Things to know before rendering in Arnold.
Arnold is pretty much a game-changer as far as rendering is concerned. Arnold is a physically based, fully optimised rendering engine which uses Monte Carlo ray tracing, allowing it to produce high quality renders with a smaller time bracket. Having only had experience in rendering with Mental ray and Keyshot in the past- I would only be limiting myself by not learning Arnold as it is noted that many large scale animation & VFX companies. Also, I can decide for myself which I believe is the most efficient for my own personal projects.
Introduction: Basics of Arnold
One of Arnold's strongest points is using the IPR render- this can be used in order to receive quick feedback on how the render will look.
A more powerful feature that Arnold offers as an alternative to this is if you click on Arnold in the tool bar then press Arnold render view, you will be able to view updates to your render in real-time. You can also view the individual passes in which you are rendering by clicking the dropdown box in the top left corner. Next to this you can chose which camera you wish to use including perspective views. You are also able to click and select objects in this render view which will then be selected in your viewport. In order to navigate around the render view using the maya key short-cuts, simply ensure that 3D manipulation is checked underneath the window tab.
As does maya, Arnold has the ability to store renders for comparisons. This feature is located in the view dropdown > store snapshot.
You can also render out certain passes by using the render> debug shading> select the pass you wish to isolate.
Sampling
"To determine the colour of each image pixel, Arnold collects information from the scene geometry, shaders, lights, etc., and traces a number of random light transport paths that connect the objects seen through the pixel to the light sources - a process called 'sampling'. The quality of the resulting image largely depends on the number of paths, or samples, generated for each pixel."
Samples play a direct role in how long the frames take to render and the quality of the frames. Begin with a scene which uses studio lighting: warm area light left, cold area light right, white area or spot light above. At the bottom of your Arnold render, you will notice Sampling, followed by 6 numbers separated by forward slashes. If you open up the render settings and click on the Arnold tab, this is what these numbers represent:
Camera AA: anti aliasing
Diffuse: Indirect illumination/global illumination
Glossy: specular highlights
Refraction: rays being reflected through transparent surfaces
SSS: sub-surface scattering rays
Volume Indirect: volume indirect rays
Save your snapshot with the 3 point lighting setup before changing the settings.
If you drop down refraction SSS and volume indirect to 0 you may notice no difference if you have nothing being noticeably effected by those rays. An important thing to remember when entering the values is that if you look just above where you are entering the values, you see that the number you enter will be squared.
To add to this, say if you had Camera AA set to value 3, the amount of rays calculated will be 9. However, if you have the diffuse or glossy values set to 2, this max value will be 36, as the squared number(4) is multiplied by the number of max anti aliasing samples(which is 9). Therefore the max value would be 36(2 squared x 3 squared). Therefore you will need to keep an eye on these values, they can go up to large numbers which means a much larger render time.
According to the Arnold documentation, some rough numbers you can use in terms of quality would be around 4 for medium quality and a 8 for high quality. In rare cases for super high quality, maybe a 16, but remember the higher these values the longer your render time will be.
Ray Depth
How does Ray-depth impact the result you see in your Arnold render?
Our camera casts out rays to sample things in our scene. These rays do not just shoot into the scene and stop- they are often times allowed to bounce and contact multiple surfaces inside of our scene.
Say we have an interior scene with 1 direct point of illumination. On the left hand side of the scene we have a red wall, and on the right hand of the scene we have a blue wall. Open up our Arnold Renderer tab in your render settings. Jump down to the Ray Depth tab. This shares some of the same controls as the sampling tab, however this would be used to control the number of bounces each type of ray is allowed to have.
The 'total' number at the top of this list is the total number of bounces allowed. So if we were to exceed 10 for our diffuse/glossy/reflection/refraction & volume, Arnold will still only calculate 10 bounces. The aim by the end of this is that we get indirect illumination coming from the red and blue wall onto our objects inside of the scene.
In the case of the number for our diffuse bounces, stick with 1/2, going any higher than this will not make much of a difference and will dramatically increase your render time. Next we look at our glossy value. Now this will mainly effect objects in the scene which have a reflective material applied. If you first take a render snapshot with the glossy value 0, you notice only the highlights will show. Compare this against when you bump the value up to 1 and there is a dramatic difference.
What may be confusing here is understanding what Arnold means when referring to "glossy" vs "reflective". This is in relation to our material. If you check through your Attribute editor, Glossy is in reference to the "specular" tab whilst reflection is in relation to the "reflection" tab. What the difference is, is that when you're dealing with specular reflections, they will reflect everything in the environment including light sources. A normal reflection does not reflect light sources it just reflects other pieces of geometry in the environment. If the reflection of the objects is black, then you will need to increase the glossy bounces. So what is learned here is that Glossy bounces include your environment/background & reflection bounces reflect the geometry.