The purpose of this document is to describe the functions contained within the Nexteer interpolation library and as an API reference in designing functional requirements, models, and software components.

Interpolation Design

Linear Interpolation

Linear interpolation is used to determine an output value between a set of known data points for a given input. In the image below, the input point (x) is between the known points (x₀,y₀) and (x₁,y₁).

The linear interpolant is defined as the straight line between the two known points (x₀,y₀) and (x₁,y₁) and is defined in equation (1).

	$$\frac{\left( y - y_{0} \right)}{\left( x - x_{0} \right)} = \frac{\left( y_{1} - y_{0} \right)}{\left( x_{1} - x_{0} \right)}$$	(1)

Solving equation (1) for the desired output (y) is described below in equation (2).

	$$y = y_{0} + (y_{1} - y_{0})\frac{\left( x - x_{0} \right)}{\left( x_{1} - x_{0} \right)}$$	(2)

If the delta between neighboring points on the X-axis is the same, equation (2) can be modified with that delta as shown in equation (3).

Linear Interpoliation Accuracy

The operations required to perform equations (2) and (3) shall be implemented using fixed point math. This is to save on execution time to perform the interpolation compared to using floating point math functions (See Appendix C). This also means that there can be information loss because the operations can produce results that have more bits than the operands. In order to keep the same number of bits as the operands, the answer must be rounded or truncated.

The interpolation library shall support both truncation and rounding methods in all linear interpolation functions. These methods are described in Appendix B. The error produced by the truncated function results can be +/- one count. Depending on the resolution of the calibration and the resolution required by the design, this may be negligible. The function designer shall use the truncation library functions in all cases where this error is acceptable in order to save on execution time.

Fixed X-Axis Linear Interpolation Functions

The following functions are available for linear interpolation with a fixed X-axis and are based on equation (3). The table below defines each argument used in the API. Note that some functions require and/or return unsigned or signed values. It is up to the designer to pick the proper interpolation function for their design.

Argument	Notes
DeltaX	Provides the delta between points in the X-axis for interpolation. A value of 0 shall return the first value in YTbl.
YTbl[]	Y table values
Size	Number of elements in YTbl
Inp	Input value (x). Values that are less than or equal to DeltaX or larger than DeltaX * (Size – 1) will return the first or last value respectively from YTbl.

API

The input and output variables are defined as uint16 or sint16 for purposes of min and max ranges. However, calibrations with different resolutions, for example u8p8, can be used because they are still represented as a 16-bit value within these functions.

Truncating Functions

Function Name	LnrIntrpn_u16_u16FixdXu16VariY	Type	Min	Max
Arguments Passed	DeltaX	uint16	0	65535
	YTbl[]	uint16[]	0	65535
	Size	uint16	1	65535
	Inp	uint16	0	65535
Return Value		uint16	0	65535

Function Name	LnrIntrpn_s16_u16FixdXs16VariY	Type	Min	Max
Arguments Passed	DeltaX	uint16	0	65535
	YTbl[]	sint16[]	-32768	32767
	Size	uint16	1	65535
	Inp	uint16	0	65535
Return Value		sint16	-32768	32767

Rounding Functions

Function Name	LnrIntrpnWithRound_u16_u16FixdXu16VariY	Type	Min	Max
Arguments Passed	DeltaX	uint16	0	65535
	YTbl[]	uint16[]	0	65535
	Size	uint16	1	65535
	Inp	uint16	0	65535
Return Value		uint16	0	65535

Function Name	LnrIntrpnWithRound_s16_u16FixdXs16VariY	Type	Min	Max
Arguments Passed	DeltaX	uint16	0	65535
	YTbl[]	sint16[]	-32768	32767
	Size	uint16	1	65535
	Inp	uint16	0	65535
Return Value		sint16	-32768	32767

Variable X-Axis Linear Interpolation Functions

The following functions are available for linear interpolation with a variable X-axis and are based on equation (2). The table below defines each argument used in the API. Note that some functions require and/or return unsigned or signed values. It is up to the designer to pick the proper interpolation function for their design.

Argument	Notes
XTbl[]	X Table
YTbl[]	Y Table
Size	Number of elements in the XTbl and YTbl
Inp	Input value (x). Values that are less than or equal to XTbl[0] or larger than XTbl[Size – 1] will return the first or last value respectively from YTbl.

