3D Game Programming

Earth, you should notice something strange (but only if you’re using the

WebGL renderer, unfortunately). Mars normally seems to be moving to the

left as time passes. But every now and then, it stops and seems to go backward

for a little while.

If you go outside and observe where Mars is in the sky for several months,

you’ll see the same phenomenon. Ancient astronomers called this “retrograde

motion.” Because they thought Earth was at the center of the universe, they

had no good explanation for why retrograde motion happened. Oh, some of

them came up with crazy explanations, but nothing as simple as we just

simulated.

But we know what’s going on, don’t we? If you switch to above_cam just as Mars

starts going backward, you’ll see that retrograde motion happens when Earth

catches up with Mars and passes it. It’s sort of like a fast car passing a

slower car—the slower car (Mars) almost looks like it’s going backward.

It took you just a hundred lines of JavaScript to solve a question that the

greatest minds in the world couldn’t solve for thousands of years. Awesome!

16.4 The Code So Far

If you would like to double-check the code in this chapter, turn to Section

A1.13, Code: Build Your Own Solar System, on page 241.

14.8 What’s Next

We have a pretty incredible view of the solar system here, which is not bad

for a quick, single-chapter project! Not only did you solve a mystery that has

perplexed great minds, but you learned some pretty useful things about 3D

programming along the way. You now know how to move objects in circular

motion. Even cooler, you can add multiple cameras to a 3D scene, move the

cameras around, and switch back and forth between them.

We’ll build on this project in the next chapter to do something that every 3D

programmer has to do at some point: simulate the phases of the moon as it

revolves around Earth.

report erratum • discuss

17.5 The Code So Far

Prepared exclusively for Michael Powell