Bilinear Interpolation

Functions
__STATIC_INLINE float32_t	arm_bilinear_interp_f32 (const arm_bilinear_interp_instance_f32 *S, float32_t X, float32_t Y)
	Floating-point bilinear interpolation.
__STATIC_INLINE q31_t	arm_bilinear_interp_q31 (arm_bilinear_interp_instance_q31 *S, q31_t X, q31_t Y)
	Q31 bilinear interpolation.
__STATIC_INLINE q15_t	arm_bilinear_interp_q15 (arm_bilinear_interp_instance_q15 *S, q31_t X, q31_t Y)
	Q15 bilinear interpolation.
__STATIC_INLINE q7_t	arm_bilinear_interp_q7 (arm_bilinear_interp_instance_q7 *S, q31_t X, q31_t Y)
	Q7 bilinear interpolation.

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Description

Bilinear interpolation is an extension of linear interpolation applied to a two dimensional grid. The underlying function f(x, y) is sampled on a regular grid and the interpolation process determines values between the grid points. Bilinear interpolation is equivalent to two step linear interpolation, first in the x-dimension and then in the y-dimension. Bilinear interpolation is often used in image processing to rescale images. The CMSIS DSP library provides bilinear interpolation functions for Q7, Q15, Q31, and floating-point data types.

Algorithm

The instance structure used by the bilinear interpolation functions describes a two dimensional data table. For floating-point, the instance structure is defined as:

   typedef struct
   {
     uint16_t numRows;
     uint16_t numCols;
     float32_t *pData;
 } arm_bilinear_interp_instance_f32;
 

where numRows specifies the number of rows in the table; numCols specifies the number of columns in the table; and pData points to an array of size numRows*numCols values. The data table pTable is organized in row order and the supplied data values fall on integer indexes. That is, table element (x,y) is located at pTable[x + y*numCols] where x and y are integers.

Let (x, y) specify the desired interpolation point. Then define:

     XF = floor(x)
     YF = floor(y)
 

The interpolated output point is computed as:

  f(x, y) = f(XF, YF) * (1-(x-XF)) * (1-(y-YF))
           + f(XF+1, YF) * (x-XF)*(1-(y-YF))
           + f(XF, YF+1) * (1-(x-XF))*(y-YF)
           + f(XF+1, YF+1) * (x-XF)*(y-YF)
 

Note that the coordinates (x, y) contain integer and fractional components. The integer components specify which portion of the table to use while the fractional components control the interpolation processor.

if (x,y) are outside of the table boundary, Bilinear interpolation returns zero output.

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Function Documentation

__STATIC_INLINE float32_t arm_bilinear_interp_f32	(	const arm_bilinear_interp_instance_f32 *	S,
		float32_t	X,
		float32_t	Y
	)

Parameters:

[in,out]	*S	points to an instance of the interpolation structure.
[in]	X	interpolation coordinate.
[in]	Y	interpolation coordinate.

Returns:

out interpolated value.

References arm_bilinear_interp_instance_f32::numCols, arm_bilinear_interp_instance_f32::numRows, and arm_bilinear_interp_instance_f32::pData.

__STATIC_INLINE q15_t arm_bilinear_interp_q15	(	arm_bilinear_interp_instance_q15 *	S,
		q31_t	X,
		q31_t	Y
	)

Parameters:

[in,out]	*S	points to an instance of the interpolation structure.
[in]	X	interpolation coordinate in 12.20 format.
[in]	Y	interpolation coordinate in 12.20 format.

Returns:

out interpolated value.

References arm_bilinear_interp_instance_q15::numCols, arm_bilinear_interp_instance_q15::numRows, and arm_bilinear_interp_instance_q15::pData.

__STATIC_INLINE q31_t arm_bilinear_interp_q31	(	arm_bilinear_interp_instance_q31 *	S,
		q31_t	X,
		q31_t	Y
	)

Parameters:

[in,out]	*S	points to an instance of the interpolation structure.
[in]	X	interpolation coordinate in 12.20 format.
[in]	Y	interpolation coordinate in 12.20 format.

Returns:

out interpolated value.

References arm_bilinear_interp_instance_q31::numCols, arm_bilinear_interp_instance_q31::numRows, and arm_bilinear_interp_instance_q31::pData.

__STATIC_INLINE q7_t arm_bilinear_interp_q7	(	arm_bilinear_interp_instance_q7 *	S,
		q31_t	X,
		q31_t	Y
	)

Parameters:

[in,out]	*S	points to an instance of the interpolation structure.
[in]	X	interpolation coordinate in 12.20 format.
[in]	Y	interpolation coordinate in 12.20 format.

Returns:

out interpolated value.

References arm_bilinear_interp_instance_q7::numCols, arm_bilinear_interp_instance_q7::numRows, and arm_bilinear_interp_instance_q7::pData.