API

Truncating Functions

Function Name	LnrIntrpn_u16_u16VariXu16VariY	Type	Min	Max
Arguments Passed	XTbl[]	uint16[]	0	65535
	YTbl[]	uint16[]	0	65535
	Size	uint16	1	65535
	Inp	uint16	0	65535
Return Value		uint16	0	65535

Function Name	LnrIntrpn_u16_s16VariXu16VariY	Type	Min	Max
Arguments Passed	XTbl[]	sint16[]	-32768	32767
	YTbl[]	uint16[]	0	65535
	Size	uint16	1	65535
	Inp	sint16	-32768	32767
Return Value		uint16	0	65535

Function Name	LnrIntrpn_s16_s16VariXs16VariY	Type	Min	Max
Arguments Passed	XTbl[]	sint16[]	-32768	32767
	YTbl[]	sint16[]	-32768	32767
	Size	uint16	1	65535
	Inp	sint16	-32768	32767
Return Value		sint16	-32768	32767

Function Name	LnrIntrpn_s16_u16VariXs16VariY	Type	Min	Max
Arguments Passed	XTbl[]	uint16[]	0	65535
	YTbl[]	sint16[]	-32768	32767
	Size	uint16	1	65535
	Inp	uint16	0	65535
Return Value		sint16	-32768	32767

Rounding Functions

Function Name	LnrIntrpnWithRound_u16_u16VariXu16VariY	Type	Min	Max
Arguments Passed	XTbl[]	uint16[]	0	65535
	YTbl[]	uint16[]	0	65535
	Size	uint16	1	65535
	Inp	uint16	0	65535
Return Value		uint16	0	65535

Function Name	LnrIntrpnWithRound_u16_s16VariXu16VariY	Type	Min	Max
Arguments Passed	XTbl[]	sint16[]	-32768	32767
	YTbl[]	uint16[]	0	65535
	Size	uint16	1	65535
	Inp	sint16	-32768	32767
Return Value		uint16	0	65535

Function Name	LnrIntrpnWithRound_s16_s16VariXs16VariY	Type	Min	Max
Arguments Passed	XTbl[]	sint16[]	-32768	32767
	YTbl[]	sint16[]	-32768	32767
	Size	uint16	1	65535
	Inp	sint16	-32768	32767
Return Value		sint16	-32768	32767

Function Name	LnrIntrpnWithRound_s16_u16VariXs16VariY	Type	Min	Max
Arguments Passed	XTbl[]	uint16[]	0	65535
	YTbl[]	sint16[]	-32768	32767
	Size	uint16	1	65535
	Inp	uint16	0	65535
Return Value		sint16	-32768	32767

Bilinear Interpolation

Bilinear interpolation is linear interpolation with expanded coverage into three dimensions. To calculate the result, three linear interpolations are required to be performed.

The first two interpolations are between the two data sets (shown in red and green in the image) and are described with the equations (4) and (5).

	$$Y_{1} = y_{0} + \left( y_{1} - y_{0} \right)\left( \frac{(x - x_{0})}{(x_{1} - x_{0})} \right)$$	(4)

	$$Y_{2} = y_{2} + \left( y_{3} - y_{2} \right)\left( \frac{(x - x_{2})}{(x_{3} - x_{2})} \right)$$	(5)

Equations (4) and (5) provide the endpoints for the interpolant between the two sets of data. The third interpolation determines how close the desired output is to each of the data sets. This is described in equation (6).

	$$y = Y_{1} + \left( Y_{2} - Y_{1} \right)\left( \frac{(BS - {BS}_{1})}{({BS}_{2} - {BS}_{1})} \right)$$	(6)

Since divisions are throughput intensive operations, equation (6) is not efficient for an embedded environment because it contains three division steps when substitutions are made for Y₁ and Y₂. However, the terms can be rearranged to reduce the amount of divisions for additional multiplications and additions, which typically are much less in throughput consumption. Staring with equations (4) and (5), the terms can be reordered into equations (7) and (8).

