The RealSoft3D ray-tracing process - Part 3: First Examples

By David Coombes
d.g.coombes@btopenworld.com

Foreword to Part 3

All this theory is all very well, but what does it actually do? In Part 3 I'll provide some example VSL materials and show how the different samples work together to affect the rendered scene. This is going to be fairly basic stuff, some of which will have been covered in the user manual so you may know some of it already; but Part 3 will concentrate on the particulars of execution to help reinforce the course of events as mentioned in Parts 1 and 2. Also, don't worry about understanding the VSL code yet. VSL will be covered in another article explaining the principles involved in material creation. For now, just trust it does what I say it does!

First Experiments

Here's that example scene from Part 1. We'll experiment on the sphere and see what we can do to it...

The sphere is red because in its properties window the value for Color is given (1,0,0). At the beginning of rendering the ray, this value is copied into the Surface:Color channel. A straight forward change of colour would be achieved by replacing this value with another.
 

The VSL material codes Surface:Color to have a new value, blue. When the camera ray is traced and intersects with the surface, the value of the Surface:Color channel is Red (1,0,0). Then the surface properties shader is executed which sets Surface:Color to (0,0,1). The subsequent surface illumination shader then calculates lighting to produce a blue sphere.

If we can change the colour by simply assigning a value to the colour channel, is the same possible with illumination? Remembering that it is the Surface:Illumination channel that is copied to Image:Color, if we set Surface:Illumination to a uniform green colour do we produce a green circle?
 

Obviously it doesn't work as intended because after adding green light to the surface, lighting calculations add red illumination, from the light source acting upon the red surface. The red and green components combine to give yellow.

If we want to create a green dot, we need to remove the lighting caused by the illumination shader. The simplest solution is to turn the surface colour black. This way whatever light reaches the surface, no illumination takes place.

And voila! A green dot.

Bumps

Perhaps the simplest shader is surface geometry for its purpose requires only modifications to Surface:Bump Height and you won't use it for anything else. By modifying the value of Surface:Bump Height the impression of variations are created in the surface.

Custom Lighting

One of the main activities of surface illumination is to create custom lighting. Rather then using the default calculations where Surface:Illumination is directly proportional to the angle of light on the surface, a material can produce a Surface:Illumination value based on a non-proportional transformation. The following material performs the same task as the RealSoft3D default illumination calculations.

Note the setting of Light:Illumination (intensity of light illuminating surface) to black to deactivate default lighting. This has the same result as setting Surface:Color to black but preserves the surface colour which will be useful later. Also note that in the VSL code I have extended annotation by including the names of the samples to which the channels belong, as in this example I'm referencing more then just the Surface sample.

The above results are the same as without the custom illumination calculations. Unlike the original calculations though, this method controls the intensity of illumination by a curve object linked to the angle of light. The default linear progression (output = input) of the curve object produces normal shading as above. By changing this curve the output is changed accordingly.

Exaggerated Lighting			Contracted Lighting

Other effects possible in surface illumination include changing the Surface:Color based on the light angle, and changing illumination based on the angle of light relative to the viewing position.

Custom Filtering

Now let's look at what's happening with the floor. Firstly, it has a chequered pattern. The material that achieves this performs a colour change in surface properties. When a cast camera ray hits the floor, Surface:Color is initialised to the object's colour property as defined in the editor, (1,1,1). Surface properties then goes on to make use of Surface:Map coords to determine what value to change Surface:Color to. Surface illumination calculates how much light is received from the light source and this gives an intensity value to Surface:Illumination. Because the light source is a 'distant' type, the angle of light remains constant so the floor is uniformly lit.

