November 2008

This installment in my GIMP Beginners series is about Rotating and Cropping images (just as the title suggests). The areas covered being basic orientation (portrait vs’ landscape), straightening horizons, corrective cropping (after leveling of horizons) and cropping to a specific ratio. This tutorial is based on GIMP version 2.6.2.

Basic Orientation (portrait vs’ landscape)

So you’ve just opened your image in GIMP but you find yourself placing your head on one shoulder to see it right. There is luckily a one step fix for this problem.

1. From the dropdown menus select Image > Transform and then select Rotate 90 clockwise or counter-clockwise as appropriate.


Tip: Rotating a .JPG image in this fashion will cause some deterioration in quality. If you are editing an image for use on the internet you will need to rotate it, however if you are editing a file for printing you may wish to simply skip this step and perform your edits in the default orientation, your printer will not know any differently.

Straightening Horizons

Now check to see that the camera was level when the shot was taken, the best check for this is the horizon. If your horizon sloping to one side or all your buildings are leaning to the left or right proceed as follows.

1. Move you mouse to an area roughly in the center of the horizon and note the coordinate given at the lower left corner of the image window.


2. From the dropdown menus select Image > Guides > New Guide…

3. In the resulting ‘Script-Fu New Guide’ dialog box select Horizontal from the dropdown box and enter the number which we memorised in step 1, then click OK. (If you are using a building to test the straightness you will select Vertical from the dropdown and enter the number from the left rather than the right).


4. From the Toolbox select the Rotate tool and mouse click anywhere in your image.

5. In the ‘Rotate’ dialog box change the Angle until the horizon runs parallel to our guide, click OK when you are done.


6. Now turn off your guides by selecting Image > Guides > Remove all Guides.

Tip: If you are using a building it is best to select one from near the center of the photo. This is due to the fact that some camera lens (wideangle) may cause buildings to slope inwards as they rise, this is called “converging verticals”, I have a feeling that we’ll be coming back to this is a later installment.

Corrective Cropping

If you have rotated your image in order to straighten it corrective cropping will your next step. To do this

1. From the Toolbox select the Crop tool. To keep the cropped image the same basic shape as the original make sure to select Fixed for ‘Aspect Ratio’ in the tool options.

2. Using your mouse, click and drag to roughly the size you wish to crop, you can now fine tune the crop area by using the mouse to drag the sides of the area to their final positions. When you are happy go back to the ‘Crop’ dialog box and click OK.


Cropping to Specific Ratio

There will of course be time when you wish to use cropping to change the basic shape of your image, for instance to fit an 8 x 10 photo frame. To do this

1. From the Toolbox select the Crop tool.

2. Make sure that Fixed is selected for ‘Aspect Ratio’ in the tool options and now use your keyboard to enter “8:10” in the box. You can use the Portrait and Landscape buttons to flip the crop area if needed.

3. Using your mouse, click and drag to roughly the size you wish to crop, you can now fine tune the crop area by using the mouse to drag the sides of the area to their final positions. When you are happy go back to the ‘Crop’ dialog box and click OK.

Happy GIMPin’


I installed GIMP on a friends machine the other day, this is when I realised that there are a few steps which I carry out before I ever edit a photo. These a just a few small changes to the GIMP’s default setup which make things run a little more smoothly, for me at least. These may be of use to GIMP beginners to help get them up and editing.

OK so you’ve just installed GIMP, if not download it for free at and install it as usual. You may wish to look at this post which explains how to set the GIMP’s file association during the install process, or afterwards if you already have GIMP installed.

Note that I am basing this on GIMP version 2.6.2

1. Organising on screen elements

The first thing to do after you launch GIMP is to arrange the onscreen elements to your liking. Here is a screen shot of my own preferred setup. Hint: (Open an image so you can better see where to place the ‘Toolbox’ and ‘Layers, Channels, Paths’ boxes.


a.) Once you have things arranged to your liking, go to the “Edit” drop down menu and select “Preferences“.

b.) Now go to “Windows Management” and click on “Save Windows positions Now“, ensure that “Save window positions on exit” is checked also and click “OK“. Your windows should appear as you like them next time GIMP is started (you may however need to maximise the image window).


2. The Grid Feature

The default setting for the grid (a very uesful tool I might add) is too small to be very useful when it comes to the large files from a digital camera. I always like to change the default to a more usable setting.

a.) Select “Edit” > “Preferences” to bring you back to the Preferences dialog and this time select “Default Grid“. in the spacing section change the “Width” and “Height” to 100 pixels (or whatever you personally like) and click on “OK“.

To activate the grid select “View” > “Show Grid“. Repeat the process to turn it off again.


3. Ensure Best Quality

You may have read that it not good practice to repeatedly save a .JPG file. This is due to the fact that .JPG is a compressed file format, as such the file is recompressed (subsampled) each time the file is saved and this will lead to deterioration of the image.