	$$Y_{1} = \frac{\left( {x - x}_{0} \right)\left( {y_{1} - y}_{0} \right) + \left( x_{1} - x_{0} \right)y_{0}}{\left( x_{1} - x_{0} \right)} = \frac{Y_{1_{Num}}}{Y_{1_{Den}}}$$	(7)

	$$Y_{2} = \frac{\left( x - x_{2} \right)\left( y_{3} - y_{2} \right) + \left( x_{3} - x_{2} \right)y_{2}}{\left( x_{3} - x_{2} \right)} = \frac{Y_{2_{Num}}}{Y_{2_{Den}}}$$	(8)

Equations (7) and (8) can be substituted back in equation (6) and the simplified result is shown in equation (9).

	$$y = \frac{\left( BS - {BS}_{1} \right)\left( Y_{2_{Num}} \right)\left( Y_{1_{Den}} \right) - \left( BS - {BS}_{1} \right)\left( Y_{1_{Num}} \right)\left( Y_{2_{Den}} \right) + \left( {BS}_{2} - {BS}_{1} \right)\left( Y_{1_{Num}} \right)\left( Y_{2_{Den}} \right)}{\left( {BS}_{2} - {BS}_{1} \right)\left( Y_{1_{Den}} \right)\left( Y_{2_{Den}} \right)}$$	(9)

Finally, the numerator in equation (9) can also be rearranged and split into multiple terms. The steps taken between equations (9) and (10) are provided in Appendix A. The final equation, requiring only one division step, is shown in equation (10).

Term_1a = (x₃−x₂)(y₁ − y₀)

Term_1b = (x − x₀)(BS₂ − BS₁)

Term_2a3a = (x₂ − x₀)(BS − BS₁)

Term_2b = (x₃ − x₂)(y₂ − y₀)

Term_3b = (x − x₃)(y₃ − y₂)

$$y = y_{0} + \left( \frac{\left( {Term}_{2a3a} \right)\left( {Term}_{3b} \right) + \left( {Term}_{2a3a} \right)\left( {Term}_{2b} \right) + \left( {Term}_{1a} \right)\left( {Term}_{1b} \right)}{\left( {BS}_{2} - {BS}_{1} \right)\left( x_{1} - x_{0} \right)\left( x_{3} - x_{2} \right)} \right)$$

(10)

Common X Axis Bilinear Interpolation Functions

The following functions are available for bilinear interpolation with a common X-axis and are based on equation (10). The table below defines each argument used in the API. Note that some functions require and/or return unsigned or signed values. It is up to the designer to pick the proper interpolation function for their design.

Argument	Notes
BilnrSeln	Bilinear Selection (BS-axis). If BilnrSeln is less than or equal to BilnrSelnTbl[0], BilnrSeln will be assigned BilnrSelnTbl[0] and the BS index will be set to 0. If BilnrSeln is greater or equal to BilnrSerlnTbl[BilnrSelnSize – 1], then the BilnrSeln will be assigned BilnrSelnTbl[BilnrSelnSize – 1] and the BS index will be set to BilnrSelnSize – 2.
Inp	Input (X-axis). If Inp is less than or equal to XTbl[0], the value will be assigned XTbl[0] and the X index will be set to 0. If it is greater than or equal to XTbl[XSize – 1], Inp will be assigned XTbl[XSize – 1] and the X index will be set to XSize – 2.
BilnrSelnTbl[]	Bilinear selection table
BilnrSelnSize	Number of elements in Bilinear selection table
XTbl[]	X table
MplYTbl[]	Y table (Number of elements = BilnrSelnSize * XSize) In most cases, Y table will be a 2-D table, and is declared as MplYTbl[BilnrSelnSize][XSize]
XSize	Size of X table

API

Function Name	BilnrIntrpnWithRound_u16_u16CmnXu16MplY	Type	Min	Max
Arguments Passed	BilnrSeln	uint16	0	65535
	Inp	uint16	0	65535
	BilnrSelnTbl[]	uint16[]	0	65535
	BilnrSelnSize	uint16	1	65535
	XTbl[]	uint16[]	0	65535
	MplYTbl[]	uint16[]	0	65535
	XSize	uint16	1	65535
Return Value		uint16	0	65535

Function Name	BilnrIntrpnWithRound_s16_u16CmnXs16MplY	Type	Min	Max
Arguments Passed	BilnrSeln	uint16	0	65535
	Inp	uint16	0	65535
	BilnrSelnTbl[]	uint16[]	0	65535
	BilnrSelnSize	uint16	1	65535
	XTbl[]	uint16[]	0	65535
	MplYTbl[]	sint16[]	-32768	32767
	XSize	uint16	1	65535
Return Value		sint16	-32768	32767

