Chapter 4. Lighting and Rendering

One of the most compelling features of UE4 is the amazing visual quality it can achieve. By leveraging physically based rendering (PBR), UE4 achieves staggering realism. Surfaces react to light and shadow in amazingly realistic ways. The Material Editor uses a visual graphing system that empowers you to bring complex, dynamic, physically based Materials to life while Material Instances and Functions let you reuse your Materials and modify them in real time. You might never want to go back to your old renderer again.

Unreal Engine 4’s renderer takes the PBR technology created for Hollywood blockbusters and presents a powerful, artist-friendly, easy-to-learn material and lighting system, combining physically based materials, lights, and reflections as a unified system to create nearly ray-traced quality images in real time.

Those who have been using physically based renderers such as Maxwell should be able to grasp the concepts in UE4s renderer very quickly. To others, this change is a bit confusing at first, but after you see how good the materials and lighting look, how straightforward they are to set up, and how remarkably well they hold up in almost all lighting conditions, you’ll have a hard time going back to your old, specular-based materials.

These values are almost never fully black or fully white. Charcoal and Fresh Asphalt are around .02 (on a scale of intensity from 0 to 1), whereas Fresh Snow should be around .81, with Fresh Concrete and Sand somewhere in the middle at .51 and .36, respectively.

Instead, you use a single, grayscale or float value to determine both the brightness and tightness of the specular reflections: Roughness (see Figure 4.2). You can drive the Roughness either through a single value or by using a texture. In Figure 4.2, note that only the Roughness values have been modified. Notice how the specular highlights and reflections are both blurred as roughness increases.

In the real world, the rougher a surface is, the more diffuse or blurred the reflected light is. This is because of microscopic imperfections on the surface. These imperfections scatter the light that hits them whereas smooth surfaces without these imperfections don’t scatter the reflections, resulting in a sharp, mirror-like reflection.

Tools such as Substance Bitmap 2 Material (B2M) can help you prepare your existing Diffuse textures for PBR rendering.

You can also generate these maps manually in Photoshop or other image-editing software, but tools such as Substance, Quixel Suite, or Substance B2M can generate far better-looking map sets much faster.

These tools use image processing techniques to generate Normal, Roughness, and Base Color maps by analyzing the shapes and lighting in the source images.

Lighting in a ray-tracing engine is slow, but typically very dynamic. You can easily move sunlight around, modulating the color to simulate a sun setting. Flawless GI fills each nook, corner, and crevice with rich light and shadow. The price you pay is time. Even a simple scene with a few basic primitives takes several seconds to render. Most real-world scenes take minutes to hours. To achieve real-time frame rates, we only have a fraction of a second to render the frame.

By restricting what lights can and can’t do, massive performance benefits have been realized. These restrictions can be difficult to cope with at first, but by knowing what to expect and what you can and can’t do, you can avoid some costly blunders.

Shadowing is expensive² for Dynamic Lights, so use it sparingly. The larger a light’s radius, the more expensive it is to render and the more Actors it will need to generate shadows for.

Dynamic shadows are low resolution and are generally very crisp edged. You cannot easily get soft attenuated shadow edges as you would with pre-calculated static shadows (see Figure 4.4).

Stationary Lights are useful for statically lit scenes because they can add detail and allow dynamic Actors to cast dynamic shadows; however, you must use them carefully to avoid causing some severe performance penalties.

Although Stationary Lights can provide the best of both worlds, there is a serious limitation to this magic. Each statically lit Mesh can only be influenced by four Stationary Lights at a time.

If more than four Stationary Lights affect a single Mesh, additional Stationary Lights will revert to Moveable Lights with dynamic shadows and have an extreme performance penalty.

To help avoid this situation, UE4 alerts you with both descriptive icons in the Editor Viewport and warnings when you build your lighting.

You can preview stationary Lightmap Overlaps by choosing the appropriate view mode in the Editor. In Figure 4.6, the four Spot Light Actors are all set to Stationary as is the Directional Light sunlight coming in from the window. The additional Spot Light cannot be rendered as a Stationary Light and will instead be rendered as more expensive Moveable Light.

You can adjust the softness of Static shadows by changing the Light Source Radius property of the Light Actor. You won’t see the effect until you render the lighting in Lightmass.

Reflections are essential for a PBR system to function. Without scene reflections, materials lose depth and are forced to rely on direct specular highlights to show surface information. This leads to the plastic, shiny look we’ve all come to associate with computer graphics.

Reflections are something that ray-tracing is great at, but incredibly slow at, especially when you start blurring those reflections. How does UE4 manage it? Mostly by faking it.

The two main reflections in a UE4 scene are static cube maps generated by Reflection Capture Actors and a screen-space reflection Post Process Effect. Both are automatically applied to materials and modulated appropriately. This helps with authoring materials and lighting scenes because you won’t have to customize materials specifically for scenes as you might have had to do with previous game engines or other renderers.

They also contribute to ambient lighting on both Dynamic and Static Meshes, making them essential for high-quality lighting.

Reflection Capture Actors cannot be updated in any way at runtime.

With traditionally rendered visualizations, you typically apply these effects to your rendered frames in a video editing or effects package such as After Effects or Nuke.

In UE4, you apply Post Process Effects in real time by defining them either in a scene Actor called a Post Process Volume or with the Post Process settings in a Camera Actor. These settings can have a dramatic effect on the scene’s look and quality and are essential for making your visualizations look as good as possible (see Figures 4.8 and 4.9).

These settings can inherit from and override each other using a priority system and can blend seamlessly from one to another.

Post Process Effects in UE4 are screen-space, meaning they only exist in the pixels being rendered onscreen. They cannot produce effects or react to anything that isn’t being directly rendered.

Unreal Engine 4 uses a custom-built temporal anti-aliasing (TAA) system that produces almost perfectly anti-aliased images at the cost of some image sharpness and some possible ghosting artifacts in motion³. TAA samples multiple rendered frames in succession and compares them to create an average.

A less effective FXAA screen-space anti-aliasing effect is also available that while better than no anti-aliasing is usually too rough to be used in visualization.

The softness of TAA, when used in conjunction with other Post Process Effects can help give your image a filmic, realistic look and gives the highest overall quality (see Figure 4.10).

Lens flares simulate the interlens reflections that are captured when cameras are exposed to high-contrast lighting conditions.

UE4 renders the scene using a linear workflow and uses that information to determine luminance to create high-quality specular blooms and lens flares. These effects if used sparingly can help accentuate the HDR lighting in your scene and elevate the quality of your visualizations.

UE4 features high-dynamic range rendering, allowing both dark and very bright areas to exist within a single scene. As the player moves from indoors to outdoors, his view will adjust brightness smoothly, simulating the human eye and/or cameras using auto exposure.

A few different types of DOF exist in UE4: Gaussian, Bokeh, and Circle.

This effect is fast and gives your images a realistic, filmic look. You can use it in almost all of your visualizations with a minimal rendering cost.

In scenes with dynamic lighting, SSAO provides an enormous visual boost. Objects gain contact shadows helping them read as connected to the surfaces they sit on and it increases the depth of shadowed areas immensely. Although SSAO is no substitute for ray-traced or pre-calculated AO, it’s a huge improvement.

Screen-space reflections help complement the Reflection Capture Actors by providing dynamic, high-detail reflections (see Figure 4.14). This helps to ground objects and provide the most accurate looking and dynamic reflections possible.

Like all other screen-space effects, these reflections cannot render anything off-screen. This can create some transition artifacts and other strange effects as you move through a scene. Whether to use them is an artistic choice.

Screen Percentage controls how many pixels are being rendered for each pixel onscreen. For example, 100% is a 1:1 ratio; 80% indicates an 80% reduction in rendered pixels; and 140% is a 40% increase. You can use this setting in two ways: To reduce the resolution of the image to allow more graphically intense scenes to run on slower machines or to increase the resolution past the screen resolution, allowing the scaler to return a very crisp image. By combining Screen Percentage with TAA you can get amazingly clear image quality, approaching ray-traced quality.

As you increase the project’s resolution, Screen-space effects become increasingly slow to use. Users on high-resolution monitors will be more acutely impacted by this setting.

These can range from the simple, such as creating your own vignette system, to the complex, such as writing your own cell-shading and outlining effects (see Figure 4.15). The sky is your limit thanks to the flexibility of the Material Editor.

Although they are called Post Process Effects, they actually happen while you’re running the project, not after. They are rendered at the end of each frame, while the next frame is beginning to render. They run in real time, reacting to a dynamically changing scene and point of view.

Post Process Effects can make your scene look like an art student’s first photo-editing assignment with gaudy blooms, vignette, and lens flares dominating the view. If used wisely, they can also give it the most filmic, realistic look possible.

PBR introduces a whole new way of looking at and defining material surfaces that are artist friendly and look great. Combined with an arsenal of lighting tricks and tools, you can achieve near-ray-traced images in real-time.

1. Scalar values are just material shader language for floating point numbers.

2. The term “expensive” is often used to describe the performance penalty of an effect. Each frame is often thought of as having a specific “budget” and each light, polygon, and shadow has a cost against that budget.

3. Ghosting most often happens with high-frequency or noisy areas that change from frame to frame. Avoiding noise in your scenes helps hide this artifact.

4. UE4 offers a Forward Rendering pipeline that allows the use of MSAA while trading many of the benefits of a deferred rendering pipeline like Dynamic Lighting.