Questions are raised all the time about irradiance map saving.
Here are the two main ones.
a: Difference between multiframe incremental and incremental add to current map.
b: How to create a camera flythrough animation with a saved irradiance map
A: If you read the help file the description of the two methods are almost exactly the same. The main difference is that everytime multiframe incremental creates a new map it will delete the previous map. However incremental add to current map will simply add the new samples to whatever map is currently in memory. I go into this further here www.vray.info/entry.aspevtryID=46
B: The basics of using incremental add for a camera flyby is this. You need your camera to take new samples asit follows it animated path so that you can fill in new irradiance map samples in the room. However there is no need to create new samples every frame. The difference between frame 1 and 2 probably isnt that different and you wont be adding that many new samples. This is all dependant on the speed of your camera move.
Step 1: Since we dont want to create new samples every frame, change the nth frame rate in the render settings to something like 10 or 15 depending on the speed of your camera. who knows it might be slow enough to have something like 20 there.
Step 2: Next. turn off your save image function. We are here to save an irradiance map, not an image.
Step 3: Lower your AA to adaptive -4 -4.
Your irradiance map settings are your own to choose.
Step4: In advanced irradiance map settings though choose incremental add to current and turn on the auto save and autoload feature.
Final Steps: When your irradiance map pass is finished, reset the nth frame to 1 and put your AA bak to the way you want it. Make sure your now using "Load" for your irradiance map and that your desired saved map is in Use.
This is a great tip to reeeeally speed up your render however if your just using a camera pan or tilt with the camera not moving possition but just rotation.
Step 1: All you need is single frame mode both for max frame rate and for Vray advanced irradiance map settings.
Step 2: Multiply your image size by 4 for height and by 3 for width
Step 3: Use adaptive AA with -4 -4 settings again
Step 4: Use a Cubic Camera
Step 5: Save this irradiance map
Step 6: Load the map and reset your camera type
Step 7: enjoy an irradiance map thats a calculation of the entire room from that camera possition at any camera angle. Therefore you can pan and tilt the camera in your animation with this single irradiance map and you only had to render 1 frame.
Instead of the -4 -4 AA trick, in later versións of VRay you can now turn off the rendering pass in the Global Switches rollout.