Our scene is lighter, but is this best we can get? Let's clone our render for a later comparison. Click the clone button (see yellow highlight on above image) to clone a copy of our render. Minimize the copy and close the original rendered image panel. In the Render Scene panel, change the Global Energy Multiplier back to its default value of 1, then just above that area, look at the area for Trace Depth. In the most simplistic of explanations, Trace Depth, with respect to Global Illumination, sets limits on how many times a photon can be reflected/refracted off of objects. Increasing trace depth also increases illumination. In the Trace Depth area of the panel, enter a value of 16 for Max. Depth, a value of 15 for Max. Reflections and lower (if necessary) the Refractions value to 1. Render. See below:
Once the render is complete, call up the cloned copy of your previous render and compare the two. Initially, your first observations would be that the newest render is slightly darker, but take a close look and comparison of the corners in both renders. The corners are better defined in the render with the increased trace depth and the artifacts are slightly less pronounced. We will follow the greater trace depth method for this tutorial. Close both of the image windows, and save your file. At this point, we are ready go to Final Gather.
Step 4: Final Gather
Final Gather is an option in global illumination, a tool that can be used alone or with your global illumination settings to help filter out artifacts and smooth the end result. In the this tutorial we have been working to help prepare the scene for Final Gather, so both will be used. If not already opened, hit F10 to call up the Render Scene panel and slide down to the Final Gather rollout. Click the Enable option box to enable Final Gather. The only value we need make an adjustment on at this time are the Samples. The value of 1000 is excessively high, and since we've made several adjustments to prepare our scene for this step, we'll start with a value of 200. Enter 200 into the Samples value, and click Render. Be prepared for a good wait.
See image below (Samples = 200):
It's possible that some of you might have gotten a flawless render, while others have a few vestiges of artifacts showing in the image, particularly in the corners. While creating this tutorial, and through practical experience in mental ray, I have found that there are times when mental ray Final Gather can be quite fickle in its renders. One time you can do a flawless render with a file, then try a second time with the exact same settings and exact same file to find artifacts present. Rather than increase the samples (thus increasing rendering time) look just beneath the Samples and you'll see Filter. If you have artifacts present, increase this value to 3 and render. Filter increases the number of rays used for filtering, thus, smoothing out the illumination further.
With this, our artifacts are fairly well removed, and the sphere is now brightly colored as it should be. This part of the tutorial was designed to show you how easy it is to work in global Illumination with a fairly simple room. Before moving on to the second part of this tutorial, take the time to create your own small room and illuminate it with what you've learned. Refer to the user reference to look up anything you're uncertain of or curious about. Experiment with settings. Try different sample values, photon sizes, lighting positions, room shapes, etc. A little experience now can save you a lot of frustration later on. Let's proceed with the second part, a real world room.