The Mysterious Opacity Mask feature in Adobe Illustrator

The Opacity Mask feature in Illustrator is a bit of a mystery because some people may not be sure when to use it or how to use it. And, even more mysterious and daunting, sometimes Illustrator decides on it's own when to use an opacity mask.

To answer the 'when to use it' question, just ask yourself "Does the graphic or logo I'm creating contain gradations which blend into transparency and will that transparency appear in front of various backgrounds?" Assume the answer is "Yes!" every time. You never know what use your graphic will have in the future. This is especially so for logos which will be used in a multitude of printed and digital situations.

An example of Opacity Mask usage in a logo is the Verizon logo. The 'v' mark and the tail of the 'z' both have gradations which fade to transparency. These gradations utilize an Opacity Mask to ensure smooth blending into any background of which the logo appears in front.

To answer the 'how to use it' question is a bit more complex. First of all, when thinking of an Opacity Mask you have to think only in black, white and grayscale, and you have to think in reverse. An Opacity Mask utilizes black for hiding and white for revealing. So if you have a square box with a linear gradient with 100% black on the left running to 100% white (0% black) on the right, the graphics on the left will not be seen, but as the gradient blends to the right, the masked elements will come into view.

And where the heck is the Opacity Mask feature? It's in the Transparency panel. The Opacity Mask works by making the topmost element a mask for any object or objects below which are selected to be masked.

There are two ways to create an Opacity Mask. The first way is to select one or more objects, whether vector or bitmap or both, then open the Transparency panel, click on the options drop down menu, then select Make Opacity Mask. A black square thumbnail will appear on the right in the Transparency panel. Click on this thumbnail to be brought into the Opacity Mask layer. At this point anything placed on the artboard will become the mask. What you'd put on this Opacity Mask layer is a shape containing a gradient. To get out of the Opacity Mask layer, click on the left thumbnail in the Transparency panel. This takes you back into the standard art layers, giving you access to the art being masked.

The second, and most probably, the best way to create an Opacity Mask is to create a graphic or logo with a topmost gradient object, select the gradient object and the object(s) behind, then open the Transparency panel, click on the options drop down menu, then select Make Opacity Mask. This time the black square thumbnail on the right will contain a thumbnail of the mask. The thumbnail on the left contains the object(s) being masked.

To edit an object with an Opacity Mask, select the object, open the Transparency panel to view the Opacity Mask options. Clicking on the right thumbnail gets you into the Opacity Mask layer. You can then move the mask object or change the gradient to update the mask. Clicking on the left thumbnail gets you back into the standard art layers.

How do you know if you have an object with an Opacity Mask in a file that you receive from somebody else? When an Opacity Mask is applied to an object, that object will appear in a sub-layer with the name of the sub-layer underlined with a dashed line. Follow the steps above to edit the mask.

What I've attempted here is just a quick explanation of the Opacity Mask feature in Illustrator. I will follow-up with a posting containing a step-by-step illustrated tutorial for creating and editing an Opacity Mask in Illustrator.