This step will limit this subsampling as much as possible and should be OK for a few svaes, however as you become more confident I suggest that you only save a .JPG file once, at the end of editing. If you are working on an important file or making a complex edit consider first saving your file in an uncompressed format, such as .TIFF or the GIMP’s native .XCF, and save to your hearts content.

a.) Open a .JPG file and select “File” > “Save As…“. In the ‘Save Image’ dialog box enter a junk filname and click on “Save“.

b.) In the ‘Save As JPEG’ dialog’ click on “Advanced Options“. Now change the “Quality:” slider to 100% and change “Subsampling” to 1X1, !X1, 1X1 (best quality). then select “Save Defaults“. You can now click on “Cancel” as we do now want to save this image at this time.


4. Use the Historgam

For photo editing I like to add the ‘Histogram’ dialog available as a reference. This setup guide is not really the place for a detailed explanation of what the image histogram is but, simply put the Histogram is a graphical representation of the tones in an image from pure Black on the left to pure White on the right.

As such it gives us some information about the exposure. If we see that the graph is clumped to the left, it suggests that the image is too dark (underexposed) and if we see that the graph is clumped to the right it suggests that the image is too bright (overexposed).

a.) To add the ‘Histogram’ dialog to your ‘Layers, Channels, Paths’ box select “Windows” > “Dockable Dialogs” > “Histogram“. this opens the ‘Histogram’ dialog in a new window.

b.) Now with your mouse grab the ‘Histogram’ dialog and drag and drop it onto the bottom of your ‘Layer, Channels, Paths’ as shown below.


5. Restoring Closed Docks

AAArrghh!, where’s that Layers thingy gone?. I have had this same experience myself. If you accidentally close the ‘Toolbox’ or ‘Layers, Channels, Paths’ boxes select “Windows” > “Recently Closed Docks” > “Whichever you closed“, and your back in business.


Happy GIMPin’,


Just had a look at the stats on this blog and I noticed “straight line” among the search engine terms.


Then I realised that there is no straight line tool in GIMP, which is strange because I use straight lines all the time. Personally I often use multiple short straight lines and a soft brush to approximate curved edges when painting a Layer Mask.


Strangely I have no idea when or how I learned to do this, must have discovered by accident at some stage. It’s is however very simple, in Windows at least, then again everything is once you know .


1. Select the “Paintbrush” or other tool.


2. Left mouse-click where you want your line to start.


3. Press and Hold the “Shift” key on your keyboard.


4. Left mouse-click where you want your line to end.


Viola, you should now have a nice straight line.


I just checked and this “Shift” key trick works with all the tools listed in the image below for GIMP 2.6.2. I was surprised to see that it even works with the “Healing tool”, once you have first defined a clone point.




Solarisation is the effect whereby the properties of a material is affected by electromagnetic radiation.

It’s effects in photography were first discribed in 1859 by H. de la Blanchere and usually refer to the skewed tones which result from exposing an already exposed (used) film to light before or during processing. This effect was popularised in the 1920’s by American painter and phtographer Man Ray.

I saw Photoshop intructions for how to reproduce this effect digitally in a UK photo magazine (can’t remember which one, sorry), so all I had to do was GIMPify these intstructions.

A word of warning, this effect can look good on some images but just doesn’t work at all on others. Image selection is important and you may have to try this effect on a few different images to get one which “works”. The guy in the print shop thought I was crazy when he seen this image first, but he changed his mind once he saw the enlarged print.

1. I started with a photo of a neighbours 1931 Norton. The first thing we do is to desaturate the original image by selecting “Colors” > “Desaturate” from the dropdown menu. I have chosen to do this using the “Average” option.

2. To replicate the skewed tones characteristic of solarisation we need to produce a radical inverted “vee” tone curve, (GIMP shows its class here, as not all applications give sufficient control over the tone curve to replicate this effect). We have two options to create the inverted “vee” curve, a.) we can use the freehand mode, which can give a good, if somewhat wobbly curve. b.) the method I prefer as outlined here. Launch the “Curves” dialog from “Colors” > “Curves” and leave it on the default “Smooth” curve type. Grab the centre of the curve (straight line as yet) and drag it up to the top of the window.

3. Next grab the right hand end of the curve and drag it down to the bottom of the window.


4. Next gab a point on the curve near the top/apex/crest and drag it up and over so it is just beside the topmost point of the curve (I say just beside as two points cannot live at the same place, your top point will have an X coordinate of about 127, so aim to place your new one just beside it at 126 or 128 depending of whether you are dragging from left to right or right to left). Viola one inverted “vee” Solarisation type curve.

You may wish to launch the “Curves” dialog again and do some fine tuning, a gentle “S” curve often works well, I did not feel that it was necessary for this image. Be sure to try this effect out on some portraits, it can really emphasise the eyes and give a very aluminum “Metropolis” type feel as you can see in this photo of my cousins kid.

Happy GIMPin’,