can simply reduce the intensity of the point light source to one half and double the film sensitivity of the view camera from the view property window.

The hierarchy with the illumination maps

The diffuse illumination material

You can naturally edit the diffuse illumination materials if you need to control illumination of a single object.

The tutorial example shows that illumination mapping is not part of the rendering process but a tool that changes the scene. This means that it is best suited for situations where it is necessary to make illumination of a static scene more realistic. The time consuming illumination maps are generated only once; after that, the camera can be animated to move in the scene. One can also add some animated objects whose contribution is not significant for indirect illumination.

	Note
	Before applying illumination maps to SDS objects, remember to initialize proper UV coordinates for them first.

The table scene with indirect illumination maps

Mapped Shadows

Shadow mapping is another texture mapping based illumination method. Mapped shadows usually have nice smooth edges which increase realism without the high rendering time penalty of ray traced smooth shadows. On the other hand, shadow mapping often suffers from a number of accuracy problems. It may take several rendering attempts to adjust them until the desired result is achieved.

Let us continue experimenting with the table scene of the previous chapter. The indirect illumination maps may be included, but you can also reload the table scene 'tutorprojects/rendering/table'.

1. Select the point light source and open the property window. Go to the Spec tab. Then change the Shadow casting selector from Ray traced shadows to Mapped shadows. Then render the view window to see the result.

Although shadows now appear smooth-edged, some quality problems can be detected easily. The shadow on the side wall has some banding, and the top part of the legs become illuminated through the table top! The default shadow map resolution is most likely not enough.

Selecting mapped shadows

2. Make sure that the light source is select and go back to the property window. The Spec tab shows that the default Shadow map resolution is 100 pixels (actually 100*100 pixels, the resolution is always the same in both dimensions). Change the value to 400 pixels and render again. Now the quality is much better.

3. We finish this example by examining how to control smoothness of mapped shadows. Take a look at the property window of the light source again. The bottom part of the window contains a Size field, which by default has the value of 0.1 meters. Double the value to 0.2 meters and render the view to see how the shadows change. The bigger the light source, the smoother the shadows. It is usually necessary to keep the size in a sensible value which matches the scene dimensions. A distant light source hundreds of meters away from the target should have a diameter of several meters to function properly.

Mapped shadows have smooth edges

Room Illumination

The next example demonstrates the same illumination techniques as the chapter above: indirect illumination maps and mapped shadows. This time the scene is a bit more complicated, a room interior shown below. The picture shows how the example scene 'tutorprojects/rendering/room' looks like when rendered with two point light sources (lamp, sun) and some flashlight.

First we generate the illumination maps:

1. Select the point light source inside the lamp object. Then multiselect the room level. We will generate the maps for only the largest surfaces of the scene, not for the furniture or other details.

A room illuminated by two point light sources

2. Activate the UVImage tool from the Materials tooltab. Change Type to Indirect Lighting, set Samples value to 3-5 and hit OK.

3. Wait until the UVImage tool has finished the map computation. Then select the spot light source behind the room. We will change its shadow type to mapped shadows: open the property window, go to the Spec tab and activate the Mapped shadows option. Increase the resolution to 400 pixels.

4. Since the spot light is far away from the room, increase its size to 1 meter. Large sizes will also make lighting appear smooth: the spotlight will simulate indirect illumination through a window on a bright day rather than direct sunlight.

5. When using mapped shadows with a spotlight, it is best to use a smooth falloff for the spot edge (sharp edge with otherwise smooth shadows looks peculiar). Therefore, increase the Angle2 value to 5 degrees greater than angle1.

6. Now select the light source in the lamp object and activate mapped shadows for it as well. Resolution can be again 200-400 pixels. The default size of 10 centimeters is quite realistic and suitable, so you do not have to change it.

7. Now comes an important step: There are some glass objects in the scene. Mapped shadows can take only the closest (transparent, translucent or non-transparent) surface into account. Therefore, select the sphere which surrounds the lamp light source and make it Shadow Invisible using the Gen tab options of the property window. Do the same for the glass window object as well.

8. Render a new image to see the changes in the illumination. Adjust shadow map resolutions and light source sizes if necessary.

A smoothly illuminated room interior

	Note
	Shadow mapping can be applied to spot lights and point lights. Of these two, a spot light works much more efficiently and faster with shadow mapping, so try using that light source type whenever possible.

	Note
	Analytic rectangle is the most accurate and memory efficient target for indirect illumination mapping. A room wall modeled using a cube requires 4-6 times more memory than a single rectangle (for a comparable quality).

Post Processed Illumination

The table scene tutorial above explained how indirect lighting can be defined using illumination maps. There is another, quite different approach to compute indirect illumination: use of post processing methods.

Example project: tutorprojects/rendering/diffuselighting

Tutorial level: advanced

1. Load first the test scene 'tutorprojects/rendering/table'.

2. Go to the Channels tab of the Select window and create two new color channels called diffuse and unshaded. These channels will carry diffuse illumination and unshaded diffuse color information from the ray tracer to the post processing system.

