How do I create an image matrix with a line drawn in it in MATLAB?

Question

I want to plot a line from one well-defined point to another and then turn it into an image matrix to use a Gaussian filter on it for smoothing. For this I use the functions

Answer 1

Here's one example of drawing a line directly into a matrix. First, we'll create a matrix of zeros for an empty image:

mat = zeros(250, 250, 'uint8');  % A 250-by-250 matrix of type uint8

Then, let's say we want to draw a line running from (30, 35) to (200, 60). We'll first compute how many pixels long the line will have to be:

x = [30 200];  % x coordinates (running along matrix columns)
y = [35 60];   % y coordinates (running along matrix rows)
nPoints = max(abs(diff(x)), abs(diff(y)))+1;  % Number of points in line

Next, we compute row and column indices for the line pixels using linspace, convert them from subscripted indices to linear indices using sub2ind, then use them to modify mat:

rIndex = round(linspace(y(1), y(2), nPoints));  % Row indices
cIndex = round(linspace(x(1), x(2), nPoints));  % Column indices
index = sub2ind(size(mat), rIndex, cIndex);     % Linear indices
mat(index) = 255;  % Set the line pixels to the max value of 255 for uint8 types

You can then visualize the line and the filtered version with the following:

subplot(1, 2, 1);
image(mat);        % Show original line image
colormap(gray);    % Change colormap
title('Line');

subplot(1, 2, 2);
h = fspecial('gaussian', 20, 10);  % Create filter
filteredImg = imfilter(mat, h);    % Filter image
image(filteredImg);                % Show filtered line image
title('Filtered line');

Answer 2

Something like this:

[linspace(30,200,numSteps); linspace(35,60,numSteps)]

Does that work for you ?

Mark

Answer 3

If you have Computer Vision System toolbox there is a ShapeInserter object available. This can be used to draw lines, circles, rectangles and polygons on the image.

mat = zeros(250,250,'uint8');
shapeInserter = vision.ShapeInserter('Shape', 'Lines', 'BorderColor', 'White');
y = step(shapeInserter, mat, int32([30 60 180 210]));
imshow(y);

http://www.mathworks.com/help/vision/ref/vision.shapeinserterclass.html

Answer 4

You can check my answer here. It is robust way to achieve what you are asking for. Advantage of my approach is that it doesn't need additional parameters to control density of the line drawn.