remove keystoning from painting photos

[broker12]

I would like to know if GIMP has this feature . . . in photo shop, it' is called removing keystoning from photos of paintings. That's not a genuine name. It's how others talk about this feature.

What removing keystoning does is allow you to straighten up the edges off a photo of a painting. Usually, it is quite difficult to set up a painting so that the photo of it is perfectly square. In photoshop, this is the procedure for removing/straightening the edges of the photo of the painting:

Removing keystoning from a photographed rectangular work is not difficult in Photoshop. Use the Crop tool to get within a quarter inch of your painting, (but not too close). With the NAVIGATOR, ZOOM IN to at least 100%, or much more. Now SELECT/SELECT ALL. Go to EDIT/TRANSFORM/DISTORT. You will find in the corners, 4 small box buttons. Drag each of these out one at a time until you get each corner of your painting pulled tight to the corners of the image file frame. Press ENTER or RETURN (Mac), to accept these changes. Any perspective distortion present in the original image will be corrected now.

To get the proportions or aspect ratio of your painting back on track, take measurements of your original painting, go to IMAGE/IMAGE SIZE, disable the CONSTRAIN PROPORTIONS checkbox, so you can alter them. Now type into the height and width dimension boxes, the actual measurements of your painting. Since this may increase your file size to something overwhelming, you can reduce the pixels per inch to something much smaller than 300dpi to something like 96 dpi, or 72 dpi. Press OK, and your painting will be flat, square, true proportioned, and looking good! Remember to go back and reset the CONSTRAIN PROPORTIONS checkbox in IMAGE SIZE, for the next time you use that function

If anyone knows how to do this in GIMP, please let me know . . . Thanks

[rich2005]

Gimp has a perspective deform tool.

To retain the original proportions, if you know them, set up some horizontal and vertical guides.
Select the perspective deform tool, click in the canvas.



Drag the handles, the squares at the corners, to suit



Click transform it to fix it, then maybe use the crop tool to size or select, invert selection, to fill in the left over edges

[Blighty]

(07-11-2017, 01:19 PM)broker12 Wrote: I would like to know if GIMP has this feature . . . in photo shop, it' is called removing keystoning from photos of paintings. That's not a genuine name. It's how others talk about this feature.

What removing keystoning does is allow you to straighten up the edges off a photo of a painting. Usually, it is quite difficult to set up a painting so that the photo of it is perfectly square. In photoshop, this is the procedure for removing/straightening the edges of the photo of the painting:

1. Use the crop tool to crop unnecessary space around the image.

2. Select the Perspective tool.
Set the mode to Corrective (Backward).
Click on the image.
Drag the 4 corners of the grid onto the corners of the painting.

   

3. Use the Scale feature to restore the original aspect ratio of the painting
From the menu bar, use Image > Scale

[broker12]

Thanks to you both. I'm on the run just now, but later today, I'll dive in and see if I can sort it out. I ran into the perspective tool several months ago, but had no luck with it . . . I suffer from SLS . . . slow learner syndrome. Again thanks.

[broker12]

Blighty . . . I tried your version first because it seemed more simple. I had some luck with it. I managed do everything you said, but I'm not sure I did it in the proper order. I tried a coupe of times. Once the picture stayed "squared up," but some extra space appeared around. Another time, I had to do it again. I'm sure your system works, but would you kindly walk me through your steps. It took me forever to realize the "scale" was a menu item and not an icon. Thanks again . . .

[Blighty]

(07-26-2017, 11:24 PM)broker12 Wrote: but would you kindly walk me through your steps.

I will put something together in a little while

[Blighty]

(07-26-2017, 11:24 PM)broker12 Wrote: Blighty . . . I tried your version first because it seemed more simple.  I had some luck with it.  I managed do everything you said, but I'm not sure I did it in the proper order.  I tried a coupe of times.  Once the picture stayed "squared up," but some extra space appeared around.  Another time, I had to do it again.  I'm sure your system works, but would you kindly walk me through your steps.  It took me forever to realize the "scale" was a menu item and not an icon.  Thanks again . . .

I am starting with this image. (Hopefully your image isn't this distorted)
http://imgur.com/a/Cjlxa

Crop off the excess surrounds
http://imgur.com/a/fqYMe

to give this
http://imgur.com/a/NL1pA

Select the Perspective tool
Set to Corrective (Backward)
and click on the image to get this
http://imgur.com/a/nkIe0

Drag the corners of the grid onto the corners of the image
http://imgur.com/a/Unqzc
and click on Transform

You now have this
http://imgur.com/a/YDGeC

Crop again to remove excess surrounds to produce this
http://imgur.com/a/sF8cd

This next step may or may not be necessary. (If the camera is well set up it might not be necessary)
Use the a tape measure to measure the original frame and determine the aspect ratio. For example, if it is 500x400mm then the aspect ration is 500/400 = 1.25:1
From the menu bar, select (Image > Scale Image)
http://imgur.com/a/qOF8J
From the Width and Height, determine the aspect ratio. If it is not the same as the original adjust either the W or H to give the correct aspect ratio. NB Make sure the chain symbol next to the W and H is broken! Click on it if necessary.

Another possible problem (not dealt with here) is that the edges of the frame are curved. This is caused by a less than perfect camera lens. This can be corrected.