August 26th, 2009 by Jeff Hipp

Adobe Illustrator Math Bug

About this post:

Senior Designer, Jeffry Hipp, talks about using the duplicate function in Adobe Illustrator - and a long-time bug that you need to be aware of.

Filed under Creativity, Tips, Tricks & Hints

One great thing in Adobe Illustrator is the ability to duplicate and move an object - then repeat that action as many times as you need. This is a handy tip, but also a "be aware of a problem".

I love a lot of Adobe products, Photoshop and Illustrator are cutting-edge software that no Designer can do without. However, there is a bug in Illustrator that has been on the top of my list of "things that piss me off" for years. It's a bug in their duplicate function - and has been there since they "joined forces" with Aldus around version 5 in 1994 or '95 (we're now on v.14.0.0 with Adobe CS4 btw)!

Here's the issue, try it yourself with this simple test so that you know: Open a new Illustrator document, turn on your rulers and place vertical guides every 1/2 inch. Draw a square that is 1/4 inch and place the right edge corner points precisely onto the first left guide. Grab & duplicate the square (holding the option & shift keys - click & drag the object to the right) being sure that you have grabbed the points so that your move is exactly 1/2 inch with the points ending on the next guide same as with the first square.

With the second square still highlighted or active, now duplicate the action (Command key & D key) and it should duplicate a new square to the next guide, repeat several times. This should always put the square exactly 1/2 inch apart - but it doesn't. After 18 steps it's off by .092" or 3/32"! Frustrating? You bet! Imagine if you have some sort of intricate pattern that you need to make using a shape - and that pattern needs to fill a certain width - it ends in the wrong place! I haven't come up with any real solution or quick fix to this problem, so I wanted you to be aware of it too. If you do have a fix, please share.

Note: I am. however, VERY GLAD to see that Adobe Illustrator appears to have very recently fixed a long-time problem with rounding-up decimals. I still had this problem in Mid May, but can't replicate it now. Here's what it was doing - Say you needed a square that was 2 1/8 inches - you'd enter 2.125, but Illustrator would round it up to 2.13. This doesn't fly when you need to be accurate in your artwork - like when creating a die-cut for a print piece or a vector shape for metal fabrication, etc.

I too have been annoyed at Illustrator rounding off decimals, but I just noticed something tonight. In preferences, under Units and Display Performace everything is set to points. I switched General to Inches and left Stroke and Text on points. Now when I resize an object it keeps all of the decimals I enter for its size. Maybe your preferences changed at some point without you realizing. Maybe it has something to do with the duplication error as well.

Posted by John Stanowski on 04/06/10 at 10:22PM
Here: I just did your duplication test up to 30 steps and it’s completely spot on.

Posted by John Stanowski on 04/06/10 at 10:25PM

Speak up.

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