Rendering an Image of a 3D Scene: an Overview

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7 mns read.

Summary

We are not going to repeat what we explained already in the last chapters. Let's just make a list of the terms or concepts you should remember from this lesson:

One of the things that we haven't talked about in the previous chapters is the difference between rendering on the CPU vs rendering on the GPU. Don't associate the term GPU with real-time rendering and the term CPU with offline rendering. Real-time and offline rendering have both very precise meanings and have nothing to do with the CPU or the GPU. We speak of real-time rendering when a scene can be rendered from 24 to 120 frames per second (24 to 30 fps is the minimum required to give the illusion of movement. A video game typically runs around 60 fps). Anything below 24 fps and above 1 frame per second is considered to be interactive rendering. When a frame takes from a few seconds to a few minutes or hours to render, we are then in the category of off-line rendering. It is very well possible to achieve interactive or even real-time frame rates on the CPU. How much time it takes to render a frame depends essentially on the scene complexity anyway. A very complex scene can take more than a few seconds to render on the GPU. Our point here is that you should not associate GPU with real-time and CPU with off-line rendering. These are different things. In the lessons of this section, we will learn how to use OpenGL to render images on the GPU, and we will implement the rasterization and the ray-tracing algorithm on the CPU. We will write a lesson dedicated to looking at the pros and cons of rendering on the GPU or the CPU.
The other thing we won't be talking about in this section is how rendering and signal processing relate to each other. This is a very important aspect of rendering, however, to understand this relationship you need to have solid foundations in signal processing which potentially also requires an understanding of Fourier analysis. We are planning to write a series of lessons on these topics once the basic section is complete. We think it's better to ignore this aspect of rendering if you don't have a good understanding of the theory behind it, rather than presenting it without being able to explain why and how it works.

Figure 1: we will also need to learn how to simulate depth of field (top) and motion blur (bottom).

Now that we have reviewed these concepts you know what you can expect to find in the different sections devoted to rendering, especially the sections on light transport, ray tracing, and shading. In the section on light transport, we will of course speak about the different ways global illumination effects can be simulated. In the section devoted to ray-tracing techniques, we will study techniques specific to ray tracing such as acceleration structures, ray differentials (don't worry if you don't know what the is for now), etc. In the section on shading, we will learn about what shaders are, we will study the most popular mathematical models developed to simulate the appearance of various materials.

We also talk about purely engineering topics such as multi-threading, multi-processing, or simply different ways the hardware can be used to accelerate rendering.

Finally and more importantly, if you are new to rendering and before you start reading any lessons from these advanced sections, we recommend that you read the next lessons from this section. You will learn about the most basic and important techniques used in rendering:

Ready?