Preliminaries
Before we can talk about the process of creating a volumetric rendering pipeline. Some key mathematics and programming ideas involved within this article have to be explained.
Distance Fields
A distance field is a scalar field that specifies the minimum distance to the surface of a shape. If we examine this in more detail, a distance field can be represented by a function F, such that any given point P will return a distance d from the object represented by the function. We store the distances returned by such a function as 3D matrices or, more commonly known within graphics programming, a 3D texture. Each texture cell stands for the closest distance from the grid element to the nearest surface. Therefore, a grid element containing a value of 0 represents the surface of a shape.
A circle is represented by a 2D distance field – Inigo Quilez
Sphere Tracing
Visualising a distance field can be achieved by using an algorithm called sphere tracing. Sphere tracing is a technique for rendering implicit surfaces using a geometric distance. To find the distance towards a shape, we need to define a distance function for it or have a generated volume available to trace against. For example, a sphere with center (x=0,y=0,z=0) situated at the world origin (P) and radius r can be represented as followed:
This equation is what is called an implicit function. A sphere represented in this form is also called an implicit shape. An implicit equation only tells us if a particular point is inside a shape (negative values), outside a shape (positive values), or precisely on the surface (value of 0). The collection of points where the implicit function equals x is called an iso-surface of value x (or iso-contour in 2 dimensions). Sphere tracing is a method of drawing a surface solely based on this data. For more information about sphere tracing, click here
Boolean Ops
Volumetric data can easily represent shapes defined via Boolean operations. These operations are often used in CAD software in collaboration with a technique called Constructive Solid Geometry (CSG), which consists of the same operations only based on surface-data not on geometry, which makes this algorithm a lot more CPU intesive as new geometry has to be constructed on the fly. Modelling complex shapes by assembling simple shapes such as spheres, cubes, planes might be hard to achieve if we modelled our geometry by hand. Being able to blend implicit shapes is a quality that parametric surfaces lack and thus one of the main motivations for using them. For more information about Boolean operations, click here
Union
Merger of two objects into one
Subtract
Subtraction of one object from another
Intersect
Portion common to both objects
Examples of Boolean Operations - Wikipedia
Deferred Shading
Deferred rendering or deferred shading is based on the idea that we defer most heavy calculations (such as light calculations) to a later stage. We can achieve deferred shading with one geometry pass and one light pass. The geometry pass renders the scene once and stores distinct data about the displayed geometry in different textures, commonly known as the G-buffer. Position vectors, color vectors, normal vectors, and/or specular values make up the majority of this data. In the second pass, we render a full-screen quad and calculate the final render using the provided G-buffer. We only need to do our light calculations once when deferring them to a later stage because the G-buffer contains all of the data from the topmost fragment. If deferred rendering is a little fuzzy I highly recommend reading the following article: Learn OpenGL: Deferred Shading from Joey De Vries.