common problem with low GI and Final Gather settings
Here's a quick step-by-step, but if you read further I have exhausted each of these steps in detail.
•Turn on Photon Map, use Read/Write File, then render
•Turn on Final Gather, use Read/Write File
•Lower samples to 1/64 - 1/64
•Render active time segment at every 10 frames
•Turn on final gather Read Only (FG Freeze)
•Increase samples to 1 - 16
•Turn on Save File for Render Output
•Render active time segment at every 1 frame
First we calculate the photon map (PM). When calculating the PM it's a good practice to have final gather (FG) off to see the pure PM results. To save your PM click on the [...] button, and if you are rendering on a renderfarm, be sure to save your PM in a location that the farm has access to (your network). Also be sure "Read/Write File" is checked.
Now go ahead and render a single frame. Mental ray will calculate the PM first, save it to the location you specified, then renders your scene. Very important to note: now the second time you render, mental ray will not re-calculate the PM, but rather read the already calculated PM from the file location you specified because you have "Read/Write File" checked. The PM is scene based rather than view/camera based. This means that when the PM is calculated it is calculating the entire scene (much like radiosity). The great thing about the PM, is that once it is calculated, a rendering can be done from any view using that same PM...wonderful for animations!
Now that we have our PM calculated, we're now going to move onto FG. Unlike the PM, FG is view/camera based. This means that when a FG map is calculated the information in the map is only of that viewing angle. So if you wanted to see both sides of an object, you would need at least 2 FG maps. This is very bad news for animations. Because every frame in an animation is different, you would need a new FG map for that frame. But we have a work around for this that I will get to.
•under FG check "Enable Final Gather"
•for the Preset choose Draft
•under Final Gather Map click on the [...] button, and choose a location to save the FG map.
Be sure "Read/Write File" is checked and "Read Only" is not checked.
Now to get back to our problem of needing different FG maps for every frame. Instead of creating a FG map for every frame, I create a FG map for a range of frames. For example, if my animation is 100 frames long, I will render every 10 frames creating a FG map for only those 10 frames. Then with that combined FG map, will go back and render every frame. Here's how to do this:
Make sure FG Map is checked on. Then in the Renderer tab, lower your samples to 1/64 - 1/64. We are doing this, because we are not concerned with the actual rendering, but just the calculation of the FG map. In the Common tab, change your Time Output to Active Time Segment, and under Every Nth Frame change it to 10.
Now click Render. You will get a warning that pops up telling you that you are rendering a sequence without saving the images to a location. That's ok, because we are just interested in FG at this point. So click Yes. Now the animation will render every 10th frame. Because we have "Read/Write File" checked and do not have "Read Only" checked, every time FG for a frame is calculated it is added to the previous FG map. After all 10 frames render, you now have a single FG map for your animation sequence.
Now go back to Indirect Illumination tab, and under Final Gather Map check "Read Only". Now when you render, it will not add to your already created FG map, but just read the one that it's locating to. Also increase your samples back up to something reasonable (1-16), and change your "Every Nth Frame" back to 1. Also be sure to set your Render Output to save to a file location.
That's it. Click render and enjoy!
Animation using PM and FG from file
Now it will start the rendering right away without calculating any indirect illumination. Even though the solution for PM and FG are low, it's not that noticeable. The noise will be even less noticeable when texture are added. Note: this technique doesn't work well for secondary animation (animation with moving objects or characters).