Function Name	BilnrIntrpnWithRound_s16_s16CmnXs16MplY	Type	Min	Max
Arguments Passed	BilnrSeln	uint16	0	65535
	Inp	sint16	-32768	32767
	BilnrSelnTbl[]	uint16[]	0	65535
	BilnrSelnSize	uint16	1	65535
	XTbl[]	sint16[]	-32768	32767
	MplYTbl[]	sint16[]	-32768	32767
	XSize	uint16	1	65535
Return Value		sint16	-32768	32767

Function Name	BilnrIntrpnWithRound_u16_s16CmnXu16MplY	Type	Min	Max
Arguments Passed	BilnrSeln	uint16	0	65535
	Inp	sint16	-32768	32767
	BilnrSelnTbl[]	uint16[]	0	65535
	BilnrSelnSize	uint16	1	65535
	XTbl[]	sint16[]	-32768	32767
	MplYTbl[]	uint16[]	0	65535
	XSize	uint16	1	65535
Return Value		uint16	0	65535

Variable X Axis Bilinear Interpolation Functions

The following functions are available for bilinear interpolation with a variable X-axis and are based on equation (10). The table below defines each argument used in the API. Note that some functions require and/or return unsigned or signed values. It is up to the designer to pick the proper interpolation function for their design.

Argument	Notes
BilnrSeln	Bilinear Selection (BS-axis). If BilnrSeln is less than or equal to BilnrSelnTbl[0], BilnrSeln will be assigned BilnrSelnTbl[0] and the BS index will be set to 0. If BilnrSeln is greater or equal to BilnrSerlnTbl[BilnrSelnSize – 1], then the BilnrSeln will be assigned BilnrSelnTbl[BilnrSelnSize – 1] and the BS index will be set to BilnrSelnSize – 2.
Inp	Input (X-axis). If Inp is less than or equal to XTbl[0], the value will be assigned XTbl[0] and the X index will be set to 0. If it is greater than or equal to XTbl[XSize – 1], Inp will be assigned XTbl[XSize – 1] and the X index will be set to XSize – 2.
BilnrSelnTbl[]	Bilinear selection table
BilnrSelnSize	Number of elements in Bilinear selection table
MplXTbl []	X table (Number of elements = BilnrSelnSize * XSize) In most cases, X table will be a 2-D table, and is declared as MplXTbl[BilnrSelnSize][XSize]
MplYTbl[]	Y table (Number of elements = BilnrSelnSize * XSize) In most cases, Y table will be a 2-D table, and is declared as MplYTbl[BilnrSelnSize][XSize]
XSize	Size of X table

API

Function Name	BilnrIntrpnWithRound_u16_u16MplXu16MplY	Type	Min	Max
Arguments Passed	BilnrSeln	uint16	0	65535
	Inp	uint16	0	65535
	BilnrSelnTbl[]	uint16[]	0	65535
	BilnrSelnSize	uint16	1	65535
	MplXTbl[]	uint16[]	0	65535
	MplYTbl[]	uint16[]	0	65535
	XSize	uint16	1	65535
Return Value		uint16	0	65535

Function Name	BilnrIntrpnWithRound_u16_s16MplXu16MplY	Type	Min	Max
Arguments Passed	BilnrSeln	uint16	0	65535
	Inp	sint16	-32768	32767
	BilnrSelnTbl[]	uint16[]	0	65535
	BilnrSelnSize	uint16	1	65535
	MplXTbl[]	sint16[]	-32768	32767
	MplYTbl[]	uint16[]	0	65535
	XSize	uint16	1	65535
Return Value		uint16	0	65535

Function Name	BilnrIntrpnWithRound_s16_s16MplXs16MplY	Type	Min	Max
Arguments Passed	BilnrSeln	uint16	0	65535
	Inp	sint16	-32768	32767
	BilnrSelnTbl[]	uint16[]	0	65535
	BilnrSelnSize	uint16	1	65535
	MplXTbl[]	sint16[]	-32768	32767
	MplYTbl[]	sint16[]	-32768	32767
	XSize	uint16	1	65535
Return Value		sint16	-32768	32767

