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imagematlabimage-processinglinematlab-cvst
Richard
Richard

Reputation: 141

MATLAB: Drawing a line over a black and white image

What is the best way to draw a line over a black and white (binary) image in MATLAB, provided the start and end coordinates are known?

Please note, I am not trying to add an annotation line. I would like the line to become part of the image.

Upvotes: 14

Views: 19018

Answers (5)

gnovice
gnovice

Reputation: 125854

You may want to look at my answer to an SO question about adding a line to an image matrix. Here's a similar example to the one I have in that answer, which will make a white line running from row and column index (10, 10) to (240, 120):

img = imread('cameraman.tif');  % Load a sample black and white image
x = [10 240];                   % x coordinates
y = [10 120];                   % y coordinates
nPoints = max(abs(diff(x)), abs(diff(y)))+1;    % Number of points in line
rIndex = round(linspace(y(1), y(2), nPoints));  % Row indices
cIndex = round(linspace(x(1), x(2), nPoints));  % Column indices
index = sub2ind(size(img), rIndex, cIndex);     % Linear indices
img(index) = 255;  % Set the line points to white
imshow(img);       % Display the image

And here's the resulting image:

enter image description here

Upvotes: 9

Dima
Dima

Reputation: 39389

If you have the Computer Vision System Toolbox, you can use insertShape.

Upvotes: 0

saastn
saastn

Reputation: 6015

It actually is just a modification on plesiv's answer. I'm drawing thousands of lines over an image and I need to increase the performance. The most improvement made by omitting interp1 calls and using integer variables made it slightly faster. It performs about 18% faster on my PC comparing to plesiv's code.

function img = drawLine(img, x1, y1, x2, y2)
x1=int16(x1); x2=int16(x2); y1=int16(y1); y2=int16(y2);
% distances according to both axes
xn = double(x2-x1);
yn = double(y2-y1);

% interpolate against axis with greater distance between points;
% this guarantees statement in the under the first point!
if (abs(xn) > abs(yn))
    xc = x1 : sign(xn) : x2;
    if yn==0
        yc = y1+zeros(1, abs(xn)+1, 'int16');
    else
    yc = int16(double(y1):abs(yn/xn)*sign(yn):double(y2));
    end
else
    yc = y1 : sign(yn) : y2;
    if xn==0
        xc = x1+zeros(1, abs(yn)+1, 'int16');
    else
    xc = int16(double(x1):abs(xn/yn)*sign(xn):double(x2));
    end
end

% 2-D indexes of line are saved in (xc, yc), and
% 1-D indexes are calculated here:
ind = sub2ind(size(img), yc, xc);

% draw line on the image (change value of '255' to one that you need)
img(ind) = 255;
end

Upvotes: 1

plesiv
plesiv

Reputation: 7028

If you are bothered by exceptional cases of other methods here's a bullet-proof method that results in a line:

  • whose pixels always touch each other during the whole length of the line (pixels are 8-neighbors to each other),
  • density of the line is not dependent on the additional parameter, but is determined flexibly to accommodate guarantee from the first point.

Inputs (convenient for making function out of this code):

  • img - matrix that contains image,
  • x1, y1, x2, y2 - coordinates of the end points of the line to be drawn.

Code:

% distances according to both axes
xn = abs(x2-x1);
yn = abs(y2-y1);

% interpolate against axis with greater distance between points;
% this guarantees statement in the under the first point!
if (xn > yn)
    xc = x1 : sign(x2-x1) : x2;
    yc = round( interp1([x1 x2], [y1 y2], xc, 'linear') );
else
    yc = y1 : sign(y2-y1) : y2;
    xc = round( interp1([y1 y2], [x1 x2], yc, 'linear') );
end

% 2-D indexes of line are saved in (xc, yc), and
% 1-D indexes are calculated here:
ind = sub2ind( size(img), yc, xc );

% draw line on the image (change value of '255' to one that you need)
img(ind) = 255;

Here's the example image with three lines drawn on it: enter image description here

Upvotes: 5

High Performance Mark
High Performance Mark

Reputation: 78316

This algorithm offers one approach.

Upvotes: 3

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