Slover: Controling the alpha (transparency) of a "split" type image

[seralouk]

I have a brain map with both positive and negative values and I would like to use slover to display it on a structural image.

My idea is to build a 3-layers slover object (layer 1: background, layer 2 & 3 will be the same file).
Layer 2 will be displayed with "hot" colors and layer 3 with "winter".

How can I control the transparency of layer 2 and 3? Both are of type "split" because "truecolour" type will display them too transparent...

My code looks like this:

obj.img(1).vol=spm_vol(structFile); 
obj.img(2).vol=spm_vol(NiftiFile);
obj.img(3).vol=spm_vol(NiftiFile);

[mx1 mn1] = slover('volmaxmin', obj.img(1).vol);
[mx mn] = slover('volmaxmin', obj.img(2).vol);

% thresholds for layer 2 and 3
lowerthreshold = 0.05; 
upperthreshold = max(abs(mx) , abs(mn)) / 2;  % custom max value for nice visualizations

% define color map range
obj.img(1).range = [0 mx1]; % do not display negatives (if any)
obj.img(2).range=[lowerthreshold, upperthreshold];    % make symmetric
obj.img(3).range=[-lowerthreshold, -upperthreshold];  % make symmetric

% define color maps
obj.img(2).cmap='hot';
obj.img(3).cmap='winter'; 

% transparency and type
obj.img(1).type = 'truecolour';
obj.img(2).type = 'split';
obj.img(3).type = 'split';

paint(obj);

Results using the above code:

Results after setting obj.img(2).type = 'truecolour'; obj.img(3).type = 'truecolour'; obj.img(2).prop = 1; obj.img(3).prop = 1;:

Desired output (like this):