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: acfTrain.m
    path of file: ./repos/MCITF/Code/MCITF/EdgeBox/piotr_toolbox/toolbox/detector
    
    the code of the file until where you have to start completion: function detector = acfTrain( varargin )
% Train aggregate channel features object detector.
%
% Train aggregate 
    
function Is = sampleWins1( I, gt, detector, stage, positive )
% Sample windows from I given its ground truth gt.
opts=detector.opts; shrink=opts.pPyramid.pChns.shrink;
modelDs=opts.modelDs; modelDsPad=opts.modelDsPad;
if( positive ), bbs=gt; bbs=bbs(bbs(:,5)==0,:); else
  if( stage==0 )
    % generate candidate bounding boxes in a grid
    [h,w,~]=size(I); h1=modelDs(1); w1=modelDs(2);
    n=opts.nPerNeg; ny=sqrt(n*h/w); nx=n/ny; ny=ceil(ny); nx=ceil(nx);
    [xs,ys]=meshgrid(linspace(1,w-w1,nx),linspace(1,h-h1,ny));
    bbs=[xs(:) ys(:)]; bbs(:,3)=w1; bbs(:,4)=h1; bbs=bbs(1:n,:);
  else
    % run detector to generate candidate bounding boxes
    bbs=acfDetect(I,detector); [~,ord]=sort(bbs(:,5),'descend