MATLAB Answers

Draw a straight line in image, given an angle

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Philip on 29 Sep 2011
Commented: Image Analyst on 26 Jun 2016
Is there a way to draw a straight line in an image from a starting (x,y) coordinate, such that the line is equivalent to a specified angle?
For example, if I start at pixel (50,50), how can I draw a line to be 30 degrees of this pixel? I would like to draw 10 pixels in this direction (obviously, the resulting line will be zig-zagged)...


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Image Analyst
Image Analyst on 29 Sep 2011
You know your line endpoints. You said you knew the (x,y) starting point. And the other endpoint's x position, if you don't know it, can't be less than 1 or more than the number of columns in your image. Therefore you know both endpoints.
Walter Roberson
Walter Roberson on 29 Sep 2011
Calculate the total angle relative to the forward Y axis (i.e., angle 0 on a polar plot.) Choose a large radius, R, such as 1000, for an imaginary line segment; don't worry if it is out of the image. Now, R*sin(theta) and R*cos(theta) give you the total delta x and total delta y if you were drawing the whole line segment. Round to integers, and add those rounded deltas to the starting point to get the imaginary end point. Now apply Bresenham's line algorithm to the imaginary line between the starting point and the identified imaginary end point, except stop when you have generated as many total pixels as you desire. The result will be a line segment drawn from the starting point at your desired angle, for the desired number of pixels.
Philip on 30 Sep 2011
Thanks for your kind suggestions. Mathematics is not my strong point, but your comments have helped me to get the otherwise unknown coordinates. I will now see if I can implement Bresenham's line algorithm to connect the two points!
Much appreciated!

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Answers (2)

Image Analyst
Image Analyst on 29 Sep 2011
How about this:
lineLength = 10;
angle = 30;
x(1) = 50;
y(1) = 50;
x(2) = x(1) + lineLength * cosd(angle);
y(2) = y(1) + lineLength * sind(angle);
hold on; % Don't blow away the image.
plot(x, y);
xlim([0 70]);
ylim([0 70]);
grid on;
If you want the line to be "burned into" your image, then you'll have to use imline() - search answers for my demo on that - or use the Bresenham line algorithm like Walter suggested if you want to do it yourself.


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Image Analyst
Image Analyst on 30 Sep 2011
Philip: Yes you can. See my demo in an answer below.
K M Ibrahim  Khalilullah
K M Ibrahim Khalilullah on 20 Jun 2016
@Image Analyst I have the same problem. Thanks for your suggestion. >> Is it possible to control end point according to the Image size? because i need end point also.
Image Analyst
Image Analyst on 26 Jun 2016
Yes, just make up your formula's for (x1,y1) and (x2,y2) using the number of rows and columns in your image and whatever else you need to do to set the endpoints wherever you want them.

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Image Analyst
Image Analyst on 30 Sep 2011
% Demo to write an ellipse and a line into the overlay of an image,
% and then to burn those overlays into the image.
%----- Initializing steps -----
% Clean up
clear all;
close all;
workspace; % Display the workspace panel.
% Change the current folder to the folder of this m-file.
hasIPT = license('test', 'image_toolbox');
if ~hasIPT
% User does not have the toolbox installed.
message = sprintf('Sorry, but you do not seem to have the Image Processing Toolbox.\nDo you want to try to continue anyway?');
reply = questdlg(message, 'Toolbox missing', 'Yes', 'No', 'Yes');
if strcmpi(reply, 'No')
% User said No, so exit.
% Display images to prepare for the demo.
monochromeImage = imread('pout.tif');
subplot(2, 4, 1);
title('Original Image');
subplot(2, 4, 2);
title('Original Image with ellipse in overlay');
subplot(2, 4, 5);
title('Original Image');
subplot(2, 4, 6);
title('Original Image with line in overlay');
set(gcf, 'units','normalized','outerposition',[0 0 1 1]); % Maximize figure.
set(gcf,'name','Demo by ImageAnalyst','numbertitle','off')
%----- Burn ellipse into image -----
% Create elliptical mask, h, as an ROI object over the second image.
subplot(2, 4, 2);
hEllipse = imellipse(gca,[10 10 50 150]); % Second argument defines ellipse shape and position.
% Create a binary image ("mask") from the ROI object.
binaryImage = hEllipse.createMask();
% Display the ellipse mask.
subplot(2, 4, 3);
title('Binary mask of the ellipse');
% Let's try to add some text. (Doesn't work)
% hText = text(50, 100, 'Line of Text');
% textMask = hText.createMask();
% binaryImage = binaryImage & textMask;
% imshow(binaryImage);
% Burn ellipse into image by setting it to 255 wherever the mask is true.
monochromeImage(binaryImage) = 255;
% Display the image with the "burned in" ellipse.
subplot(2, 4, 4);
title('New image with ellipse burned into image');
%----- Burn line into image -----
burnedImage = imread('pout.tif');
% Create line mask, h, as an ROI object over the second image in the bottom row.
subplot(2, 4, 6);
hLine = imline(gca,[10 100],[10 100]); % Second argument defines line endpoints.
% Create a binary image ("mask") from the ROI object.
binaryImage2 = hLine.createMask();
% Display the line mask.
subplot(2, 4, 7);
title('Binary mask of the line');
% Burn line into image by setting it to 255 wherever the mask is true.
burnedImage(binaryImage2) = 255;
% Display the image with the "burned in" line.
subplot(2, 4, 8);
title('New image with line burned into image');


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