An Opacity Mask refers to a mask in Illustrator that you place over an existing shape that controls its transparency based on the values of black and white gradients in the mask. It is a combination of clipping mask and layer mask. You can use the opacity mask to control the appearance (transparency of a shape) using a black to white gradient.
Let’s try it:
Make a background shape (I used dark brown).
Choose the background shape and lock it or hit Command + 2 Mac, or Control + 2 PC. Or the object menu:
Create a shape on top of the background shape with a different colour.
To make a mask for the shape, use “Offset Path,” because it is more precise shape. Keep the top shape selected and navigate to the Object menu and choose Path > Offset Path.
Offset Path Settings: set offset to 2 pixels offset with the Miter setting. (Positive pixels make the new shape bigger, negative smaller). Leave the Miter limit at the default value.
Click OK and you will get a new shape perfectly aligned with the original and 2 pixels larger on each side.
The new shape may be placed the original. So choose the new (bigger shape) and bring it to the front.
Fill the top (mask) shape with a black to white gradient.
Choose both the shapes. You can do this by clicking and dragging, since we locked the background.
With both the shapes selected, navigate to the transparency palette, and in the upper right, click on the little arrow. Choose “Make Transparency Mask.” From the Palette Options.
Result: All areas containing white gradient will be opaque. All black will be transparent, and greys a mixture (the fade). If you don’t want the shape to be all transparent, then replace the black side of the gradient with a shade of grey.
Tips for exploring:
1. You can apply this to any shape. But be aware that it increases file size quite a bit.
2. The gradient is the master. Change the gradient, change the mask, but you must use black and white.
3. Explore your options in the Transparency palette (mask is clipping) For example, you can use a simple shape and have a complex Opacity mask if “Mask is Clipping” is enabled. This gives you greater control over the starting and ending values (opacity) of your gradient.
4. There is no shortcut to apply an Opacity Mask. If you want to backup, you will have to release the mask in the same way you applied it; from the Transparency Palette Options, or undo.
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