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miniOpticalFlow.m
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miniOpticalFlow.m
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function [b, f, bu, bv, fu, fv] = miniOpticalFlow(frame0, frame1)
% Returns the plist of pixels belonging to the background and foreground
% and their velocity
regions = initregions(frame0);
% Currently only seeking background and foreground
init_num_regions = 2;
% Regions having motion error above this threshold are not
% considered to be correctly detected
region_thres = 0.4;
% Atleast 20% needs to be foreground
foreground_percentage_thres = 0.10;
num_regions = init_num_regions;
%while 1
optu = nan([length(regions), 1]);
optv = nan([length(regions), 1]);
var = nan([length(regions), 1]);
% Optimization - Need to find gradient only once per frame
[TGx, TGy] = imgradientxy(frame0);
for i=1:length(regions)
idx = regions{i};
[u,v] = LucasKanadeInverseCompositional(frame0, frame1, idx, TGx, TGy);
if (~(isnan(u) || isnan(v)))
var(i) = similarity_region_motion(frame0, frame1, idx, u, v);
if (var(i) >= region_thres)
optu(i) = u;
optv(i) = v;
end
end
if (mod(i, 100) == 0)
i
end
end
% Kmeans sometimes behaves inappropriately
% This is chiefly due to random initialization at the start
% FIXME: As a hack, we assert that percentage of points in the
% foreground needs to be greater than some threashold
while 1
[Cidx, C] = kmeans([optu(:) optv(:)], num_regions);
rerun = 0;
for i = 1 : size(C, 1)
if (sum(Cidx == i)/sum(~isnan(Cidx)) < foreground_percentage_thres)
rerun = 1;
break;
end
end
if (rerun == 0)
break;
end
end
% FIXME: Temporary hack to make the background as first element
% and foreground as second element.
% Assumption that foreground is moving faster
% (so L2 norm is greater)
% This sorting ensures that any region that is NaN in find_region()
% gets associated with background
nC = [sum(abs(C).^2,2), C];
nC = sortrows(nC, 1);
C = nC(:, 2:end);
regions = find_regions(frame0, frame1, regions, C);
foreground = get_foreground(frame0, regions);
U = C(:, 1);
V = C(:, 2);
bu = U(1);
bv = V(1);
fu = U(end);
fv = V(end);
nf = zeros(size(frame0));
nf(foreground) = 1;
se = strel('disk',10);
closeBW = imclose(nf,se);
b = find(closeBW == 0);
f = find(closeBW == 1);
% Visualize the foreground
imf = imfuse(frame0, closeBW);
imshow(imf);
%end
end
function [var] = similarity_region_motion(frame0, frame1, old_idx, u, v)
% Checks if the motion of region agrees
[old_Y old_X] = ind2sub(size(frame0), old_idx(:));
new_X = ceil(old_X - u);
new_Y = ceil(old_Y - v);
old_patch = frame0(old_idx(:));
new_patch = interp2(frame1, new_X, new_Y);
% Normalized cross-correlation
var = sum((new_patch - mean(new_patch(:))) .* (old_patch - mean(old_patch(:)))) / length(old_idx(:));
var = var / std(new_patch(:));
var = var / std(old_patch(:));
end
function [new_regions] = find_regions(frame0, frame1, regions, C)
% Based on centroids of motion (found after clustering), find which region best suits each previos region
U = C(:, 1);
V = C(:, 2);
num_centroids = size(C, 1);
nccm = nan(numel(regions), num_centroids);
for num_region = 1:numel(regions)
pregion = regions{num_region};
[pY pX] = ind2sub(size(frame0), pregion(:));
ppatch = frame0(pregion(:));
for num_centroid = 1:num_centroids
nX = pX - U(num_centroid);
nY = pY - V(num_centroid);
npatch = interp2(frame1, nX, nY);
ncc = sum(((ppatch - mean(ppatch)) .* (npatch - mean(npatch)))) / numel(pregion);
ncc = ncc / std(ppatch);
ncc = ncc / std(npatch);
nccm(num_region, num_centroid) = ncc;
end
end
[~, bestmotion] = max(nccm, [], 2);
new_regions = cell(num_centroids, 1);
for num_region = 1:numel(regions)
new_regions{bestmotion(num_region)} = [new_regions{bestmotion(num_region)}; regions{num_region}];
end
end
function [foreground] = get_foreground(frame0, regions)
% Returns list of points (index) that are part of the foreground
% regions should be cell of two. First should be background, second should be foreground points index
% New image - With all the foregorund as 1
nf = zeros(size(frame0));
nf(regions{2}) = 1;
bw = imbinarize(nf, 0.5);
cc = bwconncomp(bw);
% Return the connected component with max number of pixels
[max_size, max_index] = max(cellfun('size', cc.PixelIdxList, 1));
foreground = cc.PixelIdxList{max_index};
end