iPhone 3D Programming

Chapter 6. Blending and Augmented Reality

All colors are the friends of their neighbors and the lovers of their opposites.

—Marc Chagall

If you’ve ever used Photoshop to place yourself in front of the Taj Mahal in a paroxysm of wishful thinking, you’re probably familiar with layers and opacity. Alpha simply represents opacity on a zero-to-one scale: zero is transparent, one is fully opaque. Alpha can be used both with and without textures, and in this chapter we’ll pay special attention to textures that contain alpha. Blending is the process of compositing a source color with an existing pixel in the framebuffer.

Tangentially related to blending is anti-aliasing, or the attempt to mask “jaggies.” Antialiased vector art (such as the circle texture we generated in the previous chapter) varies the alpha along the edges of the artwork to allow it to blend into the background. Anti-aliasing is also often used for lines and triangle edges, but unfortunately the iPhone’s OpenGL implementation does not support this currently. Fret not, there are ways to get around this limitation, as you’ll see in this chapter.

Also associated with blending are heads-up displays and augmented reality. Augmented reality is the process of overlaying computer-generated imagery with real-world imagery, and the iPhone is particularly well-suited for this. We’ll wrap up the chapter by walking through a sample app that mixes OpenGL content with the iPhone’s camera interface, and we’ll use the compass and accelerometer ...