Polygon Sprite

A Polygon Sprite is also a Sprite, that is used to display a 2d image. However, unlike a normal Sprite object, which is a rectangle made of just 2 triangles, PolygonSprite objects are made of a series of triangles.

Why use a Polygon Sprite?

Simple, performance!

There is a lot of technical jargon that we can toss around here about pixel fill rate but the take home lesson is that a PolygonSprite draws based upon the shape of your Sprite, not a simple rectangle around the largest width and height. This saves a lot of unnecessary drawing. Consider this example:

Notice the difference between the left and right versions?

On the left, a typical Sprite drawn in rectangular fashion by the use of 2 triangles.

On the right, a PolygonSprite drawn with many smaller triangles.

Whether or not this trade-off is worth it for purely performance reasons depends on a number of factors (sprite shape/detail, size, quantity drawn on screen, etc.), but in general, vertices are cheaper than pixels on modern GPUs.

AutoPolygon

AutoPolygon is a helper class. It's purpose is to process an image into a 2d polygon mesh at runtime.

There are functions for each step in the process, from tracing all the points, to triangulation. The result, can be then passed to a Sprite objects create function to create a PolygonSprite. Example:

// Generate polygon info automatically.
auto pinfo = AutoPolygon::generatePolygon("filename.png");

// Create a sprite with polygon info.
auto sprite = Sprite::create(pinfo);