GIMP Photo Manipulation - Rotation, Geometric Correction, Healing, Cropping

dosdan · 06-15-2008, 01:06 AM

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

1. Overall, slightly tilted 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 to 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) on the other side of the blemish and dump it on the blemish 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.

Perhaps others can start other GIMP usage threads to make it easier for me and others to use this powerful beast of a program.

*Wombat* · 07-20-2008, 06:57 AM

I didn't know you could set Preview to Image. Thanks for that piece of info - really made my night. (How sad is that....). One thing I'd love to see in Gimp is some way to search the document file. It's painful hacking through the index and scrolling yourself silly just to find a simple fact or instruction. Am I missing something?
Great demo, mate.

Torphoto · 07-21-2008, 03:26 PM

Now that I have suse 11 installed and will be using gimp alot must say thanks for this.

Entropy · 07-25-2008, 10:43 AM

What version of GIMP are you using?

I don't remember ever seeing the healing tool... It's the one thing I keep a copy of Photoshop CS2 running in WINE for.

*Peter Zack* · 07-25-2008, 01:03 PM

Good info. I don't use GIMP but I always like these threads and learn new stuff. We need more of this.

BPT · 07-25-2008, 01:19 PM

Originally Posted by Entropy

What version of GIMP are you using?

I don't remember ever seeing the healing tool... It's the one thing I keep a copy of Photoshop CS2 running in WINE for.

The healing tool has been there a long time, I think since version 1 or before. the icon for it was a stamp before

bc_the_path · 07-25-2008, 01:43 PM

I use Gimp routinely and I am very happy with it.
The only thing I wish it had is 16 bit processing. That is why I use The Gimp only for web presentations. I am certain that you can do anything that is done using PShop using the The Gimp instead "as long as the final product will be displayed on the web only". Certainly, you can use it also for printing, archiving etc.; however, I cannot claim that it can rival other software on those regards... yet!

Let us hope The Gimp People are indeed keen on making it capable of working on 16bit images.

I don't think learning it is as cumbersome as we are led to believe; it just does not resemble its alternative!

By the way, thanks "dosdan" for the beautiful and informative post...

Fritz · 07-25-2008, 02:58 PM

Thanks for posting this, it was an informative read. I use the Gimp and find it to be a very useful tool, especially for the price!

*Wombat* · 07-26-2008, 09:55 PM

Originally Posted by bc_the_path

I use Gimp routinely and I am very happy with it.
The only thing I wish it had is 16 bit processing. That is why I use The Gimp only for web presentations. I am certain that you can do anything that is done using PShop using the The Gimp instead "as long as the final product will be displayed on the web only". Certainly, you can use it also for printing, archiving etc.; however, I cannot claim that it can rival other software on those regards... yet!

Let us hope The Gimp People are indeed keen on making it capable of working on 16bit images.

I don't think learning it is as cumbersome as we are led to believe; it just does not resemble its alternative!

By the way, thanks "dosdan" for the beautiful and informative post...

Keep an eye on the Gimp website. They're pretty close to launching 2.6, which will have 16 bit processing. We're all hanging out for it. In the meantime you can always use Cinepaint, a Gimp fork that does 16 bit. And I agree, it's not that complicated. I've never used Photoshop but I'm told that's a steep learning curve.

Entropy, the healing tool is in the toolbox - looks like a couple of Bandaids crossed over each other.

Entropy · 07-28-2008, 11:05 AM

Originally Posted by Wombat

Keep an eye on the Gimp website. They're pretty close to launching 2.6, which will have 16 bit processing. We're all hanging out for it. In the meantime you can always use Cinepaint, a Gimp fork that does 16 bit. And I agree, it's not that complicated. I've never used Photoshop but I'm told that's a steep learning curve.

Entropy, the healing tool is in the toolbox - looks like a couple of Bandaids crossed over each other.

Yeah, found it last night after looking again (prompted by this article). Will play with it tonight.

I tried Cinepaint, hated it. Crashfest, especially if you enabled color management.

emalvick · 07-28-2008, 11:57 AM

Originally Posted by Wombat

Keep an eye on the Gimp website. They're pretty close to launching 2.6, which will have 16 bit processing. We're all hanging out for it. In the meantime you can always use Cinepaint, a Gimp fork that does 16 bit. And I agree, it's not that complicated. I've never used Photoshop but I'm told that's a steep learning curve.

Entropy, the healing tool is in the toolbox - looks like a couple of Bandaids crossed over each other.

Actually, from what I've seen, the 16 bit support will not happen with 2.6. They've only implemented the GEGL framework, which will allow them to add 16 bit support later (perhaps 2.8).

*rparmar* · 07-28-2008, 01:01 PM

Great tutorial! Learning any image editor is the work of a lifetime and unfortunately I started off in Photoshop years ago. Many years. To relearn all that now would be madness, but I always encourage others to get stuck in to open source solutions whenever possible. All my programming tools (Python, Textpattern, etc.), business apps (OpenOffice) and more are open source.

*Wombat* · 07-28-2008, 08:27 PM

Originally Posted by emalvick

Actually, from what I've seen, the 16 bit support will not happen with 2.6. They've only implemented the GEGL framework, which will allow them to add 16 bit support later (perhaps 2.8).

Bummer! I must have missed something. 2.8 will be decades away. Maybe if I sent them another donation...