Changing Bounding Box Dimensions - matlab

Question

New to Matlab here. I'm trying to implement some code to detect a face in an image and crop it. I have the script running, but the bounding box that it places around the detected face is a bit small. Is there any way to change the dimensions of the bounding box to capture more of the faces?
clc;
% cd into the a folder with pictures
cd 'C:\Users\abc\Desktop\folder'
files = dir('*.jpg');
for file = files'
img = imread(file.name);
figure(1),imshow(img);
FaceDetect = vision.CascadeObjectDetector;
FaceDetect.MergeThreshold = 7;
BB = step(FaceDetect,img);
figure(2),imshow(img);
for i = 1:size(BB,1)
rectangle('Position',BB(i,:),'LineWidth',2,'LineStyle','- ','EdgeColor','r');
end
for i = 1:size(BB,1)
rectangle('Position',BB(i,:),'LineWidth',2,'LineStyle','- ','EdgeColor','r');
J = imcrop(img,BB(i,:));
figure(3);
imshow(J);
a = 'edited\'
b = file.name
output = strcat(a,b);
imwrite(J,output);
end
%Code End
end
Currently, the script finds a face like so:
And outputs an image such as this:
This is good, I just want to extend the boundaries of the cropping zone to capture more of the face (e.g., hair and chin).

Answer 1

From the MATLAB rectangle function documentation.
rectangle('Position',pos) creates a rectangle in 2-D coordinates.
Specify pos as a four-element vector of the form [x y w h] in data
units. The x and y elements determine the location and the w and h
elements determine the size. The function plots into the current axes
without clearing existing content from the axes.
If you are just looking to increase the bounding box by some scale factor about the center of the rectangle, you could scale the w and h components in BB and adjust the rectangle origin x and y by subtracting half the scale difference. The following code should work if you place it right after the BB = step(FaceDetect,img); line in your code. I don't have MATLAB available to me at the moment but I'm pretty sure this will work.
% Scale the rectangle to 1.2 times its original size
scale = 1.2;
% Adjust the lower left corner of the rectangles
BB(:,1:2) = BB(:,1:2) - BB(:,3:4)*0.5*(scale - 1)
% Adjust the width and height of the rectangles
BB(:,3:4) = BB(:,3:4)*scale;

Answer 2

You can use imresize function in Matlab as described in this link and bboxresize to resize the bounding box
Below is the simple code to resize your image into 3 times the original one
%% clean workspace
clc;
clear;
cd 'C:\Users\abc\Desktop\folder';
files = dir('*.jpg');
for file = files'
img = imread(file.name) ;
figure(1),imshow(img);
FaceDetect = vision.CascadeObjectDetector;
FaceDetect.MergeThreshold =7;
BB = step(FaceDