Geometric Transformation using OpenCV

Many of the transforamtions use Matrices as input (Computer Graphics students may already know this). Here in OpenCV we have different functions available to create those matrices.

Perspective Transformation

We need 4 points on the input image and corresponding 4 points on the output image

let these be points on input [(a,b),(a1,b1),(a2,b2),(a3,b3)] and these will be ouput points  
[(x,y),(x1,y1),(x2,y2),(x3,y3)]

the point (a,b) will be placed in position (x,y) in output image

getPerspectiveTransform will create a matrix for us using those points

Matrix = cv2.getPerspectiveTransform(pts1,pts2)

Now apply this Matrix to image using warpPerspective

Document scanners use this perspective

output = cv2.warpPerspective(image,M,(500,500))

Affine Transformation

All parallel lines in the original image will still be parallel in the output image.

To find the transformation matrix, we need three points from input image and their corresponding locations in output image

getAffineTransform will create a matrix for us.

Matrix = cv2.getAffineTransform(pts1,pts2)
output = cv2.warpAffine(img,Matrix,(width,height))

cv2.warpAffine(input_image,Transformation_Matrix,outputImageSize))

Note: output image size will be in the form of (width,height)

Rotation

getRotationMatrix2D will create Rotation Matrix

rotationMatrix=cv2.getRotationMatrix2D(center=(height/2,width/2),angle=45,scale=1)
o1=cv2.warpAffine(image,rotationMatrix,(width,height))

Scaling

We can scale images using resize function by adding interpolation

cv2.INTER_AREA for shrinking and cv2.INTER_CUBIC (slow) & cv2.INTER_LINEAR for zooming

res = cv2.resize(img,(width,height), fx=None,fy=None,interpolation = cv2.INTER_CUBIC)

or you can mention scaling factor manually

• fx- horizontal scaling factor
• fy- vertical scaling factor