If you happened to go to the PhotoWorks, Options, Advanced tab in PhotoWorks release 2 and saw a slider for Indirect illumination and was curious as to what this was for, well, you've come to the right place.

Background:

Before we can explain Indirect Illumination, we need to understand how traditional rendering engines work. Until recently most rendering engines were only capable of rendering Direct Illumination. Direct Illumination is is the light that travels in a straight path from the light source to the illuminated objects in your scene. If you were to shine a flashlight onto a red ball the illumination pattern would be caused by direct illumination. This was the only kind of illumination you had in the original PhotoWorks. The real world is not illuminated with just direct illumination. Take a look at your feet under your desk - note that it's dark under there, but you can still see your feet. There is some light under your desk because some of the light energy from the lights in the room is bouncing off the walls and floor and indirectly illuminating the space underneith your desk. Indirect Illumination is new functionality that was added to PhotoWorks release 2 to give you the ability to take into account some of this 'bounced light' in your renderings. In the images to the right you can see some of the differences between direct and indirect illumination. Note in the Direct Illumination image the shadow is completely dark and the walls look very artifical (especially in the corner). In the Indirect Ilumination image you can see that the walls appear much softer and there is red light bleeding onto the floor in front of the ball.

So how does it work?

PhotoWorks release 2 is based on the mental ray rendering engine from mental images. One of the ways that mental ray can take bounced light into account is with Final Gathering. The Indirect Illumination slider in PhotoWorks release 2 controls this Final Gathering functionality. Below is a simple scene with some exagerated colors to illustrate the effects of Indirect Illumination. The scene is simply a white sphere and a simple scene with white plastic on all walls except the blue and red walls which have their colors changed to blue and red respectively and have their "Material Type" set to: Constant. By setting these two walls to Constant we essentially exclude them from all lights and shadows and tell them to simply render at their specified color all the time.

To understand how Indirect Illumination works, we first need to understand how a renderer generates the image that you see on the screen when you render. As we all know, images on your computer screen are made up of thousands of small dots called pixels. As PhotoWorks release 2 renders an image it shoots thousands of straight rays out from the "camera" into the scene. When a ray intersects geometry in the scene it interprets the material at that point and tells the camera what that pixel of the rendered image is going to look like. In SolidWorks there is no explicit 'camera' object however to make it easier to think about, just imagine that your SolidWorks graphics viewport is you looking through the lensfinder of the virtual camera. When you enable Indirect Illumination in the scene an additional operation is performed when the ray intersects a piece of geometry. Rather than immediately return a value of what the direct illumination of that particular pixel looks like, a hemisphere of additional rays are spawned from the point of contact:

These additional rays gather up (hence "Final Gathering") the color information around the point in question. The additional color and illumination data from these secondary rays is added to the direct illumination value that was calculated for this pixel when the first ray hit and the new value is output to the screen. As you can see in the image above, the secondary hemisphere of rays that are cast out from the point of contact accumulate the red coloring from the adjacent red wall and add this to the original Direct Illumination value to give the right side of the sphere a slight red tint. That about sums it up from a technical perspective. Basically, Indirect Illumination in PhotoWorks release 2 is sort of like a 1 bounce Global Illumination solution for the diffuse illumination in your scene. Also, note that the colors used in the images above were really just for illustrative pruposes. You can get some amazing effects by using different colored pannels in different orientations. The scenes below were rendered with only 1 light each - and the light had a brightness of "0.00" - so ALL of the illumination in the scene is coming from the Indirect Illumination :

Also note that you are not limited to using the 'box' scene - you can get some great effects if you use the spherical scenes in the \Environments folder in the PhotoWorks release 2 scene editor - just make sure whenever you render with Indirect Illumination enabled you check all of the 'Reflective' checkboxes next to each piece of scenery on the Room tab of the scene editor. If you don't, light will escape from the scene and it may not look quite right. Enjoy, and happy rendering...