kaspi314/fasttrig

Arduino library with interpolated lookup for sin() and cos()

FastTrig

Arduino library with interpolated lookup for sin() and cos(). Trades speed for accuracy.

Description

Warning: The library trades speed for accuracy so use at own risk

The library provides one lookup table that is used for isin(degrees) and icos(degrees) and itan(degrees). This lookup table is optimized for interpolation so the values for whole degrees are not optimal. Furthermore the itan() on AVR has almost no performance gain over the regular tan() so on AVR one is adviced to use tan(). On ESP32 the itan(degrees) does have a serious performance gain so use it if you need speed.

These functions are to be used as replacements for sin(radians), **cos(radians)**and tan(radians). Important to know is that they are NOT direct replaceable as the parameter differs a factor (PI/180.0) or its inverse.

Similar to cos(x) == sin(x + PI) it is also true that icos(x) == isin(x + 90), so icos() can use the very same lookup table at the cost of a single addition. In fact it uses icos(x) == isin(x - 270) as that performs better, due to the folding.

The i in the names stands for int and interpolated as the core is using integer math and lookuptable of 91 uint16_t = 182 bytes. By folding and mirroring the whole 360 degrees and beyond can be handled. When isin(x) is called and x == int(x) then the library will not interpolate and this will improve performance. When x is not a whole number the library will linear interpolate between isin(int(x)) and isin(int(x+1)). Of course this introduces an error but it is quite fast (which was the goal).

Lookup tables

The lookup tables are optimized (sketch provided) to minimize the error when using the interpolation, this implies that the points in the table might not be optimal when you use only wholde degrees. A sketch that generates lookup tables is in the examples folder. This generator sketch can also generate tables with different resolution e.g. 24, 14, 12 or even 6, 5 or 4 bit lookup tables. So depending on the application these tables can be ideal, but verify they meet your requirements.

The lookup tables used by isin() can be used directly in your program, the names are:

• isinTable16[] index 0..90, values need to be (float) divided by 65535.0
• isinTable8[] index 0..90, values need to be (float) divided by 255.0

The isinTable8 is not really for doing accurate math, however it is great to use in a LEDstrip or motor movements when less accuracy is needed.

Although the tables can be written to, it is advised not to do so.

OK, the optimize example does a write to improve the table to minimize errors

Performance isin icos itan

time in us - calls 0 - 360 step 1 degree and calls 720 - 1080 (lib version 0.1.5)

function	UNO 16MHz	ESP32 240 MHz	UNO (720-1080)	ESP (720-1080)
sin	120.43	10.90	124.19	10.91
isin	44.24	1.09	85.00	1.11
cos	120.27	10.81	123.98	10.83
icos	51.40	1.16	91.42	1.18
tan	147.59	18.07	151.39	18.07
itan	126.73	1.31	129.93	1.29

*Note: itan() 0.1.3 was ( 131.23, 3.05 ) so it improved quite a bit on ESP32. *

Performance gain is most evident for the ESP32 processor, and much less on AVR. The effect of the modulo (360 degrees) can be seen explicitly in AVR. Furthermore the itan() on AVR is not faster when there is also interpolation (not in table)

The 0.1.4 version of itan() is faster for ESP32 than 0.1.3 version but the improvement on AVR is minimal. So this will stay on the TODO list.

Furthermore a lot of gain is lost when the angle is not within 0..360 and needs to be normalized ( expensive modulo on AVR ). It is worth noting that the original sin() cos() and tan() only have a small overhead for values outside the 0..360 range.

Please, verify the performance to see if it meets your requirements.

Accuracy isin icos itan

errors - based upon example sketch - lib version 0.1.5

ESP32 calls 0.0 - 360.0 step 0.1 degree

function	max abs error	avg abs error	max rel error	avg rel error
isin	0.00010264	0.00002059	0.02955145	0.00035180
icos	0.00010264	0.00002031	0.02955145	0.00034868
itan	0.69696045	0.00640957	0.00144703	0.00010100

UNO calls 0.0 - 360.0 step 0.1 degree

function	max abs error	avg abs error	max rel error	avg rel error
isin	0.00010270	0.00002059	0.02955145	0.00035171
icos	0.00010264	0.00002032	0.02949960	0.00034869
itan	0.72760009	0.00641527	0.00144703	0.00037889

*Note: 0.1.3 for AVR was bad: 17.41900634 , 0.02249339 , 0.02953807 for itan() *

Strange that the itan() on UNO and ESP32 differs (OK same order of magnitude). Different implementation of gonio / float math?

Please, verify the performance to see if it meets your requirements.

Performance iasin iacos iatan

(added 0.1.5)

time in us - calls -1 ..+1 step 0.001 degree

function	UNO 16MHz	ESP32 240 MHz
asin	149.76	16.71
iasin	107.70	2.58
acos	169.50	15.44
iacos	114.65	2.67
atan	155.75	11.68
iatan	NI	NI

• the interpolated reverse lookup is around 30% faster on UNO an 80+% on ESP32
• iatan is Not Implemented.

Please, verify the accuracy to see if it meets your requirements.

Accuracy iasin iacos iatan

(added 0.1.5)

ESP32 calls -1 ..+1 step 0.001 degree

function	max abs error	avg abs error	max rel error	avg rel error
iasin	0.22498322	0.00195790	0.00456106	0.00005727
iacos	0.22498587	0.00195794	0.64284271	0.00021902
iatan	NI	NI	NI	NI

• largest error at 0.999981 - second largest error 0.052841 at -0.999000
• iatan is Not Implemented

UNO calls -1 ..+1 step 0.001 degree

function	max abs error	avg abs error	max rel error	avg rel error
iasin	0.22499084	0.00195719	0.00456125	0.00005725
iacos	0.22498588	0.00195740	0.64284276	0.00021901
iatan	NI	NI	NI	NI

• largest error at 0.999981 - second largest error 0.052841 at -0.999000
• max rel error is high as it occured near zero.
• iatan is Not Implemented

Please, verify the accuracy to see if it meets your requirements.

0.1.4

The library (0.1.4) provides an itan() which improved accuracy upon the (0.1.3) version and performance for the ESP32.
Performance on AVR (UNO) is still an issue, accuracy is OK.

0.1.5

In (0.1.4) an error was found in the optimize algorithm, so for 0.1.5 it was ran again and accuracy improved for isin() and icos(). However itan() lost a (smaller) bit. The gain outweighs the loss and so new table is kept.

Performance has not changed.

An initial version of a reverse lookup for iasin(val) and iacos(val) is added, as it uses the same isintable16[] interpolation table.

There is no atan() or atan2() replacement.

TODO

• How to improve the accuracy of the whole degrees, as now the table is optimized for interpolation.
• investigate itan() lookup table with interpolation (see notes.txt). Interpolation is not trivial for angles between say 60-90 degrees
• investigate if atan() / atan(() can be made.

Operation

See examples