The “warp” feature can be used to correct the perspective of an image.
The following example shows a framed painting that was photographed by a digital camera. The camera was slightly off-centre, so the picture is skewed. The frame needs to be cropped, but this will also crop some of the painting. The warp feature can make the image rectangular, so that cropping only removes the frame.
This original picture has a size of 768x1024 pixels. The x and y co-ordinates of corner 1 are (0,0), corner 2 is at (0,1023), corner 3 is at (767,1023) and corner 4 is at (767,0).
To warp a picture, you move each of the corners, so that the 4 corners are aligned. You can do this either by dragging the handle at each corner, or by clicking on one of the corner numbers and typing new x and y co-ordinates. Dragging works well for moving the corners approximately, but it is easier to make fine adjustments by typing a co-ordinate value.
For the best results, only move the corners that need to be moved, and don’t move them further than necessary. Make a note of the new co-ordinates; if you decide to warp the original image again, this will easier if you can start by typing in the co-ordinates that you entered last time.
For the above image, I used the following co-ordinates:
1: (7,15) 2: (0,1003) 3: (762,1023) 4: no change
Prior to finishing the warp, the image looked like this. The white x in each corner identifies the new location of the corner. Thin jagged white lines connect the x’s; these lines are very thin, so you may not be able to see them clearly. This gives you an indication of how the warp will transform the image. Each of the white lines will become an edge of a rectangle, and the rest of the picture will be transformed accordingly.
Click on the Warp button. The image now looks like this:
If you check “Auto crop” before warping, the jagged edges are removed; this is the portion of the image that was outside the white border.
The image can now be cropped without losing any of its content. If you find that the warp was not quite right, click on “edit”, “undo warp”, adjust the parameters and warp again.
The above images are © Patricia Twist, http://firstname.lastname@example.org, and may not be reproduced without permission.