basegfx/source/color/bcolormodifier.cxx | 31 ++++++++++++++++++++++++++++---
1 file changed, 28 insertions(+), 3 deletions(-)
New commits:
commit 23e158e4c35a9f548010827adfccccd70d1928f6
Author: Noel Grandin <noelgran...@gmail.com>
AuthorDate: Fri Apr 25 08:06:05 2025 +0200
Commit: Noel Grandin <noelgran...@gmail.com>
CommitDate: Fri Apr 25 09:35:36 2025 +0200
tdf#165595 speedup BColorModifier_gamma
which shows up heavily on this profile. In this case we don't
need perfection and we can use a fast approximation for std::pow.
Thanks to the binary representation of float numbers, this new
code reduces to some basic bit-twiddling.
Change-Id: I064588f65cf8f5973ba45545fba9c5fe66500e37
Reviewed-on: https://gerrit.libreoffice.org/c/core/+/184602
Tested-by: Jenkins
Reviewed-by: Noel Grandin <noel.gran...@collabora.co.uk>
diff --git a/basegfx/source/color/bcolormodifier.cxx
b/basegfx/source/color/bcolormodifier.cxx
index e0e0ebbd6203..360a41db9285 100644
--- a/basegfx/source/color/bcolormodifier.cxx
+++ b/basegfx/source/color/bcolormodifier.cxx
@@ -324,14 +324,39 @@ namespace basegfx
return mfValue == pCompare->mfValue;
}
+ /**
+ 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)
+ */
+ static double fast_pow(double a, double b)
+ {
+ int a_exp;
+ std::frexp(a, &a_exp);
+ return std::exp2(a_exp * b);
+ }
+
::basegfx::BColor BColorModifier_gamma::getModifiedColor(const
::basegfx::BColor& aSourceColor) const
{
if(mbUseIt)
{
::basegfx::BColor aRetval(
- pow(aSourceColor.getRed(), mfInvValue),
- pow(aSourceColor.getGreen(), mfInvValue),
- pow(aSourceColor.getBlue(), mfInvValue));
+ fast_pow(aSourceColor.getRed(), mfInvValue),
+ fast_pow(aSourceColor.getGreen(), mfInvValue),
+ fast_pow(aSourceColor.getBlue(), mfInvValue));
aRetval.clamp();
return aRetval;
