Perspective corrected result (Click on the image to enlarge it).
Original photo
Often times, when taking photos, the object is very large and not possible to position the camera perfectly in a frontal position, and the resulting photo shows a perspective. i.e. Nearer is larger and further is smaller.
For example, the photo on the left are some vines grown on a flat wall near the place I lived. The wall is actually quite large, probably more than 10 meters in height and 20 meters in width. Also, It is not possible to take a perfect frontal position from a distance because of other obstacles blocking the view.
Some time ago, I have written a short program using openFrameworks and openCV library (using perspective transform, to transform a 2D plane to another 2D plane in 3-space) to perspective correct a photo so that the photo looks like taken from a view perpendicular to the wall. Tools such as Photoshop or other photo touch up utilities out there might do similar trick, but I created a short program for this only so that in future I might add some other effects for photo processing also.