Hue-Preserving Color Blending
Additional Results

Johnson Chuang, Daniel Weiskopf, Torsten Möller

Additional image comparisons from blending in different color spaces:

	Traditional blending	Hue-preserving blending in HSL	Hue-preserving blending in LCH (i.e. CIELAB)	Hue-preserving blending in CIE XYZ
Tomato
Tooth
Frog
Engine

Blending two RGB colors C1 and C2 in HSL:

HSLtrad = rgb2hsl(C1 + C2)
HSL1 = rgb2hsl(C1)
HSL2 = rgb2hsl(C2)
if(sameHue(H1, H2)) then
    Cnew = C1 + C2
else   
    if(S1 > S2) then // C1 is dominant
        H3 = H1 + 180
        S3 = S2
        L3 = L2
        Cnew = C1 + hsl2rgb(H3, S3, L3)
    else // C2 is dominant
        H3 = H2 + 180
        S3 = S1
        L3 = L1
        Cnew = hsl2rgb(H3, S3, L3) + C2
    endif

endif

Blending two RGB colors C1 and C2 in LCH:

LCHtrad = rgb2lch(C1 + C2)
LCH1 = rgb2lch(C1)
LCH2 = rgb2lch(C2)
if(sameHue(H1, H2)) then
    Cnew = C1 + C2
else
    if(C1 > C2) then // C1 is dominant
        C3 = C1-C2
        H3 = H1
    else // C2 is dominant
        C3 = C2-C1
        H3 = H2
    endif

    Cnew = lch2rgb(Ltrad, C3, H3)
endif

Blending two RGB colors C1 and C2 in XYZ:

HSLtrad = rgb2hsl(C1 + C2)
XYZ1 = rgb2xyz(C1)
XYZ2 = rgb2xyz(C2)
HSL1 = rgb2hsl(C1)
HSL2 = rgb2hsl(C2)
if(sameHue(H1, H2)) then
    Cnew = C1 + C2
else
    if(H1 closer to Htrad) then // C1 is dominant
        XYZnew = XYZ1 + rgb2xyz(hsl2rgb(H1 + 180, S2, L2))
    else // H2 closer to Htrad, and C2 is dominant
        XYZnew = XYZ2 + rgb2xyz(hsl2rgb(H2 + 180, S1, L1))
    endif

    Cnew = xyz2rgb(XYZnew)
endif

Optional: For all the above blending methods, the luminance of the final blended color can be made equal to the luminance of the traditionally-blended color by doing the following computations in CIELAB.

L*A*B*trad = rgb2lab(C1 + C2)
L*A*B*new = rgb2lab(Cnew)
Cnew = lab2rgb(L*trad, Anew, Bnew)

Last updated: July 27, 2009