This tutorial will help you to know the various uses of opacity masks.
Here we are going to create a shape having highlight using opacity mask.
What is an opacity mask?
Opacity mask is used to mask off an area of an object and selectively allow visibility to whatever is underneath. Opacity is the measure of see-through an object is. The more opaque the object is the less see through it will have. So adding a mask means you are only allowing some light to pass through. It can be used to create a realistic light effect. It is said that light defines the shape of an object by reflecting off it. You can use the opacity mask in illustrator to create such an effect.
Starting from a base shape
Let us start with a base shape. Here I have used a circle having a simple gradient and stroke.
Now we need to create a highlight. You can either use the pathfinder panel or the pen tool to create this effect.
Create a new layer. Copy the highlighted shape. Fill it with a black and white gradient. Apply the Opacity Mask. Now you can see through the white portion but the black portion will be masked. Here, the light source is coming from the top left.
Making the opacity mask
Creating the opacity mask
Select the highlight and the gradient layers. Go to the Transparency Panel (Window > Transparency). In the right hand corner at the top there is a flyout menu. From it choose Create Opacity Mask.
That’s it. This is a way you can create light effect in illustrator.