function [ind, label] = drawline(p1,p2,image_size)
%DRAWLINE Returns the geometric space (matrix indices) occupied by a line segment
%in a MxN matrix. Each line segment is defined by two endpoints.
%
% IND = DRAWLINE(P1, P2, IMAGE_SIZE) returns the matrix indices
% of the line segment with endpoints p1 and p2.
% If both points are out of the image boundary no line is drawn and an error will appear.
% If only one of the endpoints is out of the image boundary a line is still drawn.
%
% ARGUMENT DESCRIPTION:
% P1 - set of endpoints (Nx2). ([row column; ...])
% P2 - set of endpoints that connect to p1 (Nx2). ([row column; ...])
% IMAGE_SIZE - vector containing image matrix dimensions,
% where the first element is the number of rows
% and the second element is the number of
% columns.
%
% OUTPUT DESCRIPTION:
% IND - matrix indices occupied by the line segments.
% LABEL - label tag of each line drawn (from 1 to N).
%
%
% 1
% 1 _|_ _ _ _> COLUMNS
% |_|_|_|_
% |_|_|_|_
% |_|_|_|_
% V
% ROWS
%
% Example
% -------------
% BW = zeros(250,250);
% p1 = [10 10; 23 100; -14 -40];
% p2 = [50 50; 90 100; 50 50];
% [ind label] = drawline(p1,p2,[250 250]); % OR ...drawline(p2,p1,...
% BW(ind) = label;
% figure, imshow(BW,[])
%
% See also line, ind2sub.
% Credits:
% Daniel Simoes Lopes
% ICIST
% Instituto Superior Tecnico - Universidade Tecnica de Lisboa
% danlopes (at) civil ist utl pt
% http://www.civil.ist.utl.pt/~danlopes
%
% June 2007 original version.
% Input verification.
if max(size(p1) ~= size(p2))
error('The number of points in p1 and p2 must be the same.')
end
if length(size(image_size)) ~= 2
error('Image size must be bi-dimensional.')
end
% Cicle for each pair of endpoints.
ind = [];
label = [];
for line_number = 1:size(p1,1)
% Point coordinates.
p1r = p1(line_number,1); p1c = p1(line_number,2);
p2r = p2(line_number,1); p2c = p2(line_number,2);
% Image dimension.
M = image_size(1); % Number of rows.
N = image_size(2); % Number of columns.
% Boundary verification.
% A- Both points are out of range.
if ((p1r < 1 || M < p1r) || (p1c < 1 || N < p1c)) && ...
((p2r < 1 || M < p2r) || (p2c < 1 || N < p2c)),
error(['Both points in line segment n ', num2str(line_number),...
' are out of range. New coordinates are requested to fit',...
' the points in image boundaries.'])
end
% Reference versors.
% .....r..c.....
eN = [-1 0]';
eE = [ 0 1]';
eS = [ 1 0]';
eW = [ 0 -1]';
% B- One of the points is out of range.
if (p1r < 1 || M < p1r) || (p1c < 1 || N < p1c) || ...
(p2r < 1 || M < p2r) || (p2c < 1 || N < p2c),
% ....Classify the inner and outer point.
if (p1r < 1 || M < p1r) || (p1c < 1 || N < p1c)
out = [p1r; p1c]; in = [p2r; p2c];
elseif (p2r < 1 || M < p2r) || (p2c < 1 || N < p2c)
out = [p2r; p2c]; in = [p1r; p1c];
end
% Vector defining line segment.
v = out - in;
aux = sort(abs(v)); aspect_ratio = aux(1)/aux(2);
% Vector orientation.
north = v'*eN;
west = v'*eW; east = v'*eE;
south = v'*eS;
% Increments.
deltaNS = [];
if north > 0, deltaNS = -1; end
if south > 0, deltaNS = 1; end
if isempty(deltaNS), deltaNS = 0; end
deltaWE = [];
if east > 0, deltaWE = 1; end
if west > 0, deltaWE = -1; end
if isempty(deltaWE), deltaWE = 0; end
% Matrix subscripts occupied by the line segment.
if abs(v(1)) >= abs(v(2))
alpha(1) = in(1); beta(1) = in(2);
iter = 1;
while (1 <= alpha(iter)) && (alpha(iter) <= M) && ...
