Distortion Correction (Keystoning?)

This is a pic I took in Paris earlier in the week. It's not particularly interesting except that it illustrates quite nicely a problem that this particular camera, a Canon compact digital, is prone to.




I think this may be known as "keystoning", but in any case the problem is obvious: vertical lines toward the edges tend to lean inwards in a graceful, curvy manner.

Can this be corrected using an image editor such as the GIMP, or Photoshop? Are there any other (preferably free, and available for Linux) tools that will do the job?

Grateful for any advice.

You can do it in photoshop with the warp tool (or puppet warp in CS5) but that's fully manual and quite time consuming.

Don't know about the other editors.

Not sure about GIMP but Photoshop will sort this out quite easily using the distortion correction or lens correction tool depending on your version of Photoshop.

Graham

There seem to be two things going on there. Firstly, they are leaning in because you have the camera tilted up a little but secondly the straight edges are not straight (unless that lamp post really is bent like a banana ). You'll need to apply both lens correction and perspective correction...

Thanks all. I had a closer look in the GIMP and there's a built-in filter for lens distortion to fix the curvy lines, so I tried that. Then after a bit of Googling, I found that the "perspective tool" can be used to adjust the leaning. This is a first attempt, and not quite there but I think I'm on the right track.




Thanks again!

Yes, looking much better. K.

The 'curvy lines' you refer to are caused by barrel distortion. The only place I have heard the word 'keystoning' used is in relation to digital projectors