With Clipping Mask and Opacity Mask, you can quickly mask away any unwanted areas you wish to hide. Let’s take a look how it works…
In this illustration I have 3 layers. The background, sunburst effect and the sun. I will need to mask the sunburst to a square.
First, I will select both the sunburst and background and group it together. Select both with Selection Tool and press Ctrl/Command+G to group it.
Draw a rectangle to decide the area you want to show. Make sure it is at the topmost layer.
Select both the background and the path. Right click and select Make Clipping Mask (Object>Clipping Mask>Make).
Next send the background to the back by pressing Ctrl+Shift+[ / Command+Shift+[.
To make any changes to the clipping mask go under the group and select Clipping Path. You can rescale your clipping path in here. To remove the clipping path, go Object>Clipping Mask>Release.
Opacity lets your create transparency to your layer. The Intensity of black will determine the opacity of the layer. Black will be transparent and white will be opaque.
Instead of drawing a square in our previous example, I’m now going to draw a circle instead and fill it with gradient.
Now select both shapes and select Make Opacity Mask by clicking the Options icon at the corner of the Transparency Palette.
So now, I have an opacity mask that has the edges fading out. Opacity Mask is a very good way of blending edges to the background and it will prove useful in your illustrations.