The Complete Guide to Photorealism for Visual Effects, Visualization and Games

Eran Dinur

• Publisher: Routledge
• Year: 2021
• ISBN:
  9780429534348
• Edition: 1
• Topic: Media & Performing Arts
• Subtopic: Film & Video

This book offers a comprehensive and detailed guide to accomplishing and perfecting a photorealistic look in digital content across visual effects, architectural and product visualization, and games.

Emmy award-winning VFX supervisor Eran Dinur offers readers a deeper understanding of the complex interplay of light, surfaces, atmospherics, and optical effects, and then discusses techniques to achieve this complexity in the digital realm, covering both 3D and 2D methodologies. In addition, the book features artwork, case studies, and interviews with leading artists in the fields of VFX, visualization, and games. Exploring color, integration, light and surface behaviour, atmospherics, shading, texturing, physically-based rendering, procedural modelling, compositing, matte painting, lens/camera effects, and much more, Dinur offers a compelling, elegant guide to achieving photorealism in digital media and creating imagery that is seamless from real footage.

Its broad perspective makes this detailed guide suitable for VFX, visualization and game artists and students, as well as directors, architects, designers, and anyone who strives to achieve convincing, believable visuals in digital media.

PART 2

THE REAL WORLD

DOI: 10.4324/9780429244131-6

A late-afternoon view of Todi, a lovely town in Umbria. Or, in the language of this book: Sunlight photons scattering through air, humidity and dust in the atmosphere, bouncing off stone, wood and leaf surfaces, refracting in the lens, and finally hitting the sensor.

Chapter 4

Light Essentials

DOI: 10.4324/9780429244131-7

Is light particles or waves? It travels in a straight path and is reflected at a predictable angle – a particle behavior. But specific light phenomena like diffraction can only be explained through wave interference patterns. The waves/particles question was already a subject of debate back in the 17th century, when Huygens’ wave theory was pitted against Newton’s Corpuscular theory (which argued that light is particles). Newton’s approach was widely accepted, and the wave theory went out of favor until the early 19th century, when fresh ideas from Augustin-Jean Fresnel and Thomas Young revived it. The understanding of electromagnetic waves continued to evolve throughout the 19th century and solidified the light wave theory. As a result, the particle approach was nearly neglected. Then came Albert Einstein, who, along with other revolutionary physicists like Max Planck and Niels Bohr, established the theories of quantum physics. The centuries-old debate was no longer relevant, as these theories proved that light can in fact be both particles and waves.

The wave–particle duality of light is certainly a baffling concept, as it seems to contradict basic notions of classical physics (Bohr called it the “duality paradox”). But since this book is not about quantum physics, the question to be asked is: does the concept of particle/wave duality bring any benefit to the digital artist? I believe it does! Looking at light from either a wave or a particle perspective (and sometimes both), makes it easier to understand different aspects of light behavior. For example, color is best approached from a wave perspective, while the difference between specular and diffuse reflections is easier to understand from a particles perspective. Later in this book, when we explore CG rendering techniques, you will notice that some relate directly to either one of those perspectives. For example, thin film diffraction calculates light as waves, while photon mapping calculates light as particles. Digital artists, therefore, can embrace the concept of light duality to advance their understanding of light behavior and optics, even without delving into quantum physics. Here is a quick description of light from each perspective:

Light as Waves

Visible light comprises a rather small portion of the entire electromagnetic spectrum. Waves on this spectrum are defined by their frequency (measured in hertz units) and their wavelength (measured in metric units). At the very bottom of this spectrum lie extreme low frequency radio waves (around 10 hertz), which can be as long as 100,000 kilometers. At the top end, Gamma ray frequencies lie around 300 exahertz (300 × HZ¹⁸), with wavelengths as small as one trillionth of a meter (1 picometer). In between (moving from long to short waveforms) are medium and high frequency radio waves, microwaves, infrared, visible light, ultraviolet, and X-rays. The visible light portion of the spectrum starts with red (frequency around 400 terahertz) and ends with violet (frequency around 780 terahertz). There is no specific quality that differentiates visible light from other electromagnetic waves, except for the fact that this is the portion of the spectrum that the human eye can detect. Moreover, the visible light range is not consistent for other living beings. Some animals can see only a limited part of it, others can detect frequencies below or above it (some types of snakes, for example, can sense infra-red, while many insects and birds can see ultraviolet). The range of colors along the visible spectrum (red, orange, yellow, green, cyan, blue, and violet) is considered pure (fully saturated), because it represents a single light frequency. As I showed in Chapter 3, pure colors can be achieved by mixing just two of the three primary colors (red, green, or blue). However, most light sources tend to radiate a mix of different frequencies, so most of the light we see is somewhat desaturated (sunlight, which encompasses nearly all visible light frequencies, is white, or fully desaturated when unfiltered by the atmosphere).