	Note
	These channels may already exist after loading another illumination project or the default startup file. If they exist, you do not have to create duplicate ones.

3. Switch to the material tab, create a new VSL material 'globallighting'. Check the Advanced option and define the following kind of VSL structure for it:

The root level of the material contains first a color variable 'white' which is initialized to (1,1,1). The value controls the brightness of indirect illumination.
The next object in the root level is a shader level of the type Surface finishing. This shader is suitable because it is evaluated only once at the end of surface shading.
Surface finishing shader contains an If object which examines if Recursion channel is less than 1.01. In other words, diffuse illumination will be examined only at recursion depth 1 (for speed reasons).
The If block defines one local vector variable called 'dray'. It will hold the surface normal which is turned to point towards the camera.
Next a Copy object copies the diffuse color of the surface to the unshaded channel.
Then a Copy object initializes the dray variable from the surface normal.

A material that collects illumination information for post processing

Next an If shader checks if the normal points away from the camera by examining the dot product ray*normal.
If the dot product is positive, dray is flipped using an Operation object with Negate. Now dray points to camera's side of the examined surface.
The final step in the Surface finishing shader is a Raytrace object which traces 10 random rays around the surface normal at camera side of the surface; weighting signal is white because we do not want to premultiply illumination yet.

4. Go to the Geometric Objects tab, select the root level of the scene hierarchy, then switch back to the material library and drag&drop Globallighting material to the view window. The material will shade all objects of the scene.

5. Go to the Post Image Effects tab of the select window. Create a new Blur effect. On the property window, set blur level to a high value 5 - 10. The idea is to average the sampled indirect lighting over the image area. Thanks to this, random variations which would make the image dithered and grainy will disappear. Set Blurred Channel to diffuse (remember that it is the indirect lighting which was sampled by the Raytracer VSL object). Then set Geometric Weighting to level 1. This option controls the averaging so that blurring works only over continuous surface areas - illumination from the floor area that is far behind the table should not affect the table.

A blur effect averages the diffuse channel

A VSL effect adds indirect illumination to the image

6. Then create a new VSL Effect using the popup menu of the select window. Check the Advanced option. You see an Image processing shader already available. Add one Operation object to the VSL effect. The operation multiplies diffuse with unshaded and adds the result to the Color channel.

7. Select the Default Effects post image or other suitable post processing configuration and add the two new post effects to the end of the Active Effects list - blur first, vsl effect after it.

The post processing configuration finished.

8. Drag&drop the Default Effects to the view window. Select a render settings object, for example Reasonable quality, and set Undersampling values to 1. The new diffuse channel is not a trigger channel for undersampling, so it is best to sample every pixel accurately. Drag&drop the modified render settings to the view. Everything is now ready - just render the view to finish the tutorial.

Post processed illumination has some advantages compared to illumination mapping:

Indirect illumination needs to be evaluated only for visible areas of objects
Indirect illumination affects all objects of the scene
There is no need to worry about well defined UV coords of SDS objects
The technique can be applied to animated objects
It is possible to ray trace the image once and then post process several times using the backup render features. The brightness of indirect illumination can be easily rescaled in the VSL effect.

Indirect illumination added by the post processing system

There are some disadvantages as well:

Too low sampling can result to noisy illumination in animations
Post illumination is computed at every render time, whereas illumination mapping can be computed only once

The globalillumination material above uses its own private recursion depth definition of 1. Indirect illumination is usually much weaker than direct illumination and therefore one recursion level is enough. Anyway, it is easy to apply a higher recursion value. The example project contains a second material version 'GI2' which does that. The second recursion step shading is otherwise similar to step one except that:

Unshaded signal is tested against a minimal threshold. Sampling happens only if the threshold is exceeded. This both acts as an optimization and keeps indirect illumination shading separated from usual reflections etc.
The Raytrace object uses much lower sampling value than 10 to keep the render time reasonable.
This time we do the actual shading instead of only collecting channel information: the result of sampling is premultiplied with the diffuse color and added to illumination channel (the result of the shading will anyway end up to the diffuse channel at the recursion step 1).

Easy Global Illumination

The previous advanced tutorial demonstrated how to configure a global illumination system from scratch. In normal use, the standard components of the installation provide an easy work flow.

Tutorial level: medium

1. The first step is to model a scene as usual. No attention to the GI solution is required at this point. For example, load again the test scene 'tutorprojects/rendering/table'.

2. When the scene is ready (loaded), select the root level of the hierarchy. Then switch to the material tab and drag the material GI_shader onto the view window. This assigns the GI shader to all objects of the scene. If the GI_shader material, for some reason, is missing, you can use New/From Template popup menu to load it from the materials/illumination folder.

3. Next switch to the render settings tab of the select window. Drag and drop the object GI rendering into the view window. This render settings object includes an antialiasing channel setup which is tailored for global illumination. If GI rendering is not included in the settings library, select Paste from file from the popup menu and load it in from the rendersettings disk folder.

