You are an expert in writing code in many different languages. Your goal is to perform code completion for the following code, keeping in mind the rest of the code and the file meta data.
    Metadata details:
    filename: QfTree.m
    path of file: ./repos/CanlabCore/CanlabCore/External/umap/util
    
    the code of the file until where you have to start completion: %   Class for  building a dendrogram based on QF dissimilarity measurements
%
%   AUTHORSHIP
%   Primary Developer: Stephen Meehan <swmeehan@stanford.edu> 
%   Math Lead & Secondary 
    
function createImg(this, idxs, ask, which)
            if nargin<4
                which=0;
                if nargin<3
                    ask=true;
                    if nargin<2
                        idxs=[];
                    end
                end
            end
            missing=this.countMissingPngs(idxs, false, which);
            nMissing=length(missing);
            if nMissing>1 && ask
                str=String.Pluralize2('missing low res 1D <br>PathFinder image',...
                    nMissing);
                if ~askYesOrNo(Html.WrapC(['Create the ' str '?']), ...
                        str, 'south', true, 'missingLowRes1DPF')
                    return;
                end