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Fix DE_94 & DE_CMC color difference metrics #461

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May 8, 2021
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Fix DE_94 & DE_CMC color difference metrics #461

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82 changes: 44 additions & 38 deletions src/differences.jl
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Original file line number Diff line number Diff line change
Expand Up @@ -10,34 +10,52 @@ struct DE_2000 <: DifferenceMetric
kl::Float64
kc::Float64
kh::Float64
DE_2000(kl=1, kc=1, kh=1) = new(kl, kc, kh)
end
"""
DE_2000()
DE_2000(kl, kc, kh)
DE_2000(kl=1, kc=1, kh=1)

Construct a metric of the CIE Delta E 2000 recommendation, with weighting
parameters `kl`, `kc` and `kh` as provided for in the recommendation. When not
provided, these parameters default to `1`.
"""
DE_2000() = DE_2000(1,1,1)
DE_2000()


struct DE_94 <: DifferenceMetric
kl::Float64
kc::Float64
kh::Float64
k1::Float64
k2::Float64
DE_94(kl=1, kc=1, kh=1, k1=0.045, k2=0.015) = new(kl, kc, kh, k1, k2)
end
"""
DE_94()
DE_94(kl, kc, kh)
DE_94(kl=1, kc=1, kh=1, k1=0.045, k2=0.015)

Construct a metric of CIE Delta E 94 recommendation (1994), with weighting
parameters `kl`, `kc` and `kh` as provided for in the recommendation. When not
provided, these parameters default to `1`.
parameters `kl`, `kc`, `kh`, `k1`, and `k2` as provided for in the
recommendation. The `kl`, `k1`, and `k2` depend on the application:

| |Graphic Arts|Textiles|
|:--:|:-----------|:-------|
|`kl`|`1` |`2` |
|`k1`|`0.045` |`0.048` |
|`k2`|`0.015` |`0.014` |

and the default values are for graphic arts. The `kc` and `kh` default to `1`.

The `DE_94` is more perceptually uniform than the [`DE_AB`](@ref), but has some
non-uniformities resolved by the [`DE_2000`](@ref).

!!! note
The `DE_94` is a quasimetric, i.e. violates symmetry. Therefore,
`colordiff(a, b, metric=DE_94())` may not equal to
`colordiff(b, a, metric=DE_94())`.
The first argument of `colordiff` is taken as the reference (standard)
color.
"""
DE_94() = DE_94(1,1,1)
DE_94()


struct DE_JPC79 <: DifferenceMetric
Expand All @@ -55,38 +73,39 @@ DE_JPC79()
struct DE_CMC <: DifferenceMetric
kl::Float64
kc::Float64
DE_CMC(kl=1, kc=1) = new(kl, kc)
end
"""
DE_CMC()
DE_CMC(kl, kc)
DE_CMC(kl=1, kc=1)

Construct a metric using the CMC equation (CMC l:c), with weighting parameters
`kl` and `kc`. When not provided, these parameters default to `1`.

!!! note
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)
DE_CMC()


struct DE_BFD <: DifferenceMetric
wp::XYZ{Float64}
kl::Float64
kc::Float64
DE_BFD(wp::XYZ, kl=1, kc=1) = new(wp, kl, kc)
end
"""
DE_BFD()
DE_BFD([wp,] kl, kc)
DE_BFD([wp,] kl=1, kc=1)

Construct a metric using the BFD equation, with weighting parameters `kl` and
`kc`. Additionally, a whitepoint `wp` can be specified, because the BFD equation
must convert between `XYZ` and `Lab` during the computation. When not provided,
`kl` and `kc` default to `1`, and `wp` defaults to CIE D65 (`Colors.WP_D65`).
"""
DE_BFD() = DE_BFD(WP_DEFAULT,1,1)
DE_BFD(kl, kc) = DE_BFD(WP_DEFAULT,kl, kc)

DE_BFD(kl=1, kc=1) = DE_BFD(WP_DEFAULT, kl, kc)

struct DE_AB <: EuclideanDifferenceMetric{Lab}

Expand Down Expand Up @@ -244,13 +263,10 @@ function _colordiff(ai::Color, bi::Color, m::DE_94)
# Calculate H*
dh = 2 * sqrt(a.c * b.c) * sind(dh/2)

# Calculate geometric mean of chroma
mc = sqrt(a.c*b.c)

# Lightness, hue, chroma correction terms
sl = 1
sc = 1+0.045*mc
sh = 1+0.015*mc
sc = 1 + m.k1 * a.c
sh = 1 + m.k2 * a.c

sqrt((dl/(m.kl*sl))^2 + (dc/(m.kc*sc))^2 + (dh/(m.kh*sh))^2)

Expand Down Expand Up @@ -325,32 +341,22 @@ 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
Expand All @@ -365,7 +371,7 @@ end
function _colordiff(ai::Color, bi::Color, m::DE_BFD)
# Currently, support for the `wp` argument of `convert` is limited.
function to_xyz(c::Color, wp)
c isa XYZ && return ai
c isa XYZ && return c
Comment on lines -368 to +374
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😭

I am not going to backport this to v0.12. As a workaround, if the second argument is XYZ, please convert it to something other than XYZ (e.g. xyY).

(c isa xyY || c isa LMS) && return convert(XYZ, c)
(c isa Lab || c isa Luv) && return convert(XYZ, c, wp)
c isa LCHuv && return convert(XYZ, convert(Luv, c), wp)
Expand Down
14 changes: 10 additions & 4 deletions test/colordiff.jl
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Original file line number Diff line number Diff line change
@@ -1,4 +1,4 @@
using Colors, Test
using Colors, Test, FixedPointNumbers

# Test the colordiff function against example input and output from:
#
Expand Down Expand Up @@ -61,6 +61,7 @@ using Colors, Test
jl_purple = Lab{Float32}(Colors.JULIA_LOGO_COLORS.purple)
colors = (jl_red, jl_green, jl_blue, jl_purple)
pairs = ((a, b) for a in colors for b in Iterators.filter(c -> c != a,colors))
tri = (colorant"navy", colorant"peru", colorant"gold")

@testset "properties of metrics" begin
metrics = (DE_2000(), DE_94(), DE_JPC79(), DE_CMC(), DE_BFD(),
Expand All @@ -72,17 +73,22 @@ using Colors, Test
# positivity
@test all(p -> colordiff(p[1], p[2]; metric=metric) > 0, pairs)
# symmetry
if metric isa DE_CMC # quasimetric
# TODO: add test
if metric isa DE_94 || metric isa DE_CMC # quasimetric
@test all(p -> abs(colordiff(p[1], p[2]; metric=metric) -
colordiff(p[2], p[1]; metric=metric)) > eps_cdiff, pairs)
else
@test all(p -> abs(colordiff(p[1], p[2]; metric=metric) -
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)
if any(m -> metric isa m, (DE_2000, DE_94, DE_CMC)) # semimetric
@test colordiff(tri[1], tri[3]; metric=metric) >
colordiff(tri[1], tri[2]; metric=metric) +
colordiff(tri[2], tri[3]; metric=metric)
end
end
end

Expand Down