How to find an Equivalent point in a Scaled down image?

Question

I would like to calculate the corner points or contours of the star in this in a Larger image. For that I'm scaling down the size to a smaller one & I'm able to get this points clearly. Now How to map this point in original image? I'm using opencv c++.

Answer 1

Consider a trivial example: the image size is reduced exactly by half.

So, the cartesian coordinate (x, y) in the original image becomes coordinate (x/2, y/2) in the reduced image, and coordinate (x', y') in the reduced image corresponds to coordinate (x*2, y*2) in the original image.

Of course, fractional coordinates get typically rounded off, in a reduced scale image, so the exact mapping is only possible for even-numbered coordinates in this example's original image.

Generalizing this, if the image's width is scaled by a factor of w horizontally and h vertically, coordinate (x, y) becomes coordinate(x*w, y*h), rounded off. In the example I gave, both w and h are 1/2, or .5

You should be able to figure out the values of w and h yourself, and be able to map the coordinates trivially. Of course, due to rounding off, you will not be able to compute the exact coordinates in the original image.