Finding the brightness value of a specific point using interpolation

I got an exercise in class that I didn't know how to solve

I have a greyscale image that has been transformed with:

x'=0.8x - 0.8y + 170

y'=0.8x + 0.8y - 170

and I have a 10x10 fragment of the original image starting at (x,y)=(110,125) and have to find the brightness on the point (x',y')=(155,85) using bi-linear and closes neighbor interpolation.

I tried searching around but I only found threads using interpolation to resize images

Thanks for any help.

[Rik]

"I tried searching around but I only found threads using interpolation to resize images" That is essentially what you want to do. An image is not really anything else than a matrix containing the intensity at discrete positions. So what you want is finding the value at a non-integer location.

But in your situation, I think it is much easier to implement bi-linear interpolation and closest neighbor yourself.