Drawing bounding box onto an image without imshow and hold on ...

Hi there. I was wondering if its possible to draw a rectangle, whose coordinates is obtained from regoingprops. I however, am not interested to show the image first. What I intend to do is to get the bounding box coordinates form regionprops, and then just superimpose it onto an image without displaying it. I would then like to save the image (saving is ok... just drawing the rectangle got me a bit tangled up :P ). Would appreciate any advice!

The code I'm currently using is as shown above. (It's in a for loop) where 'l' is the counter.

THANKS in advance!!!

f = figure,imshow(frame, 'Border', 'tight');
hold on; 
rectangle('Position',s(l).BoundingBox,'EdgeColor','y','LineWidth',2)
print(f, '-r80', '-djpeg', [dirName 'results' '\' jpegFiles(i).name]);
close all;

Answer 1

[Walter Roberson]

If you have the Computer Vision Toolbox then you can use shapeInserter

If you do not have that, then you can just assign values to the appropriate rows and columns in the array. Watch out for the possibility that the values for the coordinates are not integral

bb = s(l).BoundingBox;
from_row = ceil(bb(2));
to_row = from_row + bb(4) - 1;
from_col = ceil(bb(1));
to_col = from_col + bb(3) - 1;
yellow = [255, 255, 0];

%left and right side
frame(from_row:to_row, [from_col, to_col], 1) = yellow(1);
frame(from_row:to_row, [from_col, to_col], 2) = yellow(2);
frame(from_row:to_row, [from_col, to_col], 3) = yellow(3);
%top and bottom
frame([from_row, to_row], from_col+1:to_col-1, 1) = yellow(1);
frame([from_row, to_row], from_col+1:to_col-1, 1) = yellow(2);
frame([from_row, to_row], from_col+1:to_col-1, 1) = yellow(3);

[Alfian Abdul Halin]

Thanks a lot Walter!!! Exactly what I was looking for. This is new knowledge for me as well as I didn't know you could play around with images like that (i.e. frame with three parameters...) I changed the code a bit tho, for the % top and bottom, and replaced the '1s' in the last two rows with a 2 and a 3, respectively.

Anywayz, again... much thanks! :D

EDIT: I guess I have to use IMWRITE to save in this case...

[Walter Roberson]

Opps, yes, I copied and pasted the lines and forgot to change the 1's.

RGB images are 3 dimensional, with (:,:,1) referring to the Red component, (:,:,2) referring to the Green component, and (:,:,3) referring to the Blue component.

[Sandipan Dey]

or slightly moifiying the abovr code to change color channels at once and draw a bounding box (e.g., one obtained using regionprops) with a given (odd) width (e.g., 3):

function frame = drawBB(bb, frame, width)
    from_row = ceil(bb(2));
    to_row = from_row + bb(4) - 1;
    from_col = ceil(bb(1));
    to_col = from_col + bb(3) - 1;
    yellow = [255, 255, 0]; 
    for i = -fix(width/2):fix(width/2)
        %left and right side
        frame(from_row:to_row, [from_col+i, to_col+i], :) = ...
            permute(repmat(yellow, to_row-from_row+1, 1, 2), [1,3,2]); 
        %top and bottom
        frame([from_row+i, to_row+i], from_col+1:to_col-1, :) = ...
            permute(repmat(yellow, 2, 1, to_col-from_col-1), [1,3,2]);
    end
end

The output can be seen in the following montage (the input image is taken from a coursera course by mathworks).