Hi Forum!
I have just tried a test of 2 frames from a 375 frame animation lowering the GI/AA balance to 4 from 16. It is a WOPPING 2 mins faster! My only concern, will this add flicker even though I’ve set it to ‘flicker free’? These animations dont have to be crystal clear. I’m also running the animations through Topaz from 25 to 50fps with slow mo. I am a bit concerned sending these to the render farm. Has anyone used a setting of 4 for animations?
Hi Johnny,
This is an interesting “oldschool” topic. 
What render limit did you use?
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Define “flicker”.
GI vs AA only affects path tracing. The “flicker-free” preset applies to the UHD Cache and it does not affect path tracing. So changing the “flicker-free” preset is unrelated to path tracing or setting it to 4, 16, or any other GI vs AA value.
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The important question here is: when does it make sense to increase or decrease GI vs AA?
The below is not exactly how things work, but let’s think of it this way to make it simpler:
Imagine Corona doing two separate things:
Point 1 affects how geometry edges look, how fine textures look, and how DOF and motion blur look. With each next render pass, the quality of those effects becomes one step higher. So at pass 1, there is pretty much no AA. With pass 2, it looks slightly better. With pass 50, most edges should look pretty nice. The more passes you render, the higher the quality. Sometimes you may need 200 passes or more. So AA quality directly depends on the number of rendered passes.
Point 2 affects how much pixel-sized noise there is in the image in the areas where we simulate light bouncing around. The GI vs AA value is how much work we put into improving the appearance of those areas per each render pass. If you decrease it, the render time of each pass will be shorter, and the overall quality of GI within given time will be lower, because we are saying “hey, let’s do less work in these areas”. At the same time, the quality of AA (edges, textures, DOF, mblur) will be higher within given time, because more passes will be rendered (because the render time of each pass is shorter).
This means that the number of rendered passes affects the overall image quality - both AA and GI.
Also: if you set a fixed time limit (e.g. 10 minutes) and render with different GI vs AA values, the overall image quality will always be similar. The difference will be in the quality of AA (higher GI vs AA = worse AA quality) and GI noise (higher GI vs AA = less GI noise).
Again: the above is not exactly how Corona works, but it is close enough for the purpose of deciding what render settings to use.
Going back to the main question: when does it make sense to increase or decrease GI vs AA?
Let’s try with a few examples:
Default settings (GI vs AA set to 16): this is the sweet spot that works well in most cases. If you don’t have a good reason, don’t change it.
Example: a static render of an interior or an exterior.
Lowering GI vs AA: if you want to get better AA quality (edges, textures, DOF, mblur) within the given render time at the cost of GI noise quality. More passes are rendered. Less work is put into improving GI noise.
Example: a static image with heavy DOF where we don’t see much GI. An animation with lots of very thin geometry elements (grass?) or textures (fabric?) where we want to make sure that the edges appear smooth without jagged pixels.
Increasing GI vs AA: if you care about GI quality more than AA.
Example: an interior image with heavy GI noise (think of sunlight entering a dark room and bouncing around it). An interesting example would be rendering an image in a very high resolution. If you render in a high resolution, you probably don’t care that much about AA because things like fine edges and textures already have enough pixels to look good. So increasing GI vs AA will make GI render faster without losing too much AA quality. I won’t specify what exactly is “high resolution” but the general rule is: the higher the render resolution, the less passes you need to get good AA quality.
Tips:
A note about the UHD Cache and the “flicker-free” preset: the UHD Cache is a secondary GI solver, meaning that it takes care of all light bounces that happen after the first bounce. It makes rendering faster by adding a slight bias/approximation to the image. The “flicker-free” preset decides about the quality of the UHD Cache. Enabling it is essential for rendering animations, otherwise you will most likely get large-scale flickering splotches (not per-pixel flickering).
Using path tracing for both the primary and the secondary GI solver usually results in much slower rendering, but can be used to get the most “unbiased” rendering.
Hope this helps!
If you need some additional help deciding what settings to use for your scene, feel free to show us what it looks like here or at https://support.chaos.com/hc/en-us/requests/new
Hi Marcin. Thank you for such a detailed break down. I will render 150 frames locally to see the results n post back 
Looking forward to the results!
Hi Marcin. The test looks almost identical to 16. Amazing! Will save a TON of money on renders. Have a good weekend
Hmm, so what configurations did you compare?
Most likely 16 vs 4 GI/AA Balance
Honestly, I also found the value of 16 a bit too high. I did some testing a few versions ago and came to the conclusion, that 8 works better for what i’m doing. So I have 8 as default and 4 when I need more AA in outdoor scenes.
I lowered GI/AA from 16 to 4