Some cmake stuff, and a cheap difference

.github/workflows/macos.yml

@@ -1,6 +1,6 @@
 name: macOS
 
-on: [push]
+on: [push, pull_request]
 
 jobs:
   build-test:
@@ -18,4 +18,4 @@ jobs:
     - name: make
       run: make
     - name: ctest
-      run: ctest
+      run: ctest --output-on-failure

.github/workflows/ubuntu.yml

@@ -1,6 +1,6 @@
 name: Ubuntu
 
-on: [push]
+on: [push, pull_request]
 
 jobs:
   build-test:
@@ -22,4 +22,4 @@ jobs:
     - name: make
       run: make
     - name: ctest
-      run: ctest
+      run: ctest --output-on-failure

.gitignore

@@ -0,0 +1 @@
+build

CMakeLists.txt

@@ -3,11 +3,13 @@ cmake_minimum_required(VERSION 3.1)
 project(color-util CXX)
 set(CMAKE_CXX_STANDARD 11)
 
-find_package(Eigen3 REQUIRED)
+find_package(Eigen3)
 if((NOT TARGET Eigen3::Eigen) AND (DEFINED EIGEN3_INCLUDE_DIR))
 	add_library(AliasEigen3 INTERFACE)
 	target_include_directories(AliasEigen3 INTERFACE ${EIGEN3_INCLUDE_DIR})
 	add_library(Eigen3::Eigen ALIAS AliasEigen3)
+else()
+	message(FATAL_ERROR "Cannot find Eigen3 and EIGEN3_INCLUDE_DIR is undefined")
 endif()
 
 file(GLOB HEADERS ${CMAKE_CURRENT_SOURCE_DIR}/include/color-util/*.hpp)

README.md

@@ -19,6 +19,12 @@ A header-only C++11 library for handling colors, including color space converter
 
 ## Color Difference Calculators
 
+### Redmean
+
+Given two sRGB colors, `calculate_redmean` calculates a difference between these two colors based on an [informal metric](https://www.compuphase.com/cmetric.htm). The function is defined in `color-util/redmean.hpp`.
+
+This simple metric is more perceptually uniform than an Euclidean distance in sRGB space, but is otherwise a bad choice.
+
 ### CIE76
 
 Given two CIELAB colors, `calculate_CIE76` calculates a *perceptual* difference between these two colors based on a metric called CIE76. This function is defined in `color-util/CIE76.hpp`.

include/color-util/redmean.hpp

@@ -0,0 +1,24 @@
+/// \file redmean.hpp
+
+#ifndef COLORUTIL_REDMEAN_HPP
+#define COLORUTIL_REDMEAN_HPP
+
+#include <cmath>
+#include <color-util/type.hpp>
+
+namespace colorutil
+{
+    /// \brief  Calculate a cheap color difference based on redmean.
+    /// \param  color_1  The first color. This should be expressed in sRGB color space.
+    /// \param  color_2  The second color. This should be expressed in sRGB color space.
+    /// \return The color difference of the two colors.
+    inline double calculate_redmean(const RGB& color_1, const RGB& color_1)
+    {
+        double redmean = (color_1(0) + color_2(0)) * 0.5;
+        Eigen::Vector3d diff    = color_1 - color_2;
+        diff = diff.cwiseProduct(Eigen::Map<const Eigen::Vector3d>(diff));
+        return sqrt((2 + redmean) * diff(0) + 4 * diff(1) + (2 - redmean) * diff(2));
+    }
+} // namespace colorutil
+
+#endif // COLORUTIL_REDMEAN_HPP