Adobe Illustrator CS

For this exercise, imagine you want to create a gradient that fades from 100-percent color to none (which is not currently possible as you can’t assign the None attribute to a color stop in a gradient). Begin by creating a rectangle and fill it with a solid color. Next, select the rectangle and copy it by pressing Command-C (Ctrl-C). Now paste it in front of the first rectangle by pressing Command-F (Ctrl-F). Fill the second rectangle with a regular black to white linear gradient. You now have two objects, stacked on top of each other, with the gradient visible on top.

Use the Selection tool to select both objects and open your Transparency palette. In order to create an Opacity mask, you have to expand the palette to show all of its options, so click the triangles to the left of the Palette tab or choose Show Options from the palette menu. You will see a thumbnail of your selection in the Transparency palette. Finally, choose Make Opacity Mask from the Transparency palette menu. The result is a mask that uses the values of the gradient to define what parts of the object are visible below it—an object that effectively fades from 100-percent color to transparent (feel free to create an object that appears beneath the rectangle if you don’t believe me).