Understanding Soft Shadows in Bryce 5

One thing that's seldom mentioned about Bryce is that it has two similar but slightly different Ray Tracers. Since Bryce 2 PC and B3 Mac it has incorporated a largely academic render model called Distributed Ray Tracing (not to be confused with Distributed Rendering, i.e. render farms). In Bryce 2 through 4, this was called the Fine Art render mode, in Bryce 5 it's called the Super mode. DRT was originally created as a better way to handle alising errors by using large oversampeling and, strangely enough, a type of controlled randomness. Along the way, some bright individuals found that DRT could also be used to simulate certain optical effects: Soft Shadows, DOF, Fuzzy transparency/reflection, Motion Blur and Color Bleed. Bryce 5 implements all these except Motion Blur and has only a very rudimentary (and pretty much useless) Color Bleed, called True Ambience. These effects are called Premium mode. Perhaps the most interesting of these effects is Soft Shadows. Shadows, both in the real world and 3D, play an essential role in how spatial relations are perceived by the viewer. But one big difference between the real and the 3D is that in 3D, a "standard" light illuminates from a single point which leads to a single hard shadow. On the other hand, most real world lights illuminate from an area or volume, which is what produces a soft shadow. The "soft" part of the shadow is called the Penumbra, the "hard" part is the Umbra. To see how soft shadows work in the real world, take a desk lamp and move your hand up and down between the light and the desk. When your hand is close to the light the shadow is almost all blur - move close to the desk top and the shadow gets sharper.

Many 3D apps that support Soft Shadows use a procedure called Shadow Mapping. Very simply put, it's largely a 2D effect that just blurs the edges of the shadows, which produces the penumbra effect. Shadow mapping is very fast but only effects the shadow (not the illumination) and every soft shadow has the same amount of blurred edge which is certainly not natural. DRT makes soft shadows by creating a bunch of temporary "feeler" lights, that are randomly placed around the primary lights center point. How far out from the light these feelers are placed is determined by the Hard/Soft control in each lights editor and how many of these feeler lights are created is determined by the Rays Per Pixel (RPP) setting in the Render Options dialog. The up side of this is a much more realistic shadowing simulation which accurately reflects how real soft shadows change based on how close the shadow casting object is to the shadow receiving object. The down side of DRT that it is very processor intensive and can dramatically slow renders. Also for many scenes, using too small an RPP will result in too few feeler lights to produce a convincing effect (noise).

Below is a simple scene, rendered without soft shadows. The lights are all parallel to the squares, and the cylinders start close to the light; half way between the light and just off the surface of the square. The BU values refer to the distance the cylinder is from the 2D square.

Below, each lights Soft Shadow value is set to 25% soft. As can be seen, soft shadows give more visual cues as to the relationship of the lights and objects in a scene because of the way the penumbra changes.

In the image below, I've created a radial light on a pole with a dark sun (just to give a little illumination to the pole itself). The left image is a point light and shows that even though the pole is tapered it still casts a large and unnatural shadow and adds no illumination to the pole. The right image shows the radial light with 25% Soft Shadow. Now the hard shadow is quite small and just as importantly, it also gives an illumination "volume" - note how the curved surfaces above the base are now illuminated all the way around. Also note that even though there are all these feeler lights, the overall illumination/range of the light doesn't change.

These last 2 images show how sizing effects soft shadows. The left image is a scene created in a 400x200x400 BU cube with a single radial light with soft shadows set to 100% . The right image shows the same scene at 10000x5000x10000. I made no real attempt to match the 2 scenes but to show that as objects change size, it changes the relationships between the lights and objects, thus changing the penumbra.

What this means in use is that: 1) With really small scenes, you always get large penumbras (which accurately reflect a small space) even with small Hard/Soft settings. Changing the Hard/Soft amount produces big changes in the softness. 2) With really large scenes the opposite is true - the hard/soft control can "bottom out" where all shadows are hard no matter the Hard/Soft setting. 3) A special case is the Bryce sun - it's set to the edge of the Bryce world, which is as far away as Bryce gets. So the sun shadows would always be hard (which is largely what the real sun does) and the Hard/Soft controls would have no effect. So to give the Bryce sun this effect Bryce 5simply inverts the whole sun soft shadow model so that the sun always creates a very soft shadow.