Overview

A texture atlas is a collection of many smaller textures in one big image. The class SPTextureAtlas is used to access textures from such an atlas.

Using a texture atlas for your textures solves two problems:

• Whenever you switch between textures, the batching of image objects is disrupted.
• Some OpenGL textures need to have side lengths that are powers of two. Sparrow hides this limitation from you, but you will nevertheless use more memory if you do not follow that rule.

By using a texture atlas, you avoid both texture switches and the power-of-two limitation. All textures are within one big “super-texture”, and Sparrow takes care that the correct part of this texture is displayed.

There are several ways to create a texture atlas. One is to use the atlas generator script that is provided with Sparrow. Here is a sample on how to use it:

# creates "atlas.xml" and "atlas.png" from the  provided images 
./generate_atlas.rb *.png output/atlas.xml

The atlas generator can be found in the ‘utils’ directory in the Sparrow package. A README file shows you how to install and use it. If you want to have more control over your atlas, you will find great alternative tools on the Internet, like Texture Packer.

Whatever tool you use, Sparrow expects the following file format:

<TextureAtlas imagePath='atlas.png'>
  <SubTexture name='texture_1' x='0'  y='0' width='50' height='50'/>
  <SubTexture name='texture_2' x='50' y='0' width='20' height='30'/> 
</TextureAtlas>

If your images have transparent areas at their edges, you can make use of the frame property of SPTexture. Trim the texture by removing the transparent edges and specify the original texture size like this:

<SubTexture name='trimmed' x='0' y='0' height='10' width='10'
            frameX='-10' frameY='-10' frameWidth='30' frameHeight='30'/>