How to add box to an image?

Hi

I have two images ('yellow.png' and 'bird.png'). See below.

How can I make a box AROUND the image borders with self-defined box colours, say all four lines with grey colour [0.5 0.5 0.5]?

See below.

Please can you help?

[Adam Danz]

After you plot the yellow patch presumable you'll have it's position, width, and height. Then you can use rectangle() to plot a frame around it and control the color, thickness, etc.

[Salad Box]

They are not plots. They are pre-saved images.

[Adam Danz]

If your question is how to add a frame around an image, you could solve this problem in powerpoint, microsoft paint, coreldraw, photoshop, or any image software. Why do you need Matlab?

[Salad Box]

If I code:

    fn = 'bird1.jpg';
    
    RGB = imread(fn);
    s = size(RGB);
    
    figure
    imshow(RGB)
    
    rectangle('position',[0 0 s(2) s(1)],'edgecolor',[1 0 0])

I got this. Instead of a box, I only got two lines, and missing the other two lines.

Can anyone help please??

[Salad Box]

Hi Adam Danz,

Because I have 273 images. Each one of different size.

The images were first generated in Matlab. I then noticed that some images have similar colour to the background. So that I need to draw a box around the image to separate it from the background. Otherwise it would look like the background eats the image.

[Adam Danz]

"The images were first generated in Matlab"

Maybe it would be easier if you altered the code that generated the image. Adding the frame is just 1 line of code. But if you have read the images back into matlab, that's going to be more work.

Based on the code and image in your other comment, it appears the image's bottom, left corner is not at (0,0) where your rectangle starts. See answer.

[DGM]

A grayscale border can be added using IPT padarray(). A colored border requires extra steps, though alternative methods exist. See the following reference answer, which shows various possibilities.

https://www.mathworks.com/matlabcentral/answers/598555-how-do-i-add-a-black-border-around-an-image

Answer 1

[Image Analyst]

If the bad lines are always the rightmost one and the bottom one, you can simply crop the image to not include those:

rgbImage = rgbImage(1:end-1, 1:end-1, :);

To pad the image with certain color, you can use padarray() on each color channel. There are other ways. Let me know if it's still a problem for you.

Answer 2

[Adam Danz]

Use the handle output from imshow to get the image's coordinates. The XData and YData properties will provide you with the coordinates of the image. Unless the image coordinates are specified in imshow() inputs, the first pixel is centered at coordinate (1,1) so the rectangle should begin at (0.5, 0.5). To expand the rectangle to the outer edges, you must add 1 to the width and height indicated by ranges of XData and YData.

% When the image is in pixel units 
corn_gray = imread('corn.tif',3);
h = imshow(corn_gray); 
rh = rectangle('position',[ ...
    h.XData(1)-.5,...
    h.YData(1)-.5,...
    range(h.XData)+1,...
    range(h.YData)+1],...
    'EdgeColor', 'r','linewidth',2); 

When the image is not in pixel units, XData and YData properties of the image object may no longer indicate the pixel centers so you must compute the location of the pixel edges.

% When the image is not in pixel units
corn_gray = imread('corn.tif',3);
h = imshow(corn_gray,'XData',200:400,'YData',110:4000); %(arbitrary xdata,ydata)
axis square
axis on % optional, to see axis scales
pixelRatio = [range(h.XData)/size(corn_gray,2), range(h.YData)/size(corn_gray,1)]; 
rh = rectangle('position',[ ...
    h.XData(1)-pixelRatio(1)/2,...
    h.YData(1)-pixelRatio(2)/2,...
    range(h.XData)+pixelRatio(1),...
    range(h.YData)+pixelRatio(2)],...
    'EdgeColor', 'r','linewidth',2);

% Expand the axis limits to show full frame
xlim(xlim()+[-.01,.01]*range(xlim())) % add 1% to the x axis limits
ylim(ylim()+[-.01,.01]*range(ylim())) % add 1% to the y axis limits

Answer 3

[Image Analyst]

Images rows and columns start with 1, not zero. so do this:

rectangle('position', [1, 1, s(2), s(1)], 'edgecolor', [1 0 0])

If some part of the rectangle is still missing (outside the image), try adding or subtracting 0.5 or 1 to the position values until you can see it. Like perhaps

rectangle('position', [1, 1, s(2) - 1, s(1) - 1], 'edgecolor', [1 0 0])

[Adam Danz]

"Images rows and columns start with 1, not zero."

The origin of the image could potentially be offset by a spatial reference object which is why I suggest using the XData and YData properties in my answer.

[Image Analyst]

True. But if you want units in something other than pixels you'd set the units with the 'XData' and 'YData' options to imshow(). Maybe you could add them to your demo.

[Salad Box]

Hi Image Analyst,

Thanks for your answer. I used your way of thinking and modified it.

'position' [0 0 s(2) s(1)] would give me

'position' [1 1 s(2) s(1)] would give me

I guess I just have to do it separately.

clear
close all
clc

RGB = imread('bird1.jpg');

s = size(RGB);

figure
imshow(RGB);

rectangle('position',[1 1 s(2) s(1)], 'edgecolor', [1 0 0])
rectangle('position',[0 0 s(2) s(1)], 'edgecolor', [1 0 0])

[Adam Danz]

Then you should have accepted Image Analyst's answer rather than your modification of another person's work.

[Salad Box]

I have accepted Image Analyst's anwser.

Answer 4

[Salad Box]

clear
close all
clc

RGB = imread('bird1.jpg');

s = size(RGB);

figure
imshow(RGB);

rectangle('position',[1 1 s(2) s(1)], 'edgecolor', [1 0 0])
rectangle('position',[0 0 s(2) s(1)], 'edgecolor', [1 0 0])

[Adam Danz]

If you zoom into the image at the rectangle corners you'll see that it doesn't frame the pixel edges because you're not offsetting it by the pixel width & height.

Answer 5

[Serge]

Here is another option that uses xlim/ylim and xline/yline.

This works even if one of the the axis is datetime or a duration.

im = imread('rice.png');

figure
imshow(im)

xline(xlim, 'color', 'r')
yline(ylim, 'color', 'r')

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