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In case there's any interest on the techniques you can use for simulating GI using a raytraced renderer, here's a brief explanation of the thought process I used in generating the indoor scene I posted above.
First, you look at your scene and decide the following:
1. What are the primary and secondary sources of direct light that illuminate your scene?
In this case it's obvious the the sunlight coming thru the sliding glass door is the main source of illumination. The secondary source of direct light is the blue skylight which bounces off the atmosphere and enters the sliding glass door.
2. What are the secondary, and brightest surfaces in the scene that will REFLECT the most amount of bounce light to illuminate the scene?
In this case, it is clear that the most important surface which will reflect light into the room is the light colored carpet on the floor near the sliding glass door. That's where there is a big bright spot from the sun's direct illumination.
Now, at this point, we have identified three primary sources of illumination: the direct sunlight, the blue skylight, and the bounce light off the carpet near the door.
And here is an image to show the placement of the two lights which represent the blue skylight and the main bounce light off the floor. Both lights were spotlights with 90 degree angles and soft shadows.
The only challenge is to decide the intensity and color of each light. The blue skylight was, obviously, set to a light blue color. The color of the bounce light off the floor was set to the color of the carpet, a light beige. And the intensity of the light depends upon how bright you think the source might be.
For example, the main bounce light of the floor is probably the biggest source of illumination in the room, so you'd give that a high intensity, maybe even 100%. And the blue skylight is also fairly intense, so you might also give that a high intensity. And you'll also have to compensate for the fact that the blue skylight is coming thru a glass surface, so you'll need to crank up the intensity a bit.
And here's an image showing a render with just the direct sunlight and the blue skylight illuminating the scene. Notice the blue tint of color on the far wall. This blue skylight color is critical if you want your daylight scenes to look "realistic". Our eyes expect daylight to make everything slightly blue, and it appears to us to be crisp and clear and real. It's just how humans react to different colors
And here's an render showing the sunlight, skylight, and main floor bounce light illuminating the scene
Now, you may be happy with just those three lights illuminating the scene, or you may want to increase the illumination by adding additional bounce light.
If you choose to add more bounce light, you can look at your scene and your renders and see that the other main sources of secondary bounce light are the light colored walls.
Now we know that the lighter the color of the surface, the more light it reflects. And if it's totally black it reflects almost nothing.
So once we've noticed that the lightest colored surfaces, which will reflect the most light, are the walls (and possibly the ceiling), we merely insert additional spotlights to simulate those light bounces.
What I did is added a spotlight behind the camera to simulate the bouncelight from the wall which will illuminate my scene the most. And since it's a secondary source of bounce light, the intensity will be less. And the color will be the color of the walls, which is a light beige. And to provide a little more illumination, I also added a spotlight on the ceiling to simulate that secondary reflection.
And the result is the final image I posted.
So for that image I chose to simulate a primary bounce (sunlight off the floor near the sliding glass door), and some secondary bounces off the walls. And since the secondary bounces are, well, secondary bounces, their intensity will be less, maybe around 30-40% for a light colored wall.
Now keep in mind that in the real world, the light keeps bouncing. From the floor to the ceiling to the wall to the chair and then into your eye. And so on. So you need to decide how many bounces you want to simulate.
In this case I chose only a primary and some secondary bounces. And the resulting render took only 3.5 minutes.
Now, once you have a render of your simulated GI scene, keep in mind there are many other techniques to tweak this to improve the render.
One way is with an ambient occlusion pass. I described in my compositing thread how an ambient occlusion pass can be used to tweak the shadows in your render. You inject an ambient occlusion pass, and dial the amount of AO and watch in real time to modify the shadows.
In your compositing app you can also adjust brightness, remove bright spots, change the color of the bounce light, and so on. And you don't need to do another 2 hour render after each adjustment to see how it looks, you can adjust it in real time to see which result you like best.
So simulating GI is not just rendering with some spotlights, there are many other techniques you can use to fine tune the render, and at the same time save possibly hours in render time.
Here's an AO pass render that can be injected into your render in a compositing app, and then you can dial the shadows in real time.
I'd give an example of that, but Carrara sometimes has some issues with its diffuse pass, and in this case it's a complete mess. Maybe cuz it's a super old scene file. So it's too much work in this case to reconstruct a final render. So I'll leave it to anyone interested to try it on their own and post their results.
Okay, help me out here. I understand in theory what AO is, but whenever I've tried to do an AO pass render, all I get is a blank white image. I must be doing something wrong? Here's a quick example - regular render, Diffuse & AO. Lighting is a mix of interior up/down wall spots and outside fake GI . . .
Render time was 3:24 on Fast/2/2
thanks,
Off the top of my head I'm not sure what the problem might be. Here's my settings FWIW. I used a 10ft occlusion radius only cuz it looks cool. In real life you probably don't want all of that shadow. But try cranking it up anyway. Maybe it's too low and that's why you're not getting any shadows.
And don't forget the quality and accuracy settings.
Rule #1 of 3D rendering: if stuff doesn't work, crank up all the quality and accuracy settings and anything else you can think of.
Thaks Joe, that seemed to get me somewhere. A little further experimenting: The AA/OA/SA settings don't seem to make any difference to the AO pass (beyond vastly increasing the render time), but the LQ & A settings make a vast difference. Attached are default and maxed out. Maxed out doubles + a bit the render time.
Rule #2 of 3D rendering: "Good stuff takes longer"
Maybe it's just too early for me, and I haven't had my coffee yet, but I was unable to decode your code.
Anti Aliasing / Object Accuracy / Shadow Accuracy and Lighting Quality / Accuracy
Early? it's nearly bed time! ;)