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This chapter is from the book

This chapter is from the book

Using the OpenGL ES 2.0 Framework

In the main function in Hello Triangle, you will see calls into several ES utility functions. The first thing the main function does is declare an ESContext and initialize it:

ESContext esContext;
UserData  userData;

esInitialize(&esContext);
esContext.userData = &userData;

Every example program in this book does the same thing. The ESContext is passed into all of the ES framework utility functions and contains all of the necessary information about the program that the ES framework needs. The reason for passing around a context is that the sample programs and the ES code framework do not need to use any global data.

Many handheld platforms do not allow applications to declare global static data in their applications. Examples of platforms that do not allow this include BREW and Symbian. As such, we avoid declaring global data in either the sample programs or the code framework by passing a context between functions.

The ESContext has a member variable named userData that is a void*. Each of the sample programs will store any of the data that are needed for the application in userData. The esInitialize function is called by the sample program to initialize the context and the ES code framework. The other elements in the ESContext structure are described in the header file and are intended only to be read by the user application. Other data in the ESContext structure include information such as the window width and height, EGL context, and callback function pointers.

The rest of the main function is responsible for creating the window, initializing the draw callback function, and entering the main loop:

esCreateWindow(&esContext, "Hello Triangle", 320, 240,
               ES_WINDOW_RGB);

if(!Init(&esContext))
   return 0;

esRegisterDrawFunc(&esContext, Draw);

esMainLoop(&esContext);

The call to esCreateWindow creates a window of the specified width and height (in this case, 320 × 240). The last parameter is a bit field that specifies options for the window creation. In this case, we request an RGB framebuffer. In Chapter 3, “An Introduction to EGL,” we discuss what esCreateWindow does in more detail. This function uses EGL to create an on-screen render surface that is attached to a window. EGL is a platform-independent API for creating rendering surfaces and contexts. For now, we will simply say that this function creates a rendering surface and leave the details on how it works for the next chapter.

After calling esCreateWindow, the next thing the main function does is to call Init to initialize everything needed to run the program. Finally, it registers a callback function, Draw, that will be called to render the frame. The final call, esMainLoop, enters into the main message processing loop until the window is closed.

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