From Anipedia

[edit] Masking

Masking consists of only showing a determined portion of a drawn layer in flash by specifiying it with another layer called the "mask".

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[edit] Masking Reflection Effects

Masking is probably one of the most usefull methods that flash posseses. Through masking we can achieve really great "reflection" illusions with a minimal effort. The example below shows this:

[edit] Small masking reflection Tutorial

1.The first step is to begin with any sort of drawing. For this example a vectorial space ship has been used, as shown below:

2.We must create then two layers above of the previous drawing (it is very important to follow the layer order shown) The first top layer is called the "mask". This will be like an open window that will display only a portion of the layer named "masked". For masked we drew a complete semicircular pattern that will be the reflection displayed. In my case I did a bunch of curved smoothed lines, but really is up to your imagination to create any pattern that you like. The mask will be in this case a drawing of the section we want only to be shown. On this case we painted it red, so it will be easily distinguishible from the drawing. I advice to paint the mask with a solid color, non gradient (gradients with alpha dont work if you are aiming for gradual masking, which is far more complicated). The result should look like this.

3.Finally we add some animation, in this case just a simple tween that rotates the "masked" layer pattern, soon after you are satisfied with your loop, it is just about to right click our mask layer, and hit the 'mask' button as shown:

The final result: