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"Roger Stafford" <ellieandrogerxyzzy@mindspring.com.invalid>
wrote in message <fvteb8$q31$1@fred.mathworks.com>...
> "jay vaughan" <jvaughan5.nospam@gmail.com> wrote in
message <fvspfn
> $m20$1@fred.mathworks.com>...
> > Hi,
> >
> > I am trying to optimize the speed of drawing a bunch of
> > bounding boxes on identified objects. I know to try to avoid
> > loops, but wasn't sure how to vectorize the code (below).
> > Any ideas on vectorizing this process or speeding it up in
> > general? Eventually it will be done many times per second,
> > so I do need the speed.
> >
> > A simulation of this below where the first part generates
> > the coordinates the objects, and the second part is a loop
> > that creates the boxes.
> >
> > % FIRST PART, CREATE (RANDOM) BOX COORDINTES
> > num_boxes= 1000;
> > box_size= 13;
> > frame_size = [512 512];
> >
> > rows = (frame_size(1)-1)*rand(num_boxes,1);
> > cols = (frame_size(2)-1)*rand(num_boxes,1);
> > box_coords = round([rows cols]);
> >
> > % SECOND PART, CREATE BOXES (IF NOT TOO CLOSE TO EDGE)
> > M = zeros(frame_size);
> > tic
> > for k = 1:size(box_coords,1)
> > dl = floor(box_size/2);
> > dr = ceil(box_size/2);
> > R1 = box_coords(k,1)-dl;
> > R2 = box_coords(k,1)+dr;
> > C1 = box_coords(k,2)-dl;
> > C2 = box_coords(k,2)+dl;
> > if R1>=1 && C1>=1 && R2<=frame_size(1) && ...
> > C2<=frame_size(2)
> > M(R1:R2,C1) = 1;
> > M(R1:R2,C2) = 1;
> > M(R1,C1:C2) = 1;
> > M(R2,C1:C2) = 1;
> > end
> > end
> > toc
> > % 0.043 seconds on my computer. I was hoping to get <0.01
> >
> > imagesc(M)
> > colormap gray
> > set(gcf,'position',[200 200 frame_size(2) frame_size(1)])
> > set(gca,'position',[0 0 1 1]);
> ---------
> A couple of thoughts come to mind. First, it seems
inefficient to not
> generate the "box_coords" in such a way that the
corresponding "boxes"
> cannot extend past the bounds of the 'frame_size'. That
should be very easy
> to do by altering the way 'rows' and 'cols' are originally
defined. Then the 'if'
> part in your for-loop would be unnecessary. The execution
of this 'if' portion
> may represent a substantial part of your execution time.
>
> The second is more difficult to describe and may not
actually save you time.
> Only experimenting would determine this. You could define
a box "template"
> centered at (0,0) just once and convert its 52 or so pairs
of subscripts into
> single indices which apply to the frame_size. The same
could be done with
> the set of 1000 'box_coords' pairs. Then you could call
'ndgrid' and add the
> outputs to obtain every possible sum of the 52 linear
indices with those of
> the 1000 centers to get an array of 52000 indices. These
can then be used to
> set the corresponding points in M to 1's. The following
shows how this can
> be done.
>
> [m,n] = size(frame_size);
>
> % Create a box template centered at (0,0)
> t = [(-dl:dr)-m*dl,(-dl:dr)+m*dr, ...
> -dl+m*(-dl+1:dr-1),dr+m*(-dl+1:dr-1)];
>
> % Convert 'box_coords' to linear indices in 'frame_size'
> b = box_coords(:,1)+m*(box_coords(:,2)-1);
>
> % Now get all 52000 possible combined sums
> [T,B] = ndgrid(t,b);
> p = T(:)+B(:); % p is 52000 by 1 in size
> M = zeros(size(frame_size));
> M(p) = 1; % Place 52000 1's in M
>
> This of course depends on your having carried out the
first suggestion so
> that no box extends beyond the boundaries of 'frame_size'.
>
> Roger Stafford
>
>
Roger,
that's a clever use of indexing...and I think it should
work. I will test it out tomorrow. Thanks!
J
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