GIMP perspective correction

[Bill Gill]

Using Paint Shop Pro, Ed Traxler corrected the perspective on one of Ray Dunakin's photos posted here:http://www.finescalerr.com/smf/index.php?topic=2565.15
and included an explanation of how he did it. Rather than hijack that thread with Ray's cool photos, here are four pictures to illustrate what Ed did, only these were done with GIMP, a free open source program that runs on PC and Mac.

Here is the before photo, taken at a low angle looking up at the building.

[Bill Gill]

This is a screen capture of the first photo, opened in GIMP and with the grid of the perspective tool superimposed on the image.

[Bill Gill]

This photo has several things going on. The dashed white rectangular outline is the original image area, that is shown automatically by GIMP. First I dragged in the two vertical, dashed blue lines to give me a guide to correct to. Next I clicked on the small, white outlined square at the very outer, upper left corner of the image and dragged it to the left until the left edge of the skewed wall looked vertical when compared to the blue guide line. Then I did the same ofn the right side. It took a liitle adjustment back on the left to true it up again after moving the right, then a little on the right to true it up. When satisfied (these adjustments were done very quickly just for demonstration purposes and could have been tweaked a little more) I clicked "TRANSFORM" and the program made the changes I had indicated.

[Bill Gill]

Final step cropped the photo back to it's original size, eliminating the trapazoidal shape.

Note, while this now looks correct, because the vertical lines are now (nearly) vertical, the image is not necessarily correct enough to scale measurements from. In this quick demonstration only the width of the image was squared up, but the height of the building was not corrected.

This brief demonstration took several times longer to do than the actual process which took about a minute or two. This is just to give an idea how the tool Ed mentioned works.

[finescalerr]

Thank you for that little tutorial, Bill, and for the courteous and very clear way you presented it.

Ray said he would check Photoshop Elements 5 for a similar tool so I checked Photoshop CS6 just now, drew a skewed trapezoid and, in the Edit menu under "Transform", I was able to turn it into a vertical rectangle in about 30 seconds. Earlier "CS" versions of Photoshop have that feature, too.

Neither Ed's program nor yours nor mine seem able to produce an image accurate enough to use for taking measurements but I can think of many other uses.

Russ

[marc_reusser]

Thanks for the Tutorial and Info Bill.

Here is a tutorial video on Russ's "transform" approach, in Photoshop CC...it also mentions the lens, correction function which can also be used for perspective/image correction. ...there is also a function under the "Filter" tab and is listed as "Vanishing Point"....which throws a perspectively adjusted/scaled grid over an image, and...thus gives you at least the scaled proportional information.

https://www.youtube.com/watch?v=pFsAxkcjFEM

[marc_reusser]

For those needing backdrop images/scenes that match their layouts, dios, etc., Photoshop CC (only avail in this version) also offers a function called "Perspective Warp"

Here are a couple of quick easy intro tutorials on that.

https://www.youtube.com/watch?v=Y2Jut0semFI

https://www.youtube.com/watch?v=yytdBHyDnUY

[Andi Little]

Read with much interest ................and thanks Marc', I try to use Photoshop but it defeats me every time. If it was the equivalent of language skills I would be at the Grunting and Primal shrug stage - and doomed to not progress much further.
I had by dumb luck figured out how to adjust for perspective distortion, but this way wasn't it .... and I have to re-teach myself every time I attempt it - and in truth I don't think I've ever done it the same way twice - (at least it feels that way). ............... Cheers bloke 

[Bill Gill]

Russ, Thanks for the additional information using PhotoShop CS6. I haven't tried this, but if the height and width of the building were known, the perspective could be adusted in proportion to the actual size and then perhaps measurements for other details could be taken from the photo? There is also the Match Photo feature in SketchUp that does allow you to scale an image if you know, or can reasonably estimate, some dimension. Ed Traxler has used that a number of times for his 3D printing projects.

Marc, Thanks for the links to the three tutorial videos, very interesting and clear (some others I've found are anything but). Wonder if GIMP will eventually include similar functions in future upgrades?

Andi, I know what you mean - sometimes I take some bizzare, meandering round-about way to get some effect with GIMp and then have no idea how to retrace the steps to do it again.