Back when I getting started with computer graphics - he says in an
elderly wise voice - it was the tail end of a CG Dark Age (or Golden Age depending on who you ask).
It was a time when most lighting and shading calculations were approximations instead of being physically accurate. Global Illumination (GI) was also called "radiosity" and was only for privileged artists and studios with cutting edge knowledge, top-of-the-line workstations, high-dollar software, and endless patience. Glossy reflections were but a dream next to "specular" reflections. HDRI's were a mystery.
Shading and lighting was a much more manual art form back then. But technology was fast evolving and physically-based render engines were just around the corner. It meant that I learned both the old ways and the new ways. It taught me 5 major lessons that I want to share with you.
1. Environment Lighting
For years I lit all my CG projects with lamps only. And for years I could never understand why none of my lighting and shading looked realistic or believable. Studying studio light setups never seemed to be enough by themselves. Then I discovered environment textures and high dynamic range images (HDRI). All of a sudden a major aspect of realistic lighting made perfect sense.
Think about it: Look out your window and ponder how many light sources and objects there are in reality. From cars, to trees, to buildings, to sunlight, to sky light, to clouds, to grass - all of it functions as sources and effectors of light as well as objects for reflection. And that's only describing an outdoor environment. The same environment depth is true indoors to varying degrees.
Yet whenever we light our projects with lamps only, we're depriving our scenes of a TON of lighting and reflection information. There's almost no chance to achieve a truly realistic render. But it's terribly impractical to build a comparable world of 3D models. This is where environment textures really shine.
While any image format can be used, including JPEG and PNG, HDRI's offer the most potential for realistic environment lighting. I cannot recommend enough that you develop a collection of HDRI images that you can cycle through whenever you're lighting your Blender projects. Don't deprive your scenes any longer! Your renders can go from horribly unrealistic to believable in literally one step.
2. Global Illumination (GI)
That's just a fancy term for light bouncing. Because when light energy hits a surface, it bounces off with less energy on to other surfaces. This happens over and over until the energy dissipates enough to not be visible. But until GI hit the scene (and could be renderable in a reasonable amount of time) all CG light calculations ended after hitting one surface. It was a limitation that wreaked havoc on realistic / believable renders. It required far too complicated light setups, struggles with overly dark shadows, and it spawned the blessing (and curse) of ambient occlusion.
If you used Blender before Cycles, you know the struggle I'm talking about. Since Cycles was introduced in 2011 it has spoiled us with accessible, physically based GI out of the box. And despite what you think you know about lengthy render times, pre-2011 was way worse for physically-based GI renders. Good riddance, Mental Ray.
Today GI is pretty much a standard among modern render engines. What once required tremendous patience and bravery now is simply enabled by default.
GI affects some situations more than others. For example character renders and object renders typically have a less noticeable effect whether GI is on or off. But GI is absolutely crucial for architectural visualization. Without it, renders wouldn't stand a chance at photo realism. Notice the tremendous difference between the images on the right.
I highly recommend you leave GI enabled for all your renders. Only in extreme situations - where you can't afford to wait a little longer - do I recommend turning off GI. Now that Cycles has denoising we have even less reason to turn it off.
When I was in college I had a teacher, Shawn Rinehart, drop one of the single most important lighting nuggets:
"Good lighting is all about gradients."
I should say that now I realize it was important. At the time I had no idea what he was talking about. Years later it finally sunk in.
If you look around you, gradients are everywhere. They're often most pronounced indoors but they also characterize outdoor lighting, some times of day more than others. For example sunset produces more gradients than high noon. But chances are you're reading this on a computer indoors which is perfect for observing gradients. Your walls and ceilings are washed in light gradients, beginning with the most intense light near your windows or lamps and gradually darkening the further away from the light sources you get.
Have you ever tried to decide on a paint color for a room? It's notoriously difficult because the paint swatch seems to look one way in your hand but the corresponding paint often looks very different once it's on the walls. The reason is light gradients. Light energy changes drastically as it travels through space and across surfaces, resulting in different values and hues of the same base color.
