Illustrator Opacity Masks

Opacity masks in Adobe Illustrator are used to allow artwork to remain complete, while providing a means to show or hide portions of the artwork non-destructively.

UPDATE 2006-07-26 It seems that you can fix all transparency problems if you use RGB black or white, instead of Grayscale black or white values.

An opacity mask can be a vector or raster object, represented in 256 shades of grey. A gradient will work, or a black and white photo, etc…

1. draw or place the main object
2. draw or place an object to use as an opacity mask
3. make sure the opacity object is above the main object
4. select both the main object and the opacity mask object
5. go to the transparency palette, and click on the top right arrow for more options
6. select “Make Opacity Mask”

* Note - the masks don't seem to work perfectly unless you use the “invert mask” checkbox, and assume black will allow 100% of the main object to be visible, while white will allow nothing to be visible. Shades in between black and white will allow varying amounts of the main object to be seen. The basis for stating that this doesn't work perfectly is from a real world issue:

1. assume a rectangle of a dark color to be placed somewhere on a web page (i.e. 563px by 40px)
2. the goal is to have 1 pixel wide lines of another color, at 45 degree angles, spaced 1 pixel apart, but to have this effect fade into just the main dark color from step 1 at some point (i.e. effect of 347px by 40px)
3. if you create a pattern for this effect, and apply an opacity mask to this object with a simple white to black gradient, it will not fade perfectly into the dark color - a visible edge will be apparent where the gradient stops; there is no known way to have the gradient block the effect 100% without using the “invert mask” checkbox, and flipping the gradient around

1. select the an object that has an opacity mask
2. in the transparency palette, click the box on the right side (you should see a thumbnail image of your mask)
3. this right side box should now be highlighted with a black line, and you are now editing the mask itself
4. to get back to the main object, click the left box in the transparency palette