OK, since you were not supplying the binary image, I made one from the picture you posted. Try this:
% Initialization Steps.
clc; % Clear the command window.
close all; % Close all figures (except those of imtool.)
clear; % Erase all existing variables. Or clearvars if you want.
workspace; % Make sure the workspace panel is showing.
format long g;
format compact;
fontSize = 18;
mask = imread('spray.png') > 128;
imshow(mask);
hold on;
axis('on', 'image')
% Define where you want the starting column to be:
% at column %1, or the last column on row 1 that is white.
startingColumn = 1;
startingColumn = find(mask(1, :), 1, 'last')
% Get the number of white pixels in the image
numWhitePixelsEntireImage = nnz(mask)
[rows, columns] = size(mask);
% Run along the bottom row getting a quadrilateral mask.
for col = columns : -1 : 1
fprintf('Col = %d. \t', col)
hLine = line([startingColumn, col], [1, rows], 'Color', 'r', 'LineWidth', 2);
% Make a mask
x = [startingColumn, columns, columns, col];
y = [1, 1, rows, rows];
hPlot = plot(x, y, 'y-', 'LineWidth', 4);
thisMask = poly2mask(x, y, rows, columns);
% And it with the image
thisMask = mask & thisMask;
% Count the number of pixels in the upper right quadrilateral.
% We're looking for the number in that quadrilateral to be 5% of the total.
numWhite = nnz(thisMask);
pctWhite = 100 * numWhite / numWhitePixelsEntireImage;
fprintf(' Number of white pixels = %d pixels = %.3f %% of %d.\n', numWhite, pctWhite, numWhitePixelsEntireImage)
if pctWhite > 5.0
% Now we have 5% in the upper right quadrilateral so we can quit.
% Go back a column to where it was less than the threshold.
col = col + 1;
delete(hLine);
break;
end
% Delete old graphics.
delete(hPlot);
delete(hLine);
end
delete(hLine);
line([startingColumn, col], [1, rows], 'Color', 'r', 'LineWidth', 2);