4. The next step is to select a suitable post processing configuration. The easiest way is to switch to the Post Image Effects tab and drag & drop the GI_effects post image into the view window. This assigns the effect as an image specific effect. As usual, if the GI_effects effect is missing from the current post effect library, just use Paste from a file popup menu and load in the posteffects/GI_effects.r3d file.

Now the GI setup is ready. Let's continue by adjusting the GI settings to an appropriate level. Since a big part of the GI is computed at post processing time, we can use some time saving tricks here:

5. Make sure that Backup Ray Trace option is set in the View/Render popup menu. Then start ray tracing. The program first ray traces an image and finally post processes the global illumination into it.

The components for global illumination: a root level mapping to a GI shader, a render settings object and a post processing configuration

6. To adjust the brightness of the global illumination into a suitable level, open the property window and select the VSL post effect GI_brightness_control from the select window. The effect has three top level controls. The gamma correction curve controls diffuse indirect illumination. Reflected GI color value scales the brightness of indirect illumination visible via reflections and transparency. GI Contrast adds some contrast to the global illumination. Change the values as you like. Then, instead of rendering everything again, just apply the Post process popup menu of the view window. Now the ray tracing part is skipped and the next image version appears much quicker.

7. The default settings include quite a low amount of indirect light sampling. Consequently, the rendered image may contain some noise which appears as an unwanted brightness variation. To reduce the noise, select the post effect called GI_blur. On the property window, increase the Iterations value by one or two units. Then apply Post process on the view window to see the improved result. If this does not remove the noise, increase the Iterations value further and post process again. A 10 pixel blur repeated 10 times will create 100 pixels wide blur.

There are other predefined GI components available as well. GI_draft post effect configuration works in a similar way, but it does not anti-aliase the result and is therefore about 4 times faster.

Important notes:

A wide GI filter may be quite slow to render (several minutes). If the scene is simple and ray traces fast, a quicker way to improve quality is to increase the RayCount1 of the applied GI material. By default, the GI_shader material takes only a couple of illumination samples per surface point. Using for example Ray count value 10 improves quality significantly.
GI blur filter does complex geometric calculations which require many channels, some of which are in the floating point form. Processing a video resolution image takes several tens of megabytes of RAM. This also means that you should not apply a GI filter as a locally rendered image effect when using a slow internet connection to a render farm. Apply the GI effect per render box.
If you have originally applied other custom post effects in your scene, remember to insert them also into the global illumination configuration you are using.
GI filtering may partly remove ray trace time antialiasing. The only reliable solution is to antialiase after post processing i.e. use output down scaling. The GI_effects post effect scales the image by 50 % to ensure a proper quality. GI_draft does not scale the output and is therefore much quicker.

Global Illumination Components

The material which takes care of Global Illumination has only one control, Ray count. It defines how many sampling rays are triggered to examine the indirect illumination of a surface. When these GI sampling rays later hit new surfaces at deeper recursions, only a single ray is launched (for speed reasons).

Usually this is the most important quality setting for indirect illumination. The higher the sampling, the lower Iterations value can be used in the GI_blur filter. The result is finer detail and higher realism in global illumination.

The tracing depth of indirect illumination obeys the general recursion depth of the applied render settings. If you need a dedicated limit for the GI tracing depth, drop the RayTrace VSL object of the GI_shader into an If VSL object, which tests surface:recursion channel value and activates sampling only at suitably low recursion steps. GI tracing depth can also be randomized by using a value computed by a Random VSL object as the input for the If block.

The post processing configuration for global illumination includes a component GI_brightness_control. As the name suggests, you can control the amount of indirect illumination using it.

Reflected GI - Adjusts the amount of global illumination visible through reflections and transparent objects. Usually the value should be slighty larger (10% - 20 %) than the end value of the control curve for diffuse GI, because the finite recursion depth of ray tracing darkens reflected GI slightly more than diffuse GI. A higher value compensates the difference.
GI Contrast - The contrast option scales GI by the viewing angle from camera. Under 'flat' lighting conditions, i.e. when a surface gets a similar amount of illumination from all directions, shadows become weak or disappear. GI Contrast adds a compensating shading factor, which helps to preserve fine surface details such as bump maps.

Control curve of Diffuse GI - With this gamma correction curve, you can, for example, enhance illumination at dark corners but leave the bright areas of the scene unchanged. To brighten indirect illumination everywhere, move the middle points and the points at the right end upwards. The start point of the curve should usually stay at zero level.

The VSL effect called blurwidth_control provides some additional control over GI filtering. When GI ray count is low (e.g. less than 10 rays per pixel) or idirect illumination is unusally noisy, there is not much change to regulate blurring, and the control curve should be close to full level 1.0 everywhere. However, if a large number of GI rays is used (several tens of samples per pixel), the blur width control curve can be adjusted towards a diagonally ascending line, as shown in the example image below. Such a control curve reduces filtering in strongly shadowed areas. This improves the contrast of GI remarkably.

Blur width curves for low sampled GI (left) and for a high number of GI samples (right).

The effect of the blur width control curves to global illumination. The image on the right side was filtered using a diagonal blur width control line as explained above.


Alpha Channel		Outline Rendering