vectorization in context of if-condition and comparison for image array

I want to implement some image compositing methods in Matlab like for example ModulusAdd defined as

if (Sca + Dca) <= 1
    Dca = Sca + Dca;
else
    Dca = (Sca + Dca) - 1;
end

Here Sca, Dca, Sa, Da are (h,w,1) arrays (gray scale images respective alpha channels) all as double (see also http://www.w3.org/TR/2014/CR-compositing-1-20140220/ for image compositing details).

Comparisons with the if-condition are defined for scalar values so using the above code in Matlab is not working directly (there are also other compositing methods that relay on other if-conditions like "if Sca == 0 && Dca == Da ..." that can not directly be ported so answering this question might also be helpful for those cases). Naively one has to deal with the pixel level and its two nested for-loops:

for i=1:h
    for j=1:w
        if (Sca(i,j) + Dca(i,j)) <= 1
            Dca(i,j) = Sca(i,j) + Dca(i,j);
        else
            Dca(i,j) = (Sca(i,j) + Dca(i,j)) - 1;
        end    
    end
end

How can I redesign this to make it work faster with an (h,w,1) (or (h,w,3)) array? Perhaps a redesign beyond arrayfun is possible (being not an expert in this):

Dca = arrayfun(@ModulusAdd_scalar, Sca, Dca);

function d = ModulusAdd_scalar(s, d)
    if (s + d) <= 1
        d = s + d;
    else
        d = (s + d) - 1;
    end   
end