[PATCH v2] libcamera: utils: Add a helper to convert floating-point to fixed-point

[Laurent Pinchart]

Hi Paul,

Thank you for the patch.

On Wed, Apr 24, 2024 at 04:44:09PM +0900, Paul Elder wrote:
> Add helper functions for converting between floating point and fixed
> point numbers. Also add tests for them.
> 
> Signed-off-by: Paul Elder <paul.elder at ideasonboard.com>
> ---
> Changes in v2:
> - added the reverse conversion function (fixed -> floating)
> - added tests
> - make the conversion code cleaner
> ---
>  include/libcamera/base/utils.h | 37 +++++++++++++++++++++++++
>  src/libcamera/base/utils.cpp   | 20 ++++++++++++++
>  test/utils.cpp                 | 49 ++++++++++++++++++++++++++++++++++

I envision more math-related helpers in the future. Could we gather
those in a new include/libcamera/base/math.h header ?

>  3 files changed, 106 insertions(+)
> 
> diff --git a/include/libcamera/base/utils.h b/include/libcamera/base/utils.h
> index 37d9af60..da0767e3 100644
> --- a/include/libcamera/base/utils.h
> +++ b/include/libcamera/base/utils.h
> @@ -9,6 +9,7 @@
>  
>  #include <algorithm>
>  #include <chrono>
> +#include <cmath>
>  #include <iterator>
>  #include <memory>
>  #include <ostream>
> @@ -369,6 +370,42 @@ decltype(auto) abs_diff(const T &a, const T &b)
>  
>  double strtod(const char *__restrict nptr, char **__restrict endptr);
>  
> +#ifndef __DOXYGEN__
> +template<unsigned int I, unsigned int F, typename R, typename T,
> +	 std::enable_if_t<std::is_integral_v<R> &&
> +			  std::is_floating_point_v<T>> * = nullptr>
> +#else
> +template<unsigned int I, unsigned int F, typename R, typename T>
> +#endif
> +constexpr R floatingToFixedPoint(T number)
> +{
> +	static_assert(I + F <= sizeof(R) * 8);
> +
> +	R maskI = (1 << I) - 1;
> +	R whole = (static_cast<R>(number) & maskI) << F;
> +
> +	R maskF = (1 << F) - 1;
> +	R frac = static_cast<R>(std::round(number * (1 << F))) & maskF;
> +
> +	return whole | frac;
> +}
> +
> +#ifndef __DOXYGEN__
> +template<unsigned int I, unsigned int F, typename R, typename T,
> +	 std::enable_if_t<std::is_floating_point_v<R> &&
> +			  std::is_integral_v<T>> * = nullptr>
> +#else
> +template<unsigned int I, unsigned int F, typename R, typename T>
> +#endif
> +constexpr R fixedToFloatingPoint(T number)
> +{
> +	static_assert(I + F <= sizeof(T) * 8);
> +
> +	int coeff = number >> (I + F - 1) ? -1 : 1;
> +
> +	return coeff * static_cast<R>(number) / static_cast<R>(1 << F);
> +}
> +
>  } /* namespace utils */
>  
>  #ifndef __DOXYGEN__
> diff --git a/src/libcamera/base/utils.cpp b/src/libcamera/base/utils.cpp
> index 3b73b442..ba36026d 100644
> --- a/src/libcamera/base/utils.cpp
> +++ b/src/libcamera/base/utils.cpp
> @@ -521,6 +521,26 @@ double strtod(const char *__restrict nptr, char **__restrict endptr)
>  #endif
>  }
>  
> +/**
> + * \fn R floatingToFixedPoint(T number)
> + * \brief Convert a floating point number to a fixed-point representation
> + * \tparam I Bit width of the integer part of the fixed-point
> + * \tparam F Bit width of the fractional part of the fixed-point
> + * \tparam R Return type of the fixed-point representation
> + * \tparam T Input type of the floating point representation
> + * \return The converted value
> + */
> +
> +/**
> + * \fn R fixedToFloatingPoint(T number)
> + * \brief Convert a fixed-point number to a floating point representation
> + * \tparam I Bit width of the integer part of the fixed-point
> + * \tparam F Bit width of the fractional part of the fixed-point
> + * \tparam R Return type of the floating point representation
> + * \tparam T Input type of the fixed-point representation
> + * \return The converted value
> + */
> +
>  } /* namespace utils */
>  
>  #ifndef __DOXYGEN__
> diff --git a/test/utils.cpp b/test/utils.cpp
> index fc56e14e..f1805207 100644
> --- a/test/utils.cpp
> +++ b/test/utils.cpp
> @@ -170,6 +170,51 @@ protected:
>  		return TestPass;
>  	}
>  
> +	template<unsigned int intPrec, unsigned fracPrec, typename T>
> +	int testSingleFixedPoint(double input, T expected)
> +	{
> +		T ret = utils::floatingToFixedPoint<intPrec, fracPrec, T>(input);
> +		if (ret != expected) {
> +			cerr << "Expected " << input << " to convert to "
> +			     << expected << ", got " << ret << std::endl;
> +			return TestFail;
> +		}
> +
> +		/*
> +		 * The precision check is fairly arbitrary but is based on what
> +		 * the rkisp1 is capable of in the crosstalk module.
> +		 */
> +		double f = utils::fixedToFloatingPoint<intPrec, fracPrec, double>(ret);
> +		if (std::abs(f - input) > 0.001) {
> +			cerr << "Reverse conversion expected " << ret
> +			     << " to convert to " << input
> +			     << ", got " << f << std::endl;
> +			return TestFail;
> +		}
> +
> +		return TestPass;
> +	}
> +
> +	int testFixedPoint()
> +	{
> +		/* These are the only cases that we know for certain */
> +		std::map<double, uint16_t> testCases = {
> +			{ 7.992, 0x3FF },
> +			{ -8, 0x400 },
> +			{ 0, 0 },
> +		};
> +
> +		int ret;
> +		for (const auto &testCase : testCases) {
> +			ret = testSingleFixedPoint<4, 7, uint16_t>(testCase.first,
> +								   testCase.second);
> +			if (ret != TestPass)
> +				return ret;
> +		}
> +
> +		return TestPass;
> +	}
> +
>  	int run()
>  	{
>  		/* utils::hex() test. */
> @@ -290,6 +335,10 @@ protected:
>  		if (testDuration() != TestPass)
>  			return TestFail;
>  
> +		/* fixed point conversion test */
> +		if (testFixedPoint() != TestPass)
> +			return TestFail;
> +
>  		return TestPass;
>  	}
>  };

-- 
Regards,

Laurent Pinchart