Drawing rectangles using matrices.

Hi, I need to draw a figure as shown in the picture using 256x256 matrix. There should be two rectangles and the rest of the matrix values should be zero. Can you please help ?

[Anvinder Singh]

So i have drawn this figure as attached here and have used this code :

xlim([0 250]);
ylim([0 200]);
x = rectangle('Position',[40 130 70 40])
y = rectangle('Position',[120 10 80 25])
imshow(y).

Now can you please suggect me how to make the same figure using a 256x256 matrix ? How can i make lines using the matrix at some particular pixels which will create the same figure as shown.

[Azzi Abdelmalek]

your question is not clear, illustrate with a 4x4 matrix for example

[Anvinder Singh]

Ok, Rephrasing the question: The image which is attached 'send.jpg' it has two rectangles. I have implemened ted using the function 'rectangle('position')'. Now i need to implement the same image with the two rectangles using matrices. In other words, how to show 2 rectangles using matrices ?

[sudharsan V]

how to draw a triangle instead of rectangle?

[Image Analyst]

Use plot() with 3 x values and 3 y values.

Answer 1

[Stephen23]

You can use MATLAB's array indexing quite effectively for this:

>> X = ones(256,256,3);
>> X(40:140,[40,140],:) = 0;
>> X([40,140],40:140,:) = 0;

and to view the rectangle:

>> image(X)

creates this:

[Anvinder Singh]

Thanks Cobel, What function is this X(40:140,[40,140],:) = 0; ? Where can i get more info on this ? Also the Y axis is reversed in the image ?

[Image Analyst]

The y axis is show normally for an image. The top row is y=1 and increases from there as it goes down. This is opposite from a graph where the origin is at the bottom left. But it matches your example of what you said you wanted. If you just want a graph, use the rectangle() function.

[Stephen23]

@Anvinder Singh: "What function is this" the first line of my answer tells you what this is: it is called array indexing. Follow that link to learn more, or start here:

http://www.mathworks.com/help/matlab/indexing.html

[Anvinder Singh]

One more question , is this a 3d array that you have used ? I am unable to see it on their website .

[Stephen23]

Yes, I used a 3d array because this represents a standard RGB image that can be displayed via image. You can find more info on multidimensional arrays here:

http://www.mathworks.com/help/matlab/math/multidimensional-arrays.html

[Anvinder Singh]

Stephen, Is there any way you can help me with the 2d matrix ? I am trying but am not able to get the result ! I want the same thing with 2d matrix

[Guillaume]

My answer is the same as Stephen's but applied to a 2D matrix.

However, I think you should pause and learn the basics of matrix indexing. If you're not able to understand Stephen's or my code you won't go very far.

In matlab's documentation follow the getting started tutorial.

[Anvinder Singh]

Guillaume you are right. I thought the rectangle function you defined was independent of the matrix and was it's values just based on the values given to it so it had nothing to do with the matrix (256*256) so got a little confused.

Answer 2

[Guillaume]

Considering your rectangles are aligned with the side of the image, all you have to do is fill two rows and two columns with 1:

function img = drawrectangle(img, xbounds, ybounds)
  img(ybounds, xbounds(1):xbounds(2)) = 1;
  img(ybounds(1):ybounds(2), xbounds) = 1;
end

And in your case, you use it like this:

img = zeros(256);
img = drawrectangle(img, [40, 140], [60, 80]); %you didn't show the y coordinates for the 1st rectangle
img = drawrectangle(img, [120, 200], [120, 200]);
imshow(img);

Answer 3

[Image Analyst]

If you want them on a graph, with the y increasing upwards, try this:

rectangle('Position', [40,40,100,100]);
hold on;
rectangle('Position', [120,120,80,80]);
axis([0,220, 0,220]);

Note, your rectangles will overlap/intersect given the coordinates you gave despite the fact that your picture incorrectly shows them not overlapping.

[Anvinder Singh]

Thanks, The rectangle function is good and i am able to implement it but the main problem is i need to do this using matrices. Like X = ones(256,256,3); X = ones(256,256,3); X(40:140,[40,140],:) = 0; X([40,140],40:140,:) = 0; but i am not sure about this function 'X([40,140],40:140,:) = 0;' and how to interpret its values. Any help is appreciated.

[Image Analyst]

Then do it the way the other two people showed you. If you want to reverse the y axis direction, you can with something like this

set(gca, 'ydir', 'reverse'); % or 'normal'

[Anvinder Singh]

how do i set a given points in a row of matrix as 1 ? example- set x coordinate 40 to 120 as 1 etc..

Answer 4

[Anvinder Singh]

The y axis of the graph is still reversed. I have tried axis([0,280, 0,280]); and set(gca, 'ydir', 'reverse'); % or 'normal'

[Image Analyst]

Explain why 120 comes after 140 in your diagram.