Function Name	BilnrIntrpnWithRound_s16_u16MplXs16MplY	Type	Min	Max
Arguments Passed	BilnrSeln	uint16	0	65535
	Inp	uint16	0	65535
	BilnrSelnTbl[]	uint16[]	0	65535
	BilnrSelnSize	uint16	1	65535
	MplXTbl[]	uint16[]	0	65535
	MplYTbl[]	sint16[]	-32768	32767
	XSize	uint16	1	65535
Return Value		sint16	-32768	32767

Known Limitations With Design

The linear interpolation functions are implemented using signed 32-bit values for the calculation of the interpolation. If the product of the X and Y terms in the numerator are greater than or equal to 2,147,483,648 or 0x80000000 the rounding functions will apply the rounding incorrectly resulting in a 1 count error due to an overflow of the sign-bit. The truncating functions should be used in these situations.
The bilinear functions are calculated using single-precision floating point math.
Values of tables for linear interpolation X-axis and each row in the x-axis in a bilinear interpolation are assumed to be increasing in value.
Consider the array of values below:

Index	0	1	2	3	4	5	6	7
X-Axis	0	1	2	2	2	2	2	2
Y-Axis	100	200	300	400	500	600	700	800

These values can be used in the interpolation library. If the input value is less than 2, then the interpolation will be performed as described in this document. If the value is greater than or equal to 2, then index 7 will be returned. This also is true for the bilinear selection table.

Consider the array of values below:

Index	0	1	2	3	4	5	6	7
X-Axis	0	1	2	2	2	2	2	4
Y-Axis	100	200	300	400	500	600	700	800

These values can also be used in the interpolation library. If the input value is less than 2, then the interpolation will be performed as described in this document. If the value is greater than or equal to 2 and less than 4, the function will interpolate between index 6 and 7 as described in this document. If the value is greater than or equal to 4, index 7 is returned. This also is true for the bilinear selection table.

All division operations in linear interpolation functions are not protected for division by zero to save on execution time. The designer shall take all efforts to ensure that the values selected for the tables will not result in a divide by zero operation.

Appendix A

The following section will demonstrate how equation (9) is recorded into the equation (10) described in the section Bilinear Interpolation.

Using equation (9) from section 5.2, the numerator and denominator can be subsitutituted for y_num and y_den as shown in equation (A1).

	$$y = \frac{\left( BS - {BS}_{1} \right)\left( Y_{2_{Num}} \right)\left( Y_{1_{Den}} \right) - \left( BS - {BS}_{1} \right)\left( Y_{1_{Num}} \right)\left( Y_{2_{Den}} \right) + \left( {BS}_{2} - {BS}_{1} \right)\left( Y_{1_{Num}} \right)\left( Y_{2_{Den}} \right)}{\left( {BS}_{2} - {BS}_{1} \right)\left( Y_{1_{Den}} \right)\left( Y_{2_{Den}} \right)} = \frac{y_{num}}{y_{den}}$$	(A1)

Substituting back in all the terms in y_num, the numerator can be simplified as shown in equations A2 to A4.

Expanding the Y_1Num and Y_2Num term:

	y_num = (BS−BS₁)(Y_{1_Den})((x−x₂)(y₃−y₂)+(Y_{2_Den})y₂) − (BS−BS₁)(Y_{2_Den})((x − x₀)(y₁ − y₀)+(Y_{1_Den})y₀) + (BS₂−BS₁)(Y_{2_Den})((x − x₀)(y₁ − y₀)+(Y_{1_Den})y₀)

	y_num = (BS−BS₁)(Y_{1_Den})(x−x₂)(y₃−y₂) + (BS−BS₁)(Y_{1_Den})(Y_{2_Den})y₂ − (BS−BS₁)(Y_{2_Den})(x − x₀)(y₁ − y₀) − (BS−BS₁)(Y_{2_Den})(Y_{1_Den})y₀ + (BS₂−BS₁)(Y_{2_Den})(x − x₀)(y₁ − y₀) + (BS₂−BS₁)(Y_{2_Den})(Y_{1_Den})y₀	(A2)

Simplifying (A2)

	y_num = (BS−BS₁)(Y_{1_Den})(x−x₂)(y₃−y₂) + (BS−BS₁)(Y_{1_Den})(Y_{2_Den})(y₂−y₀) + (BS₂−BS)(Y_{2_Den})(x − x₀)(y₁ − y₀) + (y_den)(y₀)	(A3)