Light as Particles

Like all electromagnetic waves, the energy of visible light depends on its frequency. The smaller the frequency, the higher the energy. In quantum mechanics, this energy is said to be quantized (quanta = “how much” in Latin) into defined “energy packets” called photons. Blue light photons (higher frequency) have more energy than red light photons; however, all photons travel at the same speed, regardless of their frequency (in vacuum, the speed of light is roughly 300,000 km, or 186,000 miles, per second). We are surrounded by light photons travelling in all directions as they are emitted and then scattered/reflected by surfaces. But most of this activity is completely invisible to us because our eyes can only detect light when photons hit the retina directly. This means that we can only see light if we look directly at its source (the object that is emitting radiation in the visible spectrum), or if we look at a surface that reflects photons back toward our eyes. In other words, we cannot observe photons “from the side” like watching traffic on the highway. Photons must be on a straight collision path with our eyes (or cameras for that matter) for the light to be registered by the brain (or the camera’s sensor). Therefore, we cannot see “light rays”. Those shafts of light on a concert stage, or the beams of sunlight entering a room through cracks in the shades, are visible only because light is reflected off particles like dust and water droplets and redirected toward our eyes. Without some presence of a reflecting/scattering volume, even laser beams are invisible (indeed, and despite the sci-fi convention, you cannot see laser beams in space).

Light Decay

The intensity of light falls off with distance at a quadratic rate. The inverse-square law states that light intensity is inversely proportional to the square of the distance from the light source. For example, if we consider the intensity of light at a distance of a foot from the source as one, then that intensity will drop to a quarter at 2 feet (one divided by the square of 2), and to a mere sixteenth at 4 feet (one divided by the square of 4). Light decay has nothing to do with the energy or speed of the light, as both are not affected by distance (we can see stars that are hundreds of light years away). Light decay is only related to how thinly the photons are spread out. At any given moment, a certain number of photons are emitted from a light source. These are tightly bunched together close to the emitter, but as they travel out in all directions, the distance between them becomes progressively larger. Thus, the farther away an object is from the light source, the fewer photons are likely to hit it. When we look at a star in the sky, we look directly at the source, where the photons are closest together. Traffic lights, to use another example, are too weak to even illuminate the sidewalk just a few feet below them, yet can still be easily visible from hundreds of feet away, because we see the densely packed photons at the source.

The quadratic decay of light means that there is a substantial increase in luminance in the vicinity of a light source. If you moved a light meter around a living room that is only illuminated by artificial lights, you would notice the luminance values spiking sharply when the meter is close to a lamp. If you would do the same during daytime, the values will spike even higher when you aim the meter at a window (the window is of course not a light source, but it is an opening to the considerably brighter environment outside). This is important, particularly in the context of specular reflections. Typically, light sources are much more dominant in specular reflections than the rest of the environment, due to that pronounced spike in luminosity (more on this in Chapter 5).

Photons retain their energy, but their density falls off with distance at a quadratic pace.

Pulling down the exposure shows the huge spike in luminance in the vicinity of the light sources.

Direct and Indirect Illumination

The only way our eyes (or the camera) can pick up direct light is by looking straight at the light source. When a camera is pointed at a light bulb, the photons emitted from the burning filament reach the sensor on a direct path. We can therefore say that the sensor is recording direct illumination. Everything else that is visible in the frame is the result of indirect illumination – photons that bounce off one or more surfaces before reaching the sensor. Simply put, most of the light that the camera captures is indirect (bounced light). This notion is, however, somewhat confusing for digital artists, because direct and indirect illumination is defined differently in CG jargon, where the distinction is based on the light-receiving surface, not the camera: if a certain surface is illuminated directly by a light source, it is said to be receiving direct light. If it is illuminated by light that is bouncing off surrounding surfaces, it is said to be receiving indirect light. This separation makes more sense from a rendering perspective: direct light is substantially faster to calculate and less noisy than indirect light (it is coming from a small, defined area rather than from multiple surfaces and directions). But in real-world terms, even the separation between direct and indirect light is somewhat artificial. One can argue, for example, that a camera aimed at the sun is not recording purely direct light – by the time sunlight reaches the sensor it has passed through many miles of air, water droplets, and dust, and has already been scattered, absorbed, and refracted to a certain extent. Categorizing light as direct or indirect is helpful when analyzing photographs and understanding light behavior, but it is important to remember that, unlike in CG lighting, the real-world interaction of light with different mediums is complex, irregular, and often difficult to categorize neatly.

Left: In real-world/photographic terms, the camera is capturing direct illumination only if it is aimed at a light source – everything else is reflected/scattered light. Right: in CG, the distinction is made per surface: is it receiving direct or reflected light?

What Is “Ambient Light”?

It is more of a notion than a physical definition. In other words, there really is no such thing as “ambient light”. This may sound like an odd statement, considering how much we use this term in digital (as well as traditional) art. Let me explain: first, we need to acknowledge that the term ambient light has more than one meaning, depending on the context: when photographers talk about ambient light, they usually refer to the existing, available light in the scene (as opposed to additional lighting supplemented by the photographer). For an interior designer, the term ambient light distinguishes between the basic, general lighting layer in a room and more specific lighting layers such as task lighting and accent lighting. These usages of the term ambient light are quite different from (and not related to) the particular meaning I am referring to here: the common use of the term “ambient light” in visual art (and specifically in digital art) to describe the sort of complementary, soft, uniform light in the scene that does not seem to be coming from a distinct source or direction and is not casting distinct shadows.

This categorization can be misleading, because it infers that such a light exists as a discrete entity that is separate from other “direct” lights in the scene, or that some l...