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This chapter is from the book

This chapter is from the book

Displaying the Back Buffer

We have finally gotten to the point where our triangle has been drawn into the framebuffer. There is one final detail we must address: how to actually display the framebuffer on the screen. Before we get into that, let's back up a little bit and discuss the concept of double buffering.

The framebuffer that is visible on the screen is represented by a two-dimensional array of pixel data. One possible way one could think about displaying images on the screen is to simply update the pixel data in the visible framebuffer as we draw. However, there is a significant issue with updating pixels directly on the displayable buffer. That is, in a typical display system, the physical screen is updated from framebuffer memory at a fixed rate. If one were to draw directly into the framebuffer, the user could see artifacts as partial updates to the framebuffer where displayed.

To address this problem, a system known as double buffering is used. In this scheme, there are two buffers: a front buffer and back buffer. All rendering occurs to the back buffer, which is located in an area of memory that is not visible to the screen. When all rendering is complete, this buffer is “swapped” with the front buffer (or visible buffer). The front buffer then becomes the back buffer for the next frame.

Using this technique, we do not display a visible surface until all rendering is complete for a frame. The way this is all controlled in an OpenGL ES application is through EGL. This is done using an EGL function called eglSwapBuffers:

eglSwapBuffers(esContext->eglDisplay, esContext->eglSurface);

This function informs EGL to swap the front buffer and back buffer. The parameters sent to eglSwapBuffers are the EGL display and surface. These two parameters represent the physical display and the rendering surface, respectively. In the next chapter, we explain eglSwapBuffers in more detail and further clarify the concepts of surface, context, and buffer management. For now, suffice to say that after swapping buffers we now finally have our triangle on screen!

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