Developing for Windows Phone and Xbox Live : Introduction to 3D Graphics

3D Graphics in XNA Game Studio

Developing 3D graphics is similar to creating real-time art. Even more impressive are the detailed and interactive 3D graphics found in modern real-time computer games. It is impressive when you compare it against computer graphics of ten years ago. Console and PC games were limited to a fixed set of hardware features, and resolutions were quite low and far from today’s high resolution, multisampled, antialiased, and almost realistic images.

With all of the new fancy hardware-dependent graphical improvements of the past decade also came an increase in complexity when creating graphics for games. Developers deal with many complexities, even when drawing simple graphics on the screen.

XNA Game Studio has attempted to limit these complexities in some cases and in others to solve them all together. As discussed in previous chapters, the capability to have a basic graphics window display a clear color is trivial and requires only that you create a new project for one of the supported platforms. To get to that point, using C++ with DirectX or OpenGL can take hundreds of lines of code. The graphics APIs exposed by XNA Game Studio are easy to use and often lead to the developer to fall into the correct solution using them.

In this chapter, we cover the basics developers should know before diving into creating 3D graphics. We start with defining 3D graphics and their makeup. Then, we focus on some basic math that is required when working on 3D graphics. Next, we cover the different stages that are part of the graphics pipeline. We finish by covering the XNA Game Studio GraphicsAdapter and GraphicsDevice classes and drawing the first 3D primitives on the screen.

What Are 3D Graphics?

The answer to this question might seem simple, but it might not be obvious to those who have no experience creating 3D games or applications. In reality, 3D graphics are just an illusion. They are flat 2D images on a computer monitor, television screen, or phone display.

In the real world, your eyes work together to create an image that contains visual perspective. Objects appear to be smaller the farther they are away from you physically. Imagine you are standing in the middle of perfectly straight and flat railroad tracks. If you look down at your feet, the rails appear to be some distance apart. As you let your eyes move towards the horizon and look farther down the tracks, it appears that the track rails become closer to each other the farther they are away from you. Having two eyes adds to this depth perception because each eye has a slightly different view. They help to see slightly different angles of objects. This difference is even greater for objects near to you. If you alternate closing each eye, you see objects move left and right.

When you create 3D graphics is perspective is an illusion. When you are working with 3D graphics concepts, remember that you are really creating a 2D image that is displayed on the screen. Many parts of the graphics pipeline have nothing to do with 3D at all and work only on the pixel level. A friend of mine once described to me a helpful way to look at developing 3D graphics games. He related creating 3D graphics to how great painters create stunningly realistic pieces of art. Although to a viewer standing still the painting can appear to represent real 3D objects in the world, it is still just layers of paint on flat canvas. They are illusions just like computer-generated 3D graphics.

So what are 3D graphics? They are computer-generated images that give the appearance of depth and perspective, but in reality, they are just flat 2D images when displayed.

Makeup of a 3D Image

3D graphics images are generated differently. At their core, they are a bunch of triangles drawn on the screen. If this sounds simplistic, it is in order to be fast. Modern graphics hardware can be so fast because the types of operations it performs are limited when compared to general-purpose processors used for CPUs in PCs. Graphics hardware has dedicated hardware for these operations and are designed to process millions of triangle and pixel operations.

Groups of triangles drawn on the screen are commonly called geometry. These collections of geometry can represent almost anything from a box, to a space ship, to a human marine storming the beaches of Normandy.

Each triangle is made up of three vertices. Each vertex represents one of the corners of the triangle and must contain at least the position of itself. A vertex can also contain other data such as color, texture coordinates, normals, and so on.

Although triangles determine the shape of what is displayed on the screen, there are many factors that determine the final color of pixels the triangle covers on the screen.

2D Sprites Are Actually in 3D!

When you draw 2D sprites in XNA Game Studio, you use the 3D graphics pipeline. In the past, 2D graphics were drawn using a technique called blitting, which was copying memory from a surface such as a texture to the display memory. With today’s graphics cards, it is easier and faster to just draw two-textured triangles when you want to draw a texture on the screen. Because sprites are also 3D geometry, many of the topics in this chapter are also applicable to them.