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In mathematics, bicubic interpolation is an extension of cubic interpolation for interpolating data points on a regular grid. The interpolated surface is smooth in all directions compared to bilinear interpolation and nearest neighbor interpolation. Bicubic interpolation can be accomplished using either Lagrange polynomials, cubic splines or cubic convolution algorithm.
In image processing, bicubic interpolation is often chosen over bilinear interpolation or nearest neighbor in image resampling, when speed is not an issue. Images resampled with bicubic interpolation are smoother and have fewer interpolation artifacts.
 Bicubic spline interpolation
Suppose the function values f and the derivatives fx,fy and fxy are known at the four corners (0,0), (1,0), (0,1), and (1,1) of the unit square. The interpolated surface can the be written
The interpolation problem consists of determining the 16 coefficients aij. Matching p(x,y) with the function values yields four equations,
- f(0,0) = p(0,0) = a00
- f(1,0) = p(1,0) = a00 + a10 + a20 + a30
- f(0,1) = p(0,1) = a00 + a01 + a02 + a03
Likewise, eight equations for the derivatives in the x-direction and the y-direction
- fx(0,0) = px(0,0) = a10
- fx(1,0) = px(1,0) = a10 + 2a20 + 3a30
- fx(0,1) = px(0,1) = a10 + a11 + a12 + a13
- fy(0,0) = py(0,0) = a01
- fy(1,0) = py(1,0) = a01 + a11 + a21 + a31
- fy(0,1) = py(0,1) = a01 + 2a02 + 3a03
And four equations for the cross derivative xy.
- fxy(0,0) = pxy(0,0) = a11
- fxy(1,0) = pxy(1,0) = a11 + 2a21 + 3a31
- fxy(0,1) = pxy(0,1) = a11 + 2a12 + 3a13
where the expressions above have used the following identities,
This procedure yields a surface p(x,y) on the unit square which is continuous and with continuous derivatives. Bicubic interpolation on an arbitrarily sized regular grid can then be accomplished by patching together such bicubic surfaces, ensuring that the derivatives match on the boundaries.
If the derivatives are unknown, they are typically approximated from the function values at points neighbouring the corners of the unit square, ie. using finite differences.
 Bicubic convolution algorithm
Bicubic spline interpolation requires to solve the linear system described above for each grid cell. Although similar interpolator with similar properties can be obtained by applying convolution with the following kernel in both dimensions:
where a is usually set to -0.5 or -0.75.
This approach was proposed by Keys.
 Use in computer graphics
- ^ R. Keys, (1981). "Cubic convolution interpolation for digital image processing". IEEE Transactions on Signal Processing, Acoustics, Speech, and Signal Processing.
 See also
- Bézier surface
- Bilinear interpolation
- Cubic Hermite spline, the one-dimensional analogue of bicubic spline
- Lanczos resampling
- Sinc filter
- Spline interpolation