iPhone Game Development

The Render Engine

There are several APIs for drawing in the iPhone SDK, each useful for different purposes. Cocoa uses Objective-C and focuses on manipulating high-level GUI objects such as UIImage, UIColor, and UIFont. Quartz Core Graphics uses C and is great for drawing vector graphics such as lines, gradients, and shading, and for rendering PDF content. However, most games require a high-performance rendering API, which is why OpenGL ES is available.

OpenGL ES is a very low-level API focused on 3D rendering, so we are going to have to build up some code to bridge the gap between what we want to do—draw 2D animated sprites—and what OpenGL ES can do: render textures onto 3D geometry.

2D Graphics with a 3D API

At this point, you might be asking yourself: “How are we going to use OpenGL to draw 2D sprites? I thought OpenGL was for 3D!”

Without getting into the math, 2D is a subset of 3D. That means anything you can do in 2D you can also do in 3D. Specifically, imagine a table with a piece of paper on it. The table and the paper are 3D. However, if you point a camera straight down at the paper, the camera sees what looks like a 2D scene. That is essentially what you will be doing with your 2D game. Meanwhile, when you get to the 3D example, you can use the full power of OpenGL ES with very little additional work, since it’s already set up.

Three-dimensional rendering concepts and code can become complicated and confusing very quickly, and this is not a book about OpenGL ES. For this chapter ...