Given an image of my choice, and choose an object in the image, how to move that object to another place in the image? How to stretch and shrink it?

Those problems can be well solved by Affine transformations.Making it more concrete, my job is to rotate an object of my choice 30 degrees counterclockwise, translate by a distance, and scale of 0.5.

Firstly, load an image and show it in a window. Choose a rectangle area as ROI by mouse clicking. Then do transformation for the rectangle area (the object). After transformation, some black areas will appear, flood filling is helpful here for those images that have single background color.

Matrix for affine transformation can be defined as:

scaling                  0.5    0      0

                               0      0.5   0

translation            1       0      distanceX

                              0       1      distanceY

rotation                cos(Pi/6)   -sin(Pi/6)   0

                             sin(Pi/6)    cos(Pi/6)   0

Now let me choose the first picture:

Shrink the number "0":

Translation T_x by 245 pixels from original picture:

Now rotation. A little problem for my OpenCV is that I can't set my original point to the bottom left of the image (it's always at the top-left point). But I can deal with this by some transformation .

The relation of two coordinates is: O(x,y) <----> O'(x,H-y)

calculate the new coordinate of A(x',y') in O'(x',y'), we can get the offsets

offsetX=Xo=x', offsetY=Yo=H-y'

Rotate the image in ROI, and then add the offsets, flood-fill blank areas, I can rotate the arrow in the picture by 30 degrees:

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The rotation is fine,but floodfill function leaves a black box.

Jiannan Zhang

2.9 in MN