**Part 2: Global Illumination**
[Dynamic Diffuse Global Illumination](index.html)
2019 April 16
Updated 2019 April 24
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This is a simplified introduction to the core concepts of global
illumination for real-time application developers. The focus is on
introducing the terms needed to understand Dynamic Diffuse Global
Illumination.
I'm skipping a lot of technical detail and using contemporary game
artist jargon. I mention the academic terms in passing to connect to
research publications. [The Graphics Codex](http://graphicscodex.com) has a deeper
explanation that is still friendly.
Direct Illumination
====================================================================
 The image on the right is a 2D schematic of a gray box. The left wall
is painted yellow and the right wall is painted cyan (blue-green). At the top of the
box is a light bulb, which I'll call an *emitter* to distinguish
it clearly from the "light" energy in rays. On the top right of the box
is the camera, and resting on the bottom is a gray sphere. The dark
rectangle on the floor represents the shadow of the sphere.
*Direct illumination* is the light that arrives at a surface straight
from the emitter. I've drawn the path of two direct illumination light
rays as bright white arrows. There are an infinite number of other direct illumination
rays.
When the direct illumination scatters off surfaces into the camera, it can
see those surfaces. The color that we see for a surface is the color of light
that the surface reflected. For the gray sphere, this is represented by the gray arrow.
For the yellow wall, it is a yellow arrow.
We call these surfaces that the camera sees from its viewpoint
the "[camera] primary surfaces". Note that any surface that receives
direct illumination is a primary surface _from the light's viewpoint_.
 On the right is an example of direct illumination
in a 3D scene. The sun shines through a hole in the roof on the left wall. Shadows
are completely black because there is no direct line to those locations from the sun.
The emissive skybox is also visible in the distance. The image is very dark because
most of the scene is not lit by direct illumination.
Direct illumination is what is computed with pixel shaders and shadow maps in real-time
for most games today.
Global Illumination
====================================================================
 There are also *secondary* surfaces that the camera can't see directly,
but that affect the image. For example, some direct illumination strikes
the cyan wall that is behind the camera. The cyan wall reflects cyan light
onto the sphere in the center. (Light reflection is interchangably also called
"scattering" or "bouncing", and it is the same process as "shading".)
The camera can't see the cyan wall. But it can see the side of the
sphere that is lit with cyan reflected light. That light is
indirect, or *global illumination*, abbreviated as "GI".
In this case, the cyan light on the side of the sphere
has reflected twice before reaching the camera.
 Since the light keeps reflecting off surfaces until it is completely absorbed,
there is also global illumination of three, four, five, and more reflections.
The image on the right shows a path with five reflections. It is dark green
by the end because the cyan wall absorbed all of the red and the yellow wall
absorbed all of the blue.
In the real world, global illumination can reflect infinitely.
However, some light is absorbed at each surface. After a few
reflections there is so little that it is not worth simulating any
further. Offline rendering for movies typically uses between five
and ten reflections.
Global illumination in games has previously been baked (precomputed)
offline during production for games and stored in light maps or
irradiance probes. There are some clever tricks for dynamic GI such as
screen space ray tracing, but most global illumination in previous
games can't change with the scene. With GPU accelerated geometric ray
tracing, upcoming games can now compute high-quality global illumination in real
time.
In academic terminology, "global" illumination includes the direct
light. In game terminology, "global" usually refers to only the indirect
light and direct light is computed separately.
 Here's the 3D
scene with both direct and full global illumination, taken as a screenshot
from our real-time implementation. I'm showing just the light,
without materials (textures). The global illumination has a red tint because the unseen
wall materials are red. Most of the light in this scene is from global illumination.
The renderer used thousands of reflections for each light path. But
some light is lost at each reflection and there is finite precision in the texture
map. So, only the first twenty or so reflections make a noticable difference in image quality. For example,
with 90% reflective walls, after 20 reflections 1 - 0.920 = 87% of the light has been
absorbed.
Visibility
=========================================================
 In discussing
rendering, *visibility* generalizes from "what the camera sees" to whether there is an unobstructed straight
path between any two points.
Primary surfaces have visibility _to the camera_. Points
that receive direct illumination have visibility _to the emitter_.
Shadows are locations that do not have visibility to the emitter.
For correct global illumination, it is important that objects
can cast indirect light shadows. This means computing visibility
for all points and only allowing light to reflect between ones
that have visibility.