Applying this understanding to our renders is a big step in the right direction. Consider these two images:
The left one is bad. It feels flat. Light and dark values aren't varied much on the character nor on the back wall. The colors are stale too: Purple background, grey character. There's no interaction between the 2 main colors. It's almost as if the character was cut and pasted on top of the background. For a 3D render it's ironically lacking depth.
The right one, on the other hand, is making much use of gradients. The key light is close to the characters head with a rapid falloff, resulting in a blown out head and barely lit feet. That's one gradient. Note how that upper-left key light is orangy-yellow while the far right fill light is purple. It washes the whole scene in a gradient from orange to purple. That's another gradient. Unlike the left render, the right one has GI enabled which means color bleeding is realistically creating small gradients all over the place. Finally, the background uses a cylinder instead of a plane which produces a natural vignette around the character. That's another gradient.
See what I mean? Gradients make for a far more effective result. Shawn knew what was up.
4. Glossy Reflections & Fresnel
In the age of PBR, this one is old new perhaps. But I learned about the power of glossy fresnel reflections the hard way, long before PBR and Cycles. Back then glossy reflections, which we can think of as "true" reflection, was not the default render option because it took significantly longer to render especially when roughness was involved; i.e. blurry reflections as we referred to them back in the day.
"Mirror reflections? Sure the render engine can do that well enough. Blurry reflections? Ok, sure, see you next Thursday."
In the above images, the left one features "specular reflection". It's the old standard of approximating lamp-only reflection. It's an entirely inaccurate way calculating "reflection" as it ignores all other types including world environment reflections and object to object reflections. But since the specular format was the default for so long it's what most people worked with unless they learned about the additional magic of glossy reflections - also referred to as "raytraced" or "mirror" reflections. A key component of glossy reflections was the ability to control roughness/blurriness.
However glossiness was usually disabled out of the box, probably because it was painfully slow to render, especially when roughness was involved. The right-hand image above is a comparative example of glossy reflections with varied roughness values. Notice how the sun and the environment are reflected together for a far more believable render.
Once the superior results of glossy reflections over specular reflection sunk in, there was no going back for me. It also taught me how everything is reflective. Take it from John Hable on Wikipedia: "Everything is shiny." Even fully diffuse surfaces are merely glossy reflections with a high roughness value. So yeah, glossy reflections are important if you want to make believable renders.
Fresnel was more like a mysterious trick that only a minority seemed to know about in the early 2010's. I remember when a coworker explained the trick to me. My shaders became significantly more realistic overnight. It's not a secret anymore; especially not if you've learned anything from me. I've been evangelizing fresnel since day one of my teaching career.
Wait. You don't know about fresnel?! Let me explain!
Fresnel is a phenomenon that occurs with reflection where the surfaces that face your eyes most directly reflect less than the surfaces angled away from your eyes. The image below (from Missouri State) showcases a spectrum of fresnel values on a sphere:
If that's not enough, this cool trick is my favorite example of fresnel working in reality. Note how the water on the ground hardly reflects at all when looking directly at it. But as the camera is positioned into an extreme glancing angle, the water reflects like a mirror.
Thankfully as technology has advanced, glossy + fresnel reflections are pretty much standard among modern render engines and their materials. You no longer have to go digging for them at the bottom of the toolbox. Former technology and methods are fading away. Blender Internal, may she rest in peace.
5. Colorful Lights
I see a lot of artists who have the same hesitation I had for years: A fear of getting colorful with my lights. I'd say for the first 10 years of my CG journey I only strayed away from white light in very small steps. If 0 is completely desaturated and 1 is full saturation, I would only go as far as 0.2 or 0.3. Take a look at these renders:
They're in sequential order from left to right, starting when I was in college (2009) through to today (2018). The first 5 images showcase how hesitant I was experiment with light colors. They maintain the same basic formula: Subtly warm key light (yellow/orange) with a subtly cool fill light (blue). Not that the formula is bad, it can just get boring and overused. There's more interesting color opportunities out there to discover and harness. Desaturated lights certainly have their place but so do saturated lights. In the past couple years I've finally begun to explore more colorful lighting, as seen in the final two images on the right.
I can't remember why I was hesitant to venture away from white light for so many years, but again, I see it a lot with young 3D artists as well. It's been a good lesson for me to learn and highly I recommend you try it yourself.