Fix DE_CMC color difference metric

This uses the first argument of `colordiff` as the reference color.

src/differences.jl

@@ -66,6 +66,8 @@ Construct a metric using the CMC equation (CMC l:c), with weighting parameters
     The `DE_CMC` is a quasimetric, i.e. violates symmetry. Therefore,
     `colordiff(a, b, metric=DE_CMC())` may not equal to
     `colordiff(b, a, metric=DE_CMC())`.
+    The first argument of `colordiff` is taken as the reference (standard)
+    color.
 """
 DE_CMC() = DE_CMC(1,1)
 
@@ -325,32 +327,22 @@ function _colordiff(ai::Color, bi::Color, m::DE_CMC)
     # Calculate H* from C*'s and h
     dh = 2 * sqrt(a.c * b.c) * sind(dh/2)
 
-    # Find the mean value of the inputs to use as the "standard"
-    ml, mc = (a.l + b.l)/2, (a.c + b.c)/2 # TODO: use `a.l` and `a.c` instead
-
-    # Calculate mean hue value
-    if a.c * b.c == 0
-        mh = a.h + b.h
-    else
-        mh = mean_hue(a.h, b.h)
-    end
-
     # L* adjustment term
     if (a.l <= 16)
         sl = 0.511
     else
-        sl = 0.040975*ml/(1+0.01765*ml)
+        sl = 0.040975 * a.l / (1 + 0.01765 * a.l)
     end
 
     # C* adjustment term
-    sc = 0.0638*mc/(1+0.0131*mc)+0.638
+    sc = 0.0638 * a.c / (1 + 0.0131 * a.c) + 0.638
 
-    f = sqrt((mc^4)/(mc^4 + 1900))
+    f = sqrt(a.c^4 / (a.c^4 + 1900))
 
-    if (mh >= 164 && mh < 345)
-        t = 0.56 + abs(0.2*cosd(mh+168))
+    if 164 <= a.h <= 345
+        t = 0.56 + abs(0.2 * cosd(a.h + 168))
     else
-        t = 0.36 + abs(0.4*cosd(mh+35))
+        t = 0.36 + abs(0.4 * cosd(a.h + 35))
     end
 
     # H* adjustment term

test/colordiff.jl

@@ -1,4 +1,4 @@
-using Colors, Test
+using Colors, Test, FixedPointNumbers
 
 # Test the colordiff function against example input and output from:
 #
@@ -79,7 +79,6 @@ using Colors, Test
                                    colordiff(p[2], p[1]; metric=metric)) < eps_cdiff, pairs)
             end
             # triangle inequality
-            metric isa DE_CMC && continue # FIXME
             @test all(p -> colordiff(p[1],   p[2]; metric=metric) <=
                            colordiff(p[1], jl_red; metric=metric) +
                            colordiff(jl_red, p[2]; metric=metric), pairs)