OpenCV and Image Processing - Warp Image

Warp image refers to the process of geometrically transforming an image into different shapes. It involves applying a perspective or affine transformation to the image, which can change its size, orientation, and shape.

This post introduces perspective warping, also known as perspective transformation, which is a type of image warping that transforms an image from one perspective to another. It is a geometric transformation that changes the viewpoint of an image, as if the observer's viewpoint has moved or rotated in space.

It is often used to correct the distortion caused by the camera's perspective when capturing an image. This distortion causes objects in the image to appear different in size and shape depending on their position in the image. For example, objects closer to the camera appear larger and more distorted than those farther away.

The perspective warping process includes defining four points in the original image and mapping them to four corresponding points in the output image. These points are known as the source points and destination points, respectively. Once the corresponding points are identified, a transformation matrix is calculated, which maps each pixel in the original image to its new location in the output image. As shown in Figure below, there is an original image in the left, and the output image in the right. The four source points from the original image are A, B, C and D, the perspective warping process will transform them to the output image in the right, so that A is mapped to A’, B to B’, C to C’ and D to D’.

The real-world use case is, for example, using a camera to take a picture of a document, there are some distortions that make the document looks like the left-side image where A, B, C and D are the four corners of the document. A perspective warping will be able to transform the document into the right-side image, which is corrected and aligned properly. 

Perspective warping is commonly used in computer vision and image processing applications, such as image correction, where in many cases images can be distorted due to the angle of the camera, by applying perspective warping the image can be corrected and aligned properly. Another example is image stitching where multiple images are combined to create a larger panorama, warping can be used to align the images properly so that they fit seamlessly.