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Exterior fake GI


Exterior fake Global Illumination with dirtmap and Photoshop!

Another mini-tutorial for our readers: A simple guide to faking GI in post production by Branko Jovanovic (a.k.a. d.sign).


The tutorial with sample files as attachments can be found in our forums HERE

Hi everyone!
I thaught I post some of my experiments in kind of tutorial, or better say simple guides, mostly for begginers (advenced users may find this simple and common...) and to all of you who want to make your projects look nice in a model-like style but do not have time for GI or do not have GI renderers like Vray. I assume you have Mental Ray with your MAX package, we will use it for this experiment, but without any Global Illumination calculation. Personaly, I am a big fun of GI and Vray, but also I love to experiment and I think that different situations may ask for different solutions. To get to the point - here I will try to explain how to get that daylight exterior GI look without GI calculation in three very fast steps.
I did it on a simple scene, but you can do it with quite large projects, even introduce some textures, it will not take much more time. (Howver, if you want to use reflective and refractive materials, I suggest to tur back to the real thing - Vray with real GI. In fact, my next step is to explain basic exterior GI setup for Vray with this same scene...)
1. Render main pass (layer) and save it as .tga, it took me 17 sec. for 640x480 size on a crappy computer. It may be rendered with default scanline render but later we must swich to MR so I did it in the first place, this is not so important... 
2. Render dirtmap pass which took me less than 1 minute 
3. Main thing! Postwork in photoshop, here is where your creativity and imagination can make a difference! It may take some minutes to get what you wantbut you can play with it as long as you want
Here you can see my final result and download main pass scene.
Final with Dirt Map
Well for the first step - main pass I used just one direct light as sunlight and put couple of omnis to simulate sun bounces. However I needed some more ambient light so in MAX enviroment panel I increased ambient light level to 50 and turn on logarithmic exposure control. Rendered result I saved as .tga file to preserve alpha, that is, to maintain separate bacground. Here is quite uggly result and some settings...
Do not mind the crappy result, this is just first step...
First Pass
3 Bounce Omni Set up
Light Settings
Second step - dirtmap pass. Here you will need the dirtmap shader for Mental Ray unlocked. I think that will work if you just open my scene I attached here, just save my dirtmap material in your material library, if not you should unpack dirtmap shader in your MR shaders folders...
What is dirtmap and what it does? I will not go throug theory, you may find much better explanations if you just search on the net. Basicaly, dirtmap is shader which simulate soft shadows get from ambient light. It is simmilar thing to ambient occlussion shader, but not quite the same. Ambient occlussion work only for exteriors, while dirtmap may be used in interior scenes. Both work based on QMC method, but dirtmap is much faster and depends on scene scale. In my scene, I first deleted all the lights, grouped all objects (including camera) and had to scale down to 20% to get optmal result. You will always have to do this scaling, why I do not realy know. Do not scale scene to much cause you will get result like QMC skylight effect which is good, but quite slow. (Scaling down scene cause more density in dark areas of dirtmap).
After you scaled scene you need to make dirtmap material. Creat new material from Mental Ray material preset. In surface map choose dirtmap shader from shader list. You can adjust setings as displayed.
Then you go to the proccesing panel of MR renderer and check on the material override option. Drag the dirtmap MR material to the MTL override slot. Now render the camera view. Important is to make enviroment background white! You should get something like this:
Dirt Map Pass
Dirt Map Overide Settings
Dirt Map Material Settings
Dirt Map Material Settings
We hope you found this guide useful, we are always looking for quality content, if you wish to share your knowledge with others please contact us.

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