# Bilinear interpolation

In mathematics, bilinear interpolation is an extension of linear interpolation for interpolating functions of two variables on a regular grid. The key idea is to perform linear interpolation first in one direction, and then again in the other direction.

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The four red dots show the data points and the green dot is the point at which we want to interpolate. Example of bilinear interpolation on the unit square with the z-values 0, 1, 1 and 0.5 as indicated. Interpolated values in between represented by colour.

Suppose that we want to find the value of the unknown function f at the point P = (x, y). It is assumed that we know the value of f at the four points Q11 = (x1y1), Q12 = (x1y2), Q21 = (x2y1), and Q22 = (x2y2).

We first do linear interpolation in the x-direction. This yields $f(R_1) \approx \frac{x_2-x}{x_2-x_1} f(Q_{11}) + \frac{x-x_1}{x_2-x_1} f(Q_{21}) \quad\mbox{where}\quad R_1 = (x,y_1),$ $f(R_2) \approx \frac{x_2-x}{x_2-x_1} f(Q_{12}) + \frac{x-x_1}{x_2-x_1} f(Q_{22}) \quad\mbox{where}\quad R_2 = (x,y_2).$

We proceed by interpolating in the y-direction. $f(P) \approx \frac{y_2-y}{y_2-y_1} f(R_1) + \frac{y-y_1}{y_2-y_1} f(R_2).$

This gives us the desired estimate of f(x, y). \begin{align} f(x,y) &\approx \frac{f(Q_{11})}{(x_2-x_1)(y_2-y_1)} (x_2-x)(y_2-y) \\ & + \frac{f(Q_{21})}{(x_2-x_1)(y_2-y_1)} (x-x_1)(y_2-y) \\ & + \frac{f(Q_{12})}{(x_2-x_1)(y_2-y_1)} (x_2-x)(y-y_1) \\ & + \frac{f(Q_{22})}{(x_2-x_1)(y_2-y_1)} (x-x_1)(y-y_1). \end{align}

If we choose a coordinate system in which the four points where f is known are (0, 0), (0, 1), (1, 0), and (1, 1), then the interpolation formula simplifies to $f(x,y) \approx f(0,0) \, (1-x)(1-y) + f(1,0) \, x(1-y) + f(0,1) \, (1-x)y + f(1,1) xy.$

Or equivalently, in matrix operations: $f(x,y) \approx \begin{bmatrix} 1-x & x \end{bmatrix} \begin{bmatrix} f(0,0) & f(0,1) \\ f(1,0) & f(1,1) \end{bmatrix} \begin{bmatrix} 1-y \\ y \end{bmatrix}.$

Contrary to what the name suggests, the interpolant is not linear. Instead, it is of the form $(a_1 x + a_2)(a_3 y + a_4), \,$

so it is a product of two linear functions. Alternatively, the interpolant can be written as $b_1 + b_2 x + b_3 y + b_4 x y \,$

where $b_1 = f(0,0) \,$ $b_2 = f(1,0)-f(0,0) \,$ $b_3 = f(0,1)-f(0,0) \,$ $b_4 = f(0,0)-f(1,0)-f(0,1)+f(1,1). \,$

In both cases, the number of constants (four) correspond to the number of data points where f is given. The interpolant is linear along lines parallel to either the $x$ or the $y$ direction, equivalently if $x$ or $y$ is set constant. Along any other straight line, the interpolant is quadratic.

The result of bilinear interpolation is independent of the order of interpolation. If we had first performed the linear interpolation in the y-direction and then in the x-direction, the resulting approximation would be the same.

The obvious extension of bilinear interpolation to three dimensions is called trilinear interpolation.

## Application in image processing

In computer vision and image processing, bilinear interpolation is one of the basic resampling techniques.

It is a texture mapping technique that produces a reasonably realistic image, also known as bilinear filtering or bilinear texture mapping. An algorithm is used to map a screen pixel location to a corresponding point on the texture map. A weighted average of the attributes (color, alpha, etc.) of the four surrounding texels is computed and applied to the screen pixel. This process is repeated for each pixel forming the object being textured

When an image needs to be scaled-up, each pixel of the original image needs to be moved in certain direction based on scale constant. However, when scaling up an image, there are pixels (i.e. Hole) that are not assigned to appropriate pixel values. In this case, those holes should be assigned to appropriate image values so that the output image does not have non-value pixels.

Typically bilinear interpolation can be used where perfect image transformation, matching and imaging is impossible so that it can calculate and assign appropriate image values to pixels. Unlike other interpolation techniques such as nearest neighbor interpolation and bicubic interpolation, bilinear Interpolation uses the 4 nearest pixel values which are located in diagonal direction from that specific pixel in order to find the appropriate color intensity value of a desired pixel.