bilinear interpolation sec

Bilinear interpolation example

In a bilinear interpolation, first the position of each pixel in the output map is determined. Then the values of 4 surrounding pixels of the input map are used to calculate an interpolated value for each pixel in the output map.

Figure 1 below shows the position of a 'new' pixel in the output map, and the position and values of 4 surrounding pixels in the input map.

Fig. 1: Principle of bilinear interpolation.

The value of the 'new' pixel in the output map is calculated by:

first 2 interpolations in y-direction (between values 15 and 17, and between values 16 and 19; resulting in two intermediate values which are represented in red),
then 1 interpolation in x-direction (between the two intermediate values in red).

A straight line is drawn through each set of 2 points, and from this the value of the third point is known.

A bilinear interpolation should not be used when you intend to calculate a derivative of the output map (e.g. using a DFDX, DFDY filter).