Pict 1: These beautiful window paintings were on a wall right beside the Papal Palace in Avignon - France. The sides of the paintings taper inwards because of the steep upwards angle from which I snapped the shot.
Correct the distortion in perspective that is common when taking snapshots of tall buildings. Crop a photo to exclude distractions, or to change the composition of the photo.
Yesterday, we got ourselves oriented to the user-interface of GIMP, and tried our hand at straightening a crooked photo. Today we continue with common but simple photo-editing tasks like perspective correction and cropping.
Correcting perspective distortion
When taking photos of a tall building from an oblique angle, or while standing near it, the top of the building tends to taper inwards towards the top (see Picture 1). This distortion is called converging verticals and makes the building look as if it is falling backwards.
Pict 2: Distortion in perspective corrected using GIMP
To minimise such distortions, you can either go as far away as possible from the building (unfortunately you usually don’t get such luxury of space) and try to snap the building flat-on, or invest in an expensive camera accessory called a tilt-shift lens.
In GIMP, you can correct such distortions in perspective easily and for free – simply use the Perspective tool (see Picture 2).
Select the Perspective tool from the Toolbox (see Picture 3). In the settings section in the lower half of the Toolbox, select the following options: Corrective (Backward) radio button for Direction, and “Image + Grid” from the dropdown box for Preview.
Pict 3: Pick the Perspective Tool from the Toolbox
Pict 4: Drag the gridlines until the vertical and horizontal gridlines are aligned with the horizontal and vertical edges in the photo.
Click on the photo. A grid is overlaid onto the photo and a Perspective dialog box appears (ignore it for the moment). Drag the mouse within the grid to distort the gridlines.
Distort the grid until the vertical gridlines are aligned with the sides of the building walls, and the horizontal gridlines are aligned with the horizon and rooflines in the photo (see Picture 4).
Click the Transform button in the dialog box and the distortion is corrected.
Cropping
After using the Rotate or Perspective tool on the photo, empty areas are added around the corners that need to be cropped away. Cropping is also done to remove distractions from the surrounding and fill the frame with the main subject.
Pict 5: Highlight the area in the photo to be retained by dragging with the Crop Tool.
It is also done to adjust the aspect ratio say, from 4:3 to 3:2, or to crop a flat landscape-oriented photo into a tall portrait-oriented one. Many times, the photographer may simply want to enhance the composition of the photo by improving on the original framing of the photo.
Select the Crop tool from the Toolbox. Drag a rectangular selection around the area to be retained in the photo (see Picture 5).
A rough selection is good enough because the crop selection can be adjusted easily.
Pict 6: The photo was cropped to zero into two of the paintings.
Mouse over the side or corner of the selection and the cursor changes to include arrows that show which direction the selection can be adjusted. Click and drag these adjustment handles to adjust the crop selection inwards or out.
To move the position of the crop selection itself (without altering its size and shape), move the mouse over the centre of the selection and drag it to its new location within the photo.
Once you are happy with the crop selection, press Enter on the keyboard to crop the photo (see Picture 6).
Tomorrow, we’ll look at how to make simple exposure adjustments to darken or brighten an overexposed or underexposed photo.
Tags: GIMP, image editor, photo editor, tips
Thks for the tut. Took me a week to set up and get used to GIMP – and it was worth it! Used it to crop a photo to use for my facebook profile pic.
The first step is always the most difficult. It actually takes much effort and courage to get oriented and settled into something new. The good news is that it gets easier from here. Just fire up GIMP when you’re free and follow along the tutorials. You can download the photos i used to practise along. Enjoy it!