Now equation (A3) can be grouped into smaller terms and simplified.

	y_num = (BS−BS₁)(x₁−x₀)(x−x₂)(y₃−y₂) + (BS−BS₁)(x₁−x₀)(x₃−x₂)(y₂−y₀) + (BS₂−BS)(x − x₀)(y₁ − y₀)(x₃−x₂) + (y_den)(y₀)

Term_1a = (x₃−x₂)(y₁ − y₀)

Term_1b = (BS₂ − BS₁)(x − x₀)

Term_2a3a = (BS − BS₁)(x₁ − x₀)

Term_2b = (x₃ − x₂)(y₂ − y₀)

Term_3b = (x − x₂)(y₃ − y₂)

$$y = \frac{y_{num}}{y_{den}} = y_{0} + \left( \frac{\left( {Term}_{2a3a} \right)\left( {Term}_{3b} \right) + \left( {Term}_{2a3a} \right)\left( {Term}_{2b} \right) + \left( {Term}_{1a} \right)\left( {Term}_{1b} \right)}{\left( {BS}_{2} - {BS}_{1} \right)\left( x_{1} - x_{0} \right)\left( x_{3} - x_{2} \right)} \right)$$

(A4)

Appendix B

The equations below can be used by a designer to see the impact of the truncating and rounding functions on their design.

Truncating Linear Interpolation Functions

From equations (2) and (3), the truncated result can be characterized by equations (B1) and (B2).

Fixed X-Axis Interpolation Function

	$$y = y_{0} + \left\lfloor (y_{1} - y_{0})\frac{\left( x - x_{0} \right)}{\mathrm{\Delta}x} \right\rfloor$$	(B1)

Variable X-Axis Interpolation Function

	$$y = y_{0} + \left\lfloor (y_{1} - y_{0})\frac{\left( x - x_{0} \right)}{\left( x_{1} - x_{0} \right)} \right\rfloor$$	(B2)

Rounding Linear Interpolation Functions

From equations (2) and (3), the rounding result can be characterized by equations (B3) and (B4).

Fixed X-Axis Interpolation Function

$$y = y_{0} + \frac{\left( \left( y_{1} - y_{0} \right)\left( x - x_{0} \right) + \left( \frac{\mathrm{\Delta}x}{2} \right) \right)}{\mathrm{\Delta}x},\ if\ \left( y_{1} - y_{0} \right)\left( x - x_{0} \right) \geq 0$$

$$y = y_{0} + \frac{\left( \left( y_{1} - y_{0} \right)\left( x - x_{0} \right) - \left( \frac{\mathrm{\Delta}x}{2} \right) \right)}{\mathrm{\Delta}x},\ if\ \left( y_{1} - y_{0} \right)\left( x - x_{0} \right) < 0$$

(B3)

Variable X-Axis Interpolation Function

$$y = y_{0} + \frac{\left( \left( y_{1} - y_{0} \right)\left( x - x_{0} \right) + \left( \frac{\left( x_{1} - x_{0} \right)}{2} \right) \right)}{\left( x_{1} - x_{0} \right)},\ if\ \left( y_{1} - y_{0} \right)\left( x - x_{0} \right) \geq 0$$

$$y = y_{0} + \frac{\left( \left( y_{1} - y_{0} \right)\left( x - x_{0} \right) - \left( \frac{\left( x_{1} - x_{0} \right)}{2} \right) \right)}{\left( x_{1} - x_{0} \right)},\ if\ \left( y_{1} - y_{0} \right)\left( x - x_{0} \right) < 0$$

(B4)

Appendix C

The chart below shows the estimated number of clock cycles that are required for the fixed x-axis linear interpolation for various calculation methods. The source file was converted to assembly code for the Renesas RH850 microcontroller targeted for EA4 applications. The number of cycles for each instruction was determined based on the software user’s manual [1]. The best/worst case values are based on all predictive branching passing or failing and all other optimizations are ignored.

Based on the information, use of floating point calculations is too costly in terms of clock cycles for use throughout the project. Where the truncating and rounding functions offer the same results for almost half the execution time.

Last modified October 12, 2025: Initial commit (1fadfc4)