basegfx/source/color/bcolormodifier.cxx | 30 +++++++++++-------------------
basegfx/test/BColorModifierTest.cxx | 21 +++++++++++++++++++++
include/basegfx/color/bcolormodifier.hxx | 2 +-
3 files changed, 33 insertions(+), 20 deletions(-)
New commits:
commit c03d752c614b3b19ed89a510d7791bc46604a452
Author: Noel Grandin <noel.gran...@collabora.co.uk>
AuthorDate: Fri Apr 25 13:49:14 2025 +0200
Commit: Noel Grandin <noel.gran...@collabora.co.uk>
CommitDate: Fri Apr 25 17:47:09 2025 +0200
improve fast_pow in BColorModifier_gamma
And add tests to demonstrate accuracy.
This is taken from
https://martin.ankerl.com/2007/10/04/optimized-pow-approximation-for-java-and-c-c/
and results in a maximum error of 3% over our input range.
Change-Id: I1568dabb552d52a3eff085fd327ed3c1c246edcc
Reviewed-on: https://gerrit.libreoffice.org/c/core/+/184623
Reviewed-by: Noel Grandin <noel.gran...@collabora.co.uk>
Tested-by: Jenkins
diff --git a/basegfx/source/color/bcolormodifier.cxx
b/basegfx/source/color/bcolormodifier.cxx
index 360a41db9285..44da250acdbf 100644
--- a/basegfx/source/color/bcolormodifier.cxx
+++ b/basegfx/source/color/bcolormodifier.cxx
@@ -18,6 +18,7 @@
*/
#include <sal/config.h>
+#include <sal/mathconf.h>
#include <algorithm>
#include <float.h>
#include <basegfx/color/bcolormodifier.hxx>
@@ -325,28 +326,19 @@ namespace basegfx
}
/**
- A fast and approximate std::pow(), good enough for gamma calculations.
-
- std::pow() is basically implemented using log's:
- pow(a,b) = x^(logx(a) * b)
- So we need a fast log and fast exponent - it doesn't matter what x is so
we use 2.
- pow(a,b) = 2^(log2(a) * b)
- The trick is that a floating point number is already in a log style
format:
- a = M * 2^E
- Taking the log of both sides gives:
- log2(a) = log2(M) + E
- or more simply:
- log2(a) ~= E
- In other words if we take the floating point representation of a number,
- and extract the Exponent we've got something that's a good starting
point as its log.
- And then we can do:
- pow(a,b) = 2^(E * b)
+ A fast and approximate std::pow(), good enough for gamma calculations,
+ given that the parameter a is in the range [0,1), and b is in the range
(0, 0.1]
+
+ Google for "optimised power approximation". The below function is the
result
+ of reducing various bit-twiddling tricks into fewer operations.
*/
static double fast_pow(double a, double b)
{
- int a_exp;
- std::frexp(a, &a_exp);
- return std::exp2(a_exp * b);
+ sal_math_Double u;
+ u.value = a;
+ u.w32_parts.msw = static_cast<int>(b *
(static_cast<int>(u.w32_parts.msw) - 1072632447) + 1072632447);
+ u.w32_parts.lsw = 0;
+ return u.value;
}
::basegfx::BColor BColorModifier_gamma::getModifiedColor(const
::basegfx::BColor& aSourceColor) const
diff --git a/basegfx/test/BColorModifierTest.cxx
b/basegfx/test/BColorModifierTest.cxx
old mode 100755
new mode 100644
index 4a84c3662a6b..12c5af660138
--- a/basegfx/test/BColorModifierTest.cxx
+++ b/basegfx/test/BColorModifierTest.cxx
@@ -395,6 +395,26 @@ public:
CPPUNIT_ASSERT(aBColorModifier->operator==(*aBColorModifier2));
}
+ // Verify that our shortcut gamma calculation produces reasonably accurate
results
+ void testGamma()
+ {
+ for (int i = 1; i < 10; i++)
+ {
+ BColorModifier_gamma g(i);
+ for (int j = 0; j < 100; j++)
+ {
+ double inputRed = j / 100.0;
+ // this is the "slow but correct" gamma calculation
+ double expectedOutputRed = std::pow(inputRed, 1 / double(i));
+ BColor col = g.getModifiedColor(BColor(inputRed, 0, 0));
+ auto msg = OString("col is " + OString::number(inputRed) + "
and gamma is "
+ + OString::number(i));
+ CPPUNIT_ASSERT_DOUBLES_EQUAL_MESSAGE(msg.getStr(),
expectedOutputRed, col.getRed(),
+ 0.029);
+ }
+ }
+ }
+
CPPUNIT_TEST_SUITE(bcolormodifier);
CPPUNIT_TEST(testGray);
CPPUNIT_TEST(testInvert);
@@ -407,6 +427,7 @@ public:
CPPUNIT_TEST(testMatrixShift);
CPPUNIT_TEST(testIdentityMatrix);
CPPUNIT_TEST(testBlackAndWhite);
+ CPPUNIT_TEST(testGamma);
CPPUNIT_TEST_SUITE_END();
};
diff --git a/include/basegfx/color/bcolormodifier.hxx
b/include/basegfx/color/bcolormodifier.hxx
index e74d6b43726e..f6052067767f 100644
--- a/include/basegfx/color/bcolormodifier.hxx
+++ b/include/basegfx/color/bcolormodifier.hxx
@@ -351,7 +351,7 @@ namespace basegfx
col(r,g,b) = clamp(pow(col(r,g,b), 1.0 / gamma), 0.0, 1.0)
*/
- class SAL_WARN_UNUSED UNLESS_MERGELIBS(BASEGFX_DLLPUBLIC)
BColorModifier_gamma final : public BColorModifier
+ class SAL_WARN_UNUSED BASEGFX_DLLPUBLIC BColorModifier_gamma final :
public BColorModifier
{
private:
double mfValue;
