Drawing rectangles around vertical groups of pixels having the same value - matlab

Question

Consider the following visualization of a 7x5 matrix consisting of 3 distinct regions/values:
bL = toeplitz( [zeros(1,5) -2*ones(1,2)], [0 -ones(1,4)] );
hF = figure(); hA = axes(hF);
imagesc(hA,bL); axis(hA,'image'); set(hA,'XTick',[],'YTick',[]);
N = 4; cmap = parula(N); colormap(cmap(1:end-1,:));
Now let's say I "select" 0 or more pixels in each column such that:
Selected pixels can only be chosen in the green region.
Selected pixels are always contiguous.
Selection is performed by assigning a constant new value, which is different from the 3 initial regions.
Several examples of selection (using the value 1):
%Example 1:
cSF = toeplitz([ones(1,1) zeros(1,4) -2*ones(1,2)],[1 -ones(1,4)]);
%Example 2:
oSF = toeplitz( [zeros(1,5) -2*ones(1,2)], [0 -ones(1,4)] );
oSF(end-2:end,find(any(oSF==-2,1),1,'last')+1:end) = 1;
%Example 3:
iSF = toeplitz([ones(1,3) zeros(1,2) -2*ones(1,2)],[1 -ones(1,4)]);
% Plot:
hF = figure();
hP(1) = subplot(1,3,1); imagesc(cSF);
hP(2) = subplot(1,3,2); imagesc(oSF);
hP(3) = subplot(1,3,3); imagesc(iSF);
axis(hP,'image'); set(hP,'XTick',[],'YTick',[]);
My objective is to draw a set of rectangles encompassing "selected" (yellow) pixels belonging to the same column. For the examples above, the results should look like this (respectively):
The way I see it, for the code to be general it should accept: (1) an axes handle where the imagesc should be plotted; (2) a data array; (3) a value found in the data array, representing "chosen" pixels; and optionally the color of the enclosed pixels.
I found some ways of doing this using patch and rectangle (see own answer), but I'm wondering if this can be achieved with fewer function calls or in other ways I hadn't thought of.

Answer 1

Loopless solution using patch:
Here's a solution that generates coordinates for patch without needing a loop:
function column_highlight(hA, data, selectionVal)
assert(nargin >= 2);
if (nargin < 3) || isempty(selectionVal)
selectionVal = 1;
end
nCol = size(data, 2);
data = diff([false(1, nCol); (data == selectionVal); false(1, nCol)]);
[r, c] = find(data);
r = reshape(r-0.5, 2, []);
c = c(1:2:end);
X = [c-0.5 c+0.5 c+0.5 c-0.5].';
Y = r([1 1 2 2], :);
patch(hA, 'XData', X, 'YData', Y, 'FaceColor', 'none');
end
Solution using regionprops:
If you have the Image Processing Toolbox, you can solve this by labeling each masked column section and getting the 'BoundingBox' shape measure using regionprops:
function column_highlight(hA, data, selectionVal)
assert(nargin >= 2);
if (nargin < 3) || isempty(selectionVal)
selectionVal = 1;
end
labelMat = bsxfun(#times, (data == selectionVal), 1:size(data, 2));
coords = regionprops(labelMat, 'BoundingBox');
coords = vertcat(coords.BoundingBox);
coords(:, 3:4) = coords(:, 1:2)+coords(:, 3:4);
X = coords(:, [1 3 3 1]).';
Y = coords(:, [4 4 2 2]).';
patch(hA, 'XData', X, 'YData', Y, 'FaceColor', 'none');
end

Answer 2

A solution using rectangle:
function markStreaksRect(hA, data, selectionVal)
% Check inputs:
assert(nargin >= 2); if nargin < 3 || isempty(selectionVal), selectionVal = 1; end
% Create a mask for "selected" values:
oneMask = data == selectionVal;
% Find the first encountered "selected" element from both the top and the bottom:
[~,I1] = max(oneMask,[],1); [~,I2] = max(flipud(oneMask),[],1);
% Express the "selected" extent as a 2 row vector:
firstLast = [I1; size(oneMask,1)-I2+1].*any(oneMask,1);
% For nonzero extents, plot shifted rectangles:
for ind1 = find(all(firstLast,1))
rectangle(hA,'Position',[ind1-0.5, firstLast(1,ind1)-0.5, 1, diff(firstLast(:,ind1))+1 ]);
end
A solution using patch:
function markStreaksPatch(hA, data, selectionVal)
% Check inputs:
assert(nargin >= 2); if nargin < 3 || isempty(selectionVal), selectionVal = 1; end
% Create a mask for "selected" values:
oneMask = data == selectionVal;
% Find the first encountered "selected" element from both the top and the bottom:
[~,I1] = max(oneMask,[],1); [~,I2] = max(flipud(oneMask),[],1);
% Express the "selected" extent as a 2 row vector:
firstLast = [I1; size(oneMask,1)-I2+1].*any(oneMask,1);
% For nonzero extents, plot shifted patches:
for ind1 = find(all(firstLast,1))
[XX,YY] = meshgrid(ind1-0.5 + [0 1], firstLast(1,ind1)-0.5+[0 diff(firstLast(:,ind1))+1]);
patch(hA, XX(:), [YY(1:2) YY(4:-1:3)], 'y', 'FaceAlpha', 0);
end
The above solutions can be tested using:
function q45965920
iSF = toeplitz([ones(1,3) zeros(1,2) -2*ones(1,2)],[1 -ones(1,4)]);
hF = figure(); hA = axes(hF); imagesc(hA,iSF);
axis(hA,'image'); set(hA,'XTick',[],'YTick',[]);
...then running either markStreaksRect(hA, iSF, 1); or markStreaksPatch(hA, iSF, 1); produces the desired result.