It seems obvious that objects should block indirect light as well
as direct light, since they do in the real world. Computing visibility
for global illumination can be tricky, however. Just as 3D games before
the year 2000 often did not have correct shadows, many games today
either intentionally omit visibility for global illumination or struggle
to hide errors in that visibility computation. We call these *light leaks*
or *shadow leaks* depending on whether the visual error is bright or dark.
Glossy Reflection
=========================================================
The diagram on the right shows a schematic of a "flat" surface
viewed under a magnifying glass. Everything
is usually a little bit rough microscopically.
The surface acts like a broken mirror even for surfaces that you
wouldn't think of as mirror-like, such as rocks, apples, or leather.
There are lots of mirror fragments called *microfacets*.
When light hits the surface and reflects right off a microfacet, that
creates a glossy reflection. (The terms specular and shiny are also
used for this, and GGX and Blinn-Phong are two microfacet models that you
might be familiar with.)
!!! Info Glossy Reflection
(Specular, shiny, microfacet, roughness/smoothness, GGX, Blinn-Phong, highlights, etc.)
- Light scatters *at* the surface
- Appearance changes with angle
- Reflection color is often unchanged by the material
The reflection direction is in the mirror direction of the
microfacet--it bounces like a billiard ball. If the surface is
microscopically smooth, then you see a perfect mirror reflection. When
the surface is rough, that reflection gets blurry or dull because
different facets are sending light in different directions.
If you look directly at something really bright
in the reflection, such as an emitter, then you see a highlight. But really everything
is being reflected, it is just sometimes too dark to notice
unless you look carefully.
DDGI will light the surfaces seen in glossy reflections, and it can
incorporate second-order glossy reflections into the GI paths that
lead to a final diffuse reflections. But it does not render glossy
reflections for the camera's view. This article briefly describes a
technique for glossy ray tracing but does not discuss how to optimize
it in production as the focus here is on diffuse GI.
Diffuse Reflection
=========================================================
For some surfaces, such as office walls, you don't see much glossy
surface reflection. Instead of scattering off the surface, most of
the light that shines on those surfaces penetrates a little under the
surface. The light scatters around and then comes back out in a random
direction. This is *diffuse* reflection.
Because the light interacts heavily with the material, the color of
light that emerges has been filtered by the material. This works a
little differently in the real world than in real-time rendering
because most rendering only tracks the three color contributions of
red, green, and blue. But almost all of the time that approximation is
good enough, at least for entertainment applications.
!!! Info Diffuse Reflection
(Matte, Lambertian, Oren-Nayar, Henyey-Greenstein, Diffuse BRDF, etc.)
- Light scatters just below the surface
- Visible from all directions
- Color is the product of the light and material colors
The same physical process gives rise to deep subsurface scattering and
transmission, which we don't need for describing DDGI.
The picture on the right
shows the diffuse portion of global illumination in our 3D scene. This includes some diffuse light that is scattered
off tiny dust and fog particles in the air, creating *volumetric* effects.
In physics terminology, the diffuse global illumination I've described
is the irradiance field. It is all kinds of reflection, where last
reflection before the camera is diffuse. Modern light maps/radiosity normal maps,
voxels, and irradiance
probes store this kind of lighting, although often with some problems
handling visibility. The diffuse GI/irradiance field is different from
*radiosity*, which requires all reflection everywhere to be diffuse.
That's what Quake III and older light maps stored.
Summary
=========================================================
We've now covered all of the terms needed to understand what the DDGI
technique provides:
- *Dynamic* - Everything can change at runtime: camera position, emitters, geometry, and even materials.
- *Diffuse* - The last reflection into the camera is from the diffuse part of the material. The GI paths up to the last reflection can be both glossy and diffuse.
The glossy part of the last reflection is handled separately with glossy ray tracing or screen-space ray tracing.
- *Global Illumination* - Light that has reflected off at least two surfaces for reaching the camera, and up to infinite reflections within texture map precision.
as well as the key idea of *visibility* for global illumination, which DDGI computes
in a new way to prevent light leaks and speed workflow.
[ <== Previous Page: Introduction](index.html)
[Next Page: DDGI Overview ==> ](overview.html)
* * * *
I wrote this blog article in collaboration with my colleagues [Zander Majercik](https://research.nvidia.com/person/zander-majercik)
and [Adam Marrs](http://www.visualextract.com/) at NVIDIA.