draw a line on an image

Hello

I want draw a line on an image, just by giving the coordinates of the extremities.

I tryed the function line(...) but it didn't give me the right result

Answer 1

[Fangjun Jiang]

Pay attention of the syntax of line().

line([x1,x2],[y1,y2]), not line([x1,y1],[x2,y2]);

figure(1);line([0 1],[0 1]); will show a line

figure(2);line([0 0],[1 1]); won't show a line

[Fatma Gargouri]

hi

yes I know

but the problem is that the line is shown alone and not on an image

[Fangjun Jiang]

use "hold on"

[Fatma Gargouri]

thank you :))

[Sumit Nishar]

then how to save the image after hold on

....as its unable to save the image

[Amber Elferink]

you have to do hold off at the end again (for next viewers)

[Walter Roberson]

I suspect Sumit either wanted to know about getframe() https://www.mathworks.com/help/matlab/ref/getframe.html or else wanted to know how to draw a line directly into an array with the intent to save the array as an image but without having to display on the screen.

The easiest way to draw a line directly into an array is to use Computer Vision Toolbox insertShape()

Answer 2

[David Young]

Perhaps your problem is that figure coordinates and array indices have different conventions. So, for example

imshow(im)
line([50 100], [30 30])

draws a line from im(30, 50) to im(30, 100).

Arrays are indexed by (ROW,COLUMN). Positions in a figure are addressed by (X,Y). When you display an array as an image, using the usual conventions, ROW corresponds to Y and COLUMN corresponds to X.

[Fatma Gargouri]

when I wrote this only the image is shown without line

im=imread('img.jpg');

line ([50, 40],[100, 100]);

imshow (im);

[Image Analyst]

Issue the hold on command

im=imread('img.jpg');

imshow (im);

hold on;

line ([50, 40],[100, 100]);

[Walter Roberson]

You never need to use "hold on" when you are using line() afterwards.

However, you do need "hold on" if you are using imshow() after line()

[Fatma Gargouri]

thank you :))

[Jes]

I have a follow up question here. After this line command, how to see the line as a pixel values in the same image?

[Image Analyst]

What does that mean? Post your reply/clarification in a new question. When you say "see the line" do you mean that you want to burn the line into the image instead of being plotted in the graphical overlay? Or do you want to print out the pixel values of the image underneath the line into the command window or a file?

[Jes]

Yes I want to burn the line into the image instead of graphical overlay

[Image Analyst]

See attached demos that do that.

[Somsubhro Chaudhuri]

Hello - follow up question.

Can I add a line that is defined in the code rather than being an interactive line?

[Image Analyst]

Of course. Just hard-code in the x and y values of the endpoints of the line.

[Somsubhro Chaudhuri]

Thank you @Image Analyst. I managed doing that now. I now have a binary image with a line between two points (attached). Could you please suggest a way to only change the area right below the line to black?

I was thinking of defining the equation of the line, and then running a for loop to cover these pixels, but I want to enquire about a shorter way of doing this as I'll have to run this code for a very large number of images.

Thank you! Regards, Somsubhro

[Image Analyst]

You could take the endpoints of the line, then add on the bottom of the image coordinates, then call poly2mask() to get a binary image mask. Then use that to erase the image in that quadrilateral.