If we look more closely at this step of the scene's illumination, we see the involvement of the surface filtering shader. During the tracing of the light rays reaching the floor, there are instances of when there is an object (the sphere) between light source and the floor target. As such, when surface illumination is invoked it executes first surface filtering before any other calculations. Surface filtering simply transforms the intensity of light by the transparency of the interim surface with the calculation

Light:Illumination=Light:Illumination * Filter:Transparency

where Filter:Transparency is initialised at the beginning of surface filtering to the value of the Surface:Transparency channel of the interim surface. If you look at the rendering diagram in Part 2 you will see the values of Surface Properties are copied to the Surface Filter sample. In this scene the transparency of the sphere is uniformly the default (0,0,0). So when surface illumination is returned to, the original value of Light:Illumination has been changed by

Light:Illumination=Light:Illumination * Filter:Transparency =
Light:Illumination=Light:Illumination * (0,0,0)

The result; Light:Illumination is (0,0,0) and so surface illumination adds nothing to Surface:Illumination. What we can do in the surface filtering shader of the shadow-casting object is change the value of Filter:Transparency to change the appearance of the shadow. Let's try a simple constant value change with our original scene.

With this change to Filter:Transparency some light gets through to illuminate the floor. Note that the light passes through two surfaces either side of the sphere, as it enters and exits the volume of the sphere. This means the actual shadowing might not be as expected. A Filter:Transparency value of 50% (0.5,0.5,0.5) results in a quarter of the light reaching the final surface in this instance.  For the first surface, Light:Illumination is multilied by (0.5,0.5,0.5) to be halved, and then again at the second surface.

Just as we could change the illumination by the light angle, we can change the shadowing by angle. The following works with a calculation of the angle of the surface relative to the light source, and a transformation of Filter:Transparency relative to this angle. The transformation curve used modifies red, green and blue components separately to produce a colour separation in the shadow

Finishing Off

The last shader of this part is surface finishing. This versatile shader is a very welcome addition to version 4.5 of RealSoft3D. It allows processing of the shading results (the value of Surface:Illumination) after illumination calculations executed in surface illumination. A good example of an effect possible through surface finishing is a "cartoon" or binary shader.

We can determine the amount of illumination acting upon the surface by dividing the resulting pixel's illumination by the colour of the surface lit. Using this we can "reverse engineer" illumination, changing the coefficient by which the surface is lit with a boolean curve and multiplying colour by this new value to generate a new Surface:Illumination value. All channels used are from the Surface sample

Creating a Material

What we've got so far is a collection of examples of the application of different shaders. Each shader handles a different side of the image creation process. A material may consist of custom VSL code in one or more shaders. Where in the examples so far we've only used one shader at a time, we can combine the above different shaders to produce a more complex material. This is the sphere rendering process in the order the shaders are executed...

At the end of that, what we have to show isn't too pretty but the choice effects show obvious results. Quite often VSL materials apply more subtle, realistic effects, or a value change in one shader is used to affect the results of another shader. With these extreme changes I can now offer one last look at the RealSoft3D rendering pipeline as so far covered with a walkthrough of the value changes of two points in the final image. The following table records the values of the channels of the points A and B marked in the final image, showing data changes as a result of custom VSL code and default RealSoft3D operations. Red data signifies a value change. A dash in a column shows no instance of that sample at that point in the render process. Also, some values are approxmate (leave the 64-bit precision floating-point maths to the computers!)

 Summary

You should now understand the practical role of the different shaders. If you want to affect an object's colouring, how it filters light, or how to modify it's appearance after lighting has taken place, you now know where your VSL code needs to be placed and when in the order of rendering events that code will be executed. If this is the case, you understand RealSoft3D!

Still to come

The first three parts of this article have taken you through the basic rendering pipeline of RealSoft3D. You should now understand that rays are cast, these access collections of data as channels, with different groups of data being collected together as samples. Different shaders are executed to fill in different aspects of a scene's construction, such as defining surface properties or lighting effects. At the end of the process, whatever is in Surface:Illumination is copied to Image:Color and determines the final pixel colour.

In Part 4 the rest of the shaders are included to give a full picture of the steps contributing to an image's creation. The later Parts will introduce recursion which should provide a breakthrough in understanding the full RealSoft3D process, as this is such a key concept to RealSoft3D. Still lots to come, all of it worthwhile!