Bilinear interpolation?

duck_soup

Hi folks!

I wanted to use bilinear interpolation (just a simple, not an optimized version) to zoom an image, but I don't know exactly how it works.

What I know is that one loop runs over the new, enlarged image and in this image every cell imgOut[j][k] gets a new value, which is calculated from the surrounding 4 or 8 pixels of the input image.
My central question is: Do I have to resample this input image like the output image before I start calculating the interpolated values?

Thanks in advance.