ipa: rkisp1: Add a helper to convert floating-point to fixed-point

Add helper functions for converting between floating point and fixed
point numbers. Also add tests for them.

Signed-off-by: Paul Elder <paul.elder@ideasonboard.com>
Reviewed-by: Stefan Klug <stefan.klug@ideasonboard.com>
Reviewed-by: Kieran Bingham <kieran.bingham@ideasonboard.com>

1 files changed, 66 insertions, 0 deletions

diff --git a/src/ipa/rkisp1/utils.h b/src/ipa/rkisp1/utils.h
new file mode 100644
index 00000000..450f2244
--- /dev/null
+++ b/src/ipa/rkisp1/utils.h
@@ -0,0 +1,66 @@
+/* SPDX-License-Identifier: LGPL-2.1-or-later */
+/*
+ * Copyright (C) 2024, Paul Elder <paul.elder@ideasonboard.com>
+ *
+ * Miscellaneous utility functions specific to rkisp1
+ */
+
+#pragma once
+
+#include <cmath>
+#include <limits>
+#include <type_traits>
+
+namespace libcamera {
+
+namespace ipa::rkisp1::utils {
+
+#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(sizeof(int) >= sizeof(R));
+	static_assert(I + F <= sizeof(R) * 8);
+
+	/*
+	 * The intermediate cast to int is needed on arm platforms to properly
+	 * cast negative values. See
+	 * https://embeddeduse.com/2013/08/25/casting-a-negative-float-to-an-unsigned-int/
+	 */
+	R mask = (1 << (F + I)) - 1;
+	R frac = static_cast<R>(static_cast<int>(std::round(number * (1 << F)))) & mask;
+
+	return 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(sizeof(int) >= sizeof(T));
+	static_assert(I + F <= sizeof(T) * 8);
+
+	/*
+	 * Recreate the upper bits in case of a negative number by shifting the sign
+	 * bit from the fixed point to the first bit of the unsigned and then right shifting
+	 * by the same amount which keeps the sign bit in place.
+	 * This can be optimized by the compiler quite well.
+	 */
+	int remaining_bits = sizeof(int) * 8 - (I + F);
+	int t = static_cast<int>(static_cast<unsigned>(number) << remaining_bits) >> remaining_bits;
+	return static_cast<R>(t) / static_cast<R>(1 << F);
+}
+
+} /* namespace ipa::rkisp1::utils */
+
+} /* namespace libcamera */