Using an atlas

As I have mentioned already, texture memory is one of your core resources. In fact, it is common to run out of memory because of all the textures required to animate a typical 2D game. It is also time-consuming to load inpidual textures rather than loading on a larger texture. So, we have to come up with methods to use texture memory more efficiently.

One common technique designed to pack more textures into less space is known as atlasing. A texture atlas works much like a sprite sheet described earlier in this chapter. Instead of storing each texture as its own image, we pack all of the textures for the entire game into one or more textures known as atlases.

As the word suggests, an atlas works much like a map. We simply need to know the location of any particular image, and we can find and extract it out of the atlas. Every atlas consists of two parts:

• The texture that contains all of the images
• A text file that contains the positions of each image in the atlas

As you can imagine, efficiently packing thousands of images into an atlas and then keeping track of each image's position within the atlas would be almost impossible to manage manually. This is why there are programs to do this for us.

I use a free texture atlas tool called Texture Atlas Generator. You can download this at http://www.gogo-robot.com/2010/03/20/texture-atlas-sprite-sheet-generator/.

A detailed example of atlasing is beyond the scope of this chapter. If you want to explore this on your own, here are the steps that you require:

1. Use a program, such as the one just mentioned, to create your atlas.
2. Save your data as an XML file.
3. Write a class to parse the XML saved in the previous step (I suggest TinyXML at http://www.grinninglizard.com/tinyxml/ as a starter).
4. Using the code to work with sprite sheets, modify the sprite class to be able to handle sub-textures from any arbitrary position in a larger texture.