对于第一个简单效果，将在视频流中的固定位置绘制一个阿尔法混合(半透明的)圆。

For our first simple effect, we will draw an alpha-blended (semitransparent) circle at a fixed position over the video stream. The first step is to add the drawing functions to the screen.c.

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RolloverIcon 实例将包装我们希望用 图 1中所示的图形效果绘制的初始图标。

The RolloverIcon instance is wrapping the original icon we want to paint with the graphics effect shown in Figure 1.

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在实时绘制中，在性能上至少有3个目标：每秒有更多的帧、更高的分辨率、更多(比较好的真实感效果)的场景物体。

In the graphics rendering, we have at least 3 goals: more frames in one second, higher screen resolution and more scene objects with good third dimension effect.

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