AGG第二十课 agg::ellipse 方法approximation_scale()

 献给：任何一种方案都是一种折中的选择，任何一种替代方案都可能会实现同样的效果。而这完全取决于你是如何思考定位的。请教是学，自学是学，只不过可以站在巨人的肩膀上，你可以看的更加清楚或许走的更远。

    In the previous section we have seen agg::path_storage object. While this object is very flexible and can be used to describe all kind of geometric shapes there are a few other vertex source primitive types. Actually there are not so many of them and we will just discuss the agg::ellipse object because it will be also useful to illustrate some important idea.

    At this point we need to explain the approximation_scale() method. approximation_scale() we need to go back to the coordinates specifications. The coordinates are actually given as double precision floating point number and they may or may not map directly to pixel coordinates. For example we can map some logical coordinates ranging from 0 to 1 to an huge viewport of size 1280x1280. The problem at this point is that an agg::ellipse object does not know the mapping between logical coordinates and viewport coordinates. So the idea is that to let agg::ellipse adjust correctly the level of approximation you should call the method approximation_scale() and pass as an argument the ratio between the viewport coordinates and the logical coordinates. So in the example of the viewport of size 1280x1280 we would need to use approximation_scale() with an scale argument of 1280.

We will see later that the approximation_scale() is also important with some kind of transformation like Bezier curves that are supposed to generate smooth curves based on a few vertices and some mathematical equations.

     本文转自fengyuzaitu 51CTO博客，原文链接：http://blog.51cto.com/fengyuzaitu/1961908，如需转载请自行联系原作者