Setting the Viewport and Clearing the Color Buffer

Now that we have created a rendering surface with EGL and initialized and loaded shaders, we are ready to actually draw something. The Draw callback function draws the frame. The first command that we execute in Draw is glViewport, which informs OpenGL ES of the origin, width, and height of the 2D rendering surface that will be drawn to. In OpenGL ES, the viewport defines the 2D rectangle in which all OpenGL ES rendering operations will ultimately be displayed.

// Set the viewport
glViewport(0, 0, esContext->width, esContext->height);

The viewport is defined by an origin (x, y) and a width and height. We cover glViewport in more detail in Chapter 7, “Primitive Assembly and Rasterization,” when we discuss coordinate systems and clipping.

After setting the viewport, the next step is to clear the screen. In OpenGL ES, there are multiple types of buffers that are involved in drawing: color, depth, and stencil. We cover these buffers in more detail in Chapter 11, “Fragment Operations.” In the Hello Triangle example, only the color buffer is drawn to. At the beginning of each frame, we clear the color buffer using the glClear function.

// Clear the color buffer
glClear(GL_COLOR_BUFFER_BIT);

The buffer will be cleared to the color specified with glClearColor. In the example program at the end of Init, the clear color was set to (0.0, 0.0, 0.0, 1.0) so the screen is cleared to black. The clear color should be set by the application prior to calling glClear on the color buffer.