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GIMP Photo Manipulation - 1. Rotation, Geometric Correction, Healing, Cropping
Posted By: dosdan, 11-28-2009, 01:56 PM

GIMP is a powerful, freeware image editor. It's challenging to learn (at least for a dummy like me). Here's some examples of its power. The following photo has 3 problems that I'll deal with here:

1. Overall, slight tilt to the right.
2. Geometric distortion: the left & right tilt towards each other at the top of the picture.
3. A slight blemish in the sky at the top left of centre.




Particularly with buildings I find I'm checking overall orientation often. GIMP does not have a predefined hotkey for switching the grid on & off, and I got tired of using the menu (View | Show Grid) since I switch this on and off so often. So I think it's a good idea to define a hotkey for grid display. Click on Edit (the one on the image-being-edited window) | Keyboard Shortcuts | View | Show Grid. I highlighted this line and pressed "G" to use it as the hotkey. Make sure you've tick "Save keyboard shortcuts on exit". Now you can easily bring up the grid, check, adjust it if needed, put it away:




The first step is to check for overall tilt. I applied the grid. Since the image was 2002 pixels wide. I checked around the 1,000 pixel vertical centre. See the crop below:




The vertical edge is slightly tilted clockwise. I need to rotate it anti-clockwise. I pressed Shift-R (or Tools | Transform Tools | Rotate). The rotation options & settings are in the next two screens:







I set the angle to -.5 degrees and clicked on Rotate. The centre section after being aligned:



Now, let's check the geometric distortion. There are four large vertical buildings in this shot. The left edge of the leftmost building:




You can see the clockwise tilt here. Next the left edge of the 3rd building from the left:



An anti-clockwise tilt is visible, as it is the right side of the rightmost building:



To bring up the Prospective tool, press Shift-P (or Tools | Transform Tools | Perspective). Options shown below:




I found it easiest to set Preview to "Image" so I could see the change on the screen. There are grab boxes at each of the 4 corners of the image. I needed to drag the top-left & top-right boxes outwards enough to visibly correct the distortion without throwing the central orientation out:





When you're happy with your correction, click on the Transform button:




Now the image is aligned correctly, it's time to fix the blemish. I pressed G to remove the grid, and then H for the Healing tool ( or Tools | Paint Tools | Heal). The options are shown below. It's a big blemish so I selected a large spot diameter, scaled it up a further 3.8x times, and applied jitter & a large fade out distance so the edge of the fixed area would not be obvious in the sky:




Initially, nothing is selected:




Hold down the Ctrl-key and click on a clean area close to the blemish to get a replacement (shown as a "plus"). Then move the circle over the blemish (on the left here):




and click:




If the blemish is not hidden completely, click on it a few more times. If necessary, you can also take another sample (Ctrl-click) from the other side (left) of the blemish and dump it on the spot as well.

The Perspective Tools cropped off the excess, but the Rotation tool didn't (at least on the Clipping setting I used), so a final crop is necessary. Press Shift-C (or Tools | Transform Tools | Crop), drag and mark out the area to keep:




Finally, click on the highlighted area to activate the crop.

Last edited by dosdan; 03-12-2010 at 02:25 PM.
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12-04-2009, 11:04 AM   #2
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Thanks a lot! I find GIMP quite difficult to use, but this looks great. I'll have to give it more attention. Maybe these dark days at the end of the year?

Thanks again!!
01-25-2010, 01:26 AM   #3
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I love GIMP, with Picasa to sort my photos. Is there a way to get the lens info in GIMP that you get with the Pentax software?
02-01-2010, 03:24 PM   #4
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excellent article... but Gimp is not freeware! It's open source software. There's a huge distinction.

02-27-2010, 01:20 AM   #5
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Original Poster
QuoteOriginally posted by GoremanX Quote
excellent article... but Gimp is not freeware! It's open source software. There's a huge distinction.
GoremanX, thanks for the correction. As I understand it:

Freeware
- ownership kept by 1 or more programmers; source code not released; the public (but not necessarily commercial users) are free to use it.

Open Source - ownership & source code is released into the public arena; both the public & commercial users are free to use it, modify it or incorporate it into other products.

Dan.
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