Smooth Out Jagged Lines in a Bitmap Image

I was thrilled to receive this success story in my e-mail:

Your tutorial "How to Smooth Out Jagged Lines in a Bitmap Image" saved my yearbook cover! I'm doing the yearbook for my kid's elementary school this year. We had a t-shirt design contest last year, and I wanted to use the winner's design on the cover of the yearbook. Unfortunately, the winner had moved away, and all I had to go on was a small jpg of the design. Needless to say, when blown up the the proper proportions, the jaggies came out to play! With your tutorial, I was able to resurrect the design and now have a beautiful cover! Thank you!

This trick works in most image editing software, so give it a try the next time you're in a bind.
• How to Smooth Out Jagged Lines in a Bitmap Image