Fix conversion from RGB to signed CMYK

include/boost/gil/color_convert.hpp

@@ -150,47 +150,50 @@ struct default_color_converter_impl<rgb_t,gray_t> {
 /// by Burger, Wilhelm, Burge, Mark J.
 /// and it is a gross approximation not precise enough for professional work.
 ///
-/// \todo FIXME: The original implementation did not handle properly signed CMYK pixels as destination
-///
 template <>
 struct default_color_converter_impl<rgb_t, cmyk_t>
 {
     template <typename SrcPixel, typename DstPixel>
     void operator()(SrcPixel const& src, DstPixel& dst) const
     {
         using src_t = typename channel_type<SrcPixel>::type;
-        src_t const r = get_color(src, red_t());  
+        src_t const r = get_color(src, red_t());
         src_t const g = get_color(src, green_t());
         src_t const b = get_color(src, blue_t());
 
-        using dst_t   = typename channel_type<DstPixel>::type;
-        dst_t const c = channel_invert(channel_convert<dst_t>(r)); // c = 1 - r
-        dst_t const m = channel_invert(channel_convert<dst_t>(g)); // m = 1 - g
-        dst_t const y = channel_invert(channel_convert<dst_t>(b)); // y = 1 - b
-        dst_t const k = (std::min)(c, (std::min)(m, y));           // k = minimum(c, m, y)
+        using uint_t = typename channel_type<cmyk8_pixel_t>::type;
+        uint_t c = channel_invert(channel_convert<uint_t>(r)); // c = 1 - r
+        uint_t m = channel_invert(channel_convert<uint_t>(g)); // m = 1 - g
+        uint_t y = channel_invert(channel_convert<uint_t>(b)); // y = 1 - b
+        uint_t k = (std::min)(c,(std::min)(m,y));              // k = minimum(c, m, y)
 
         // Apply color correction, strengthening, reducing non-zero components by
         // s = 1 / (1 - k) for k < 1, where 1 denotes dst_t max, otherwise s = 1 (literal).
-        dst_t const dst_max = channel_traits<dst_t>::max_value();
-        dst_t const s_div   = dst_max - k;
+        uint_t const dst_max = channel_traits<uint_t>::max_value();
+        uint_t const s_div   = dst_max - k;
         if (s_div != 0)
         {
-            double const s              = dst_max / static_cast<double>(s_div);
-            get_color(dst, cyan_t())    = static_cast<dst_t>((c - k) * s);
-            get_color(dst, magenta_t()) = static_cast<dst_t>((m - k) * s);
-            get_color(dst, yellow_t())  = static_cast<dst_t>((y - k) * s);
+            double const s = dst_max / static_cast<double>(s_div);
+            c = (c - k) * s;
+            m = (m - k) * s;
+            y = (y - k) * s;
         }
         else
         {
             // Black only for k = 1 (max of dst_t)
-            get_color(dst, cyan_t())    = channel_traits<dst_t>::min_value();
-            get_color(dst, magenta_t()) = channel_traits<dst_t>::min_value();
-            get_color(dst, yellow_t())  = channel_traits<dst_t>::min_value();
+            c = channel_traits<uint_t>::min_value();
+            m = channel_traits<uint_t>::min_value();
+            y = channel_traits<uint_t>::min_value();
         }
-        get_color(dst, black_t()) = k; 
+        using dst_t   = typename channel_type<DstPixel>::type;
+        get_color(dst, cyan_t())    = channel_convert<dst_t>(c);
+        get_color(dst, magenta_t()) = channel_convert<dst_t>(m);
+        get_color(dst, yellow_t())  = channel_convert<dst_t>(y);
+        get_color(dst, black_t())   = channel_convert<dst_t>(k);
     }
 };
 
+
 /// \ingroup ColorConvert
 /// \brief CMYK to RGB (not the fastest code in the world)
 ///

test/core/color/CMakeLists.txt

@@ -8,7 +8,8 @@
 foreach(_name
   concepts
   color_spaces_are_compatible
-  default_color_converter_impl)
+  default_color_converter_impl
+  default_color_converter_rgb_to_cmyk)
   set(_test t_core_color_${_name})
   set(_target test_core_color_${_name})
 

test/core/color/Jamfile

@@ -13,3 +13,4 @@ compile color_spaces_are_compatible.cpp ;
 compile-fail default_color_converter_impl_fail.cpp ;
 
 run default_color_converter_impl.cpp ;
+run default_color_converter_rgb_to_cmyk.cpp ;