(1 <= beta(iter)) && (beta(iter) <= N),
alpha(iter+1) = alpha(iter) + deltaNS; % alpha grows throughout the column direction.
beta(iter+1) = beta(iter) + aspect_ratio*deltaWE; % beta grows throughout the row direction.
iter = iter + 1;
end
alpha = round(alpha(1:end-1)); beta = round(beta(1:end-1));
ind = cat(2,ind,sub2ind(image_size,alpha,beta));
label = cat(2,label,line_number*ones(1,max(size(alpha))));
end
% ...
if abs(v(1)) < abs(v(2))
alpha(1) = in(2); beta(1) = in(1);
iter = 1;
while (1 <= alpha(iter)) && (alpha(iter) <= N) &&...
(1 <= beta(iter)) && (beta(iter) <= M),
alpha(iter+1) = alpha(iter) + deltaWE; % alpha grows throughout the row direction.
beta(iter+1) = beta(iter) + aspect_ratio*deltaNS; % beta grows throughout the column direction.
iter = iter + 1;
end
alpha = round(alpha(1:end-1)); beta = round(beta(1:end-1));
ind = cat(2,ind,sub2ind(image_size,beta,alpha));
label = cat(2,label,line_number*ones(1,max(size(alpha))));
end
clear alpha beta
continue
end
% C- Both points are in range.
in = [p1r; p1c]; out = [p2r; p2c]; % OR in = p2; out = p1;
% Vector defining line segment.
v = out - in;
aux = sort(abs(v)); aspect_ratio = aux(1)/aux(2);
% Vector orientation.
north = v'*eN;
west = v'*eW; east = v'*eE;
south = v'*eS;
% Increments.
deltaNS = [];
if north > 0, deltaNS = -1; end
if south > 0, deltaNS = 1; end
if isempty(deltaNS), deltaNS = 0; end
deltaWE = [];
if east > 0, deltaWE = 1; end
if west > 0, deltaWE = -1; end
if isempty(deltaWE), deltaWE = 0; end
% Matrix subscripts occupied by the line segment.
row_range = sort([p1r p2r]);
col_range = sort([p1c p2c]);
if abs(v(1)) >= abs(v(2))
alpha(1) = in(1); beta(1) = in(2);
iter = 1;
while (row_range(1) <= alpha(iter)) && (alpha(iter) <= row_range(2)) && ...
(col_range(1) <= beta(iter)) && (beta(iter) <= col_range(2)),
alpha(iter+1) = alpha(iter) + deltaNS; % alpha grows throughout the column direction.
beta(iter+1) = beta(iter) + aspect_ratio*deltaWE; % beta grows throughout the row direction.
iter = iter + 1;
end
alpha = round(alpha(1:end-1)); beta = round(beta(1:end-1));
ind = cat(2,ind,sub2ind(image_size,alpha,beta));
label = cat(2,label,line_number*ones(1,max(size(alpha))));
end
% ...
if abs(v(1)) < abs(v(2))
alpha(1) = in(2); beta(1) = in(1);
iter = 1;
while (col_range(1) <= alpha(iter)) && (alpha(iter) <= col_range(2)) &&...
(row_range(1) <= beta(iter)) && (beta(iter) <= row_range(2)),
alpha(iter+1) = alpha(iter) + deltaWE; % alpha grows throughout the row direction.
beta(iter+1) = beta(iter) + aspect_ratio*deltaNS; % beta grows throughout the column direction.
iter = iter + 1;
end
alpha = round(alpha(1:end-1)); beta = round(beta(1:end-1));
ind = cat(2,ind,sub2ind(image_size,beta,alpha));
label = cat(2,label,line_number*ones(1,max(size(alpha))));
end
clear alpha beta
continue
end