How to perform basic image post-processing?

Screencast shows how to adjust some image features using GIMP and Picasa software.

1. Fixing rotation, cropping images (using Gimp)
   
2. Fixing image contrast, white balance (using Gimp)

Rotating and cropping with GIMP

This section presents the post-processing of images created during the scanning process. If the object has not been properly placed on the scanner or scanning parameters, such as the scan area, have not been adjusted, you will get the image that will not meet your expectations. However, you do not have to scan again. If the image shows the whole object, you can use the software for image processing to improve images from the scanner.

In this example we will use GIMP software (http://www.gimp.org/) version 2.6.10. This is free software available under the GNU General Public License for Unix, Windows and Mac OS X. You can download it here: http://www.gimp.org/downloads/.

In this example we use the image of a carelessly scanned object. The exemplary graphic file is accessible in TIFF format here.


Figure 1: Image used in the example.

You can see that the object has not been located straight on the scanner. In addition, the scan area has not been adjusted and is too big.

Start by launching the GIMP software. This step depends on the operating system you use.

Open the file using the menu File -> Open or using the Ctrl+O key combination. This will open the Open image dialog, allowing you to navigate the file system and select the file.
More about opening files in GIMP you can be found here.

Figure 2: Image after opening.

First rotate the image 90 degrees clockwise. To do this use Image -> Transform -> Rotate 90° clockwise.

Figure 3: Image rotation 90 degrees to the right.

Now the picture is more readable, but is still slightly rotated. To straighten the image, we use the Rotate tool that will allow to set the angle of rotation.

Figure 4: Image after rotating 90° clockwise.

You can access to the Rotate tool by using the menu Tools -> Transform Tools -> Rotate, by using the Shift+R key combination or by clicking the tool icon (and clicking on the image). More about the Rotate tool and options of this tool can be found here and here.

Now change default values of the rotation tool. In the toolbox change Direction to Corrective (Backward) and Preview to Grid. The rest of the options can be left with default values.

You have to align grid lines to the edges of leaf. Set the rotation angle by using the slider in the dialog or by dragging the mouse pointer on the image or the rotation point.

Figure 5: Custom image rotation with the Rotate Tool.

To correctly adjust the rotation angle you can zoom in the image and increase the number of grid lines. Zoom in using the drop-down list located under the image. In the toolbox change Number of grid lines to Grid line spacing and set spacing between lines.
After adjusting the rotation angle push the Rotate button.

Figure 6: Adjusting the angle of rotation.

Now we have to remove excess space from the image. We can do this with Crop tool.

Figure 7: Image after rotation.

To get this tool push the button on the toolbox, use the menu Tools -> Transform Tools -> Crop or use Shift+C key combination. Now you can select the area you want to crop.

Figure 8: Removing edge areas from image using Crop Tool.

After you select the accurate area, run crop transformation by double clicking on this area or by pressing Enter. Do not forget to save the file.

Figure 9: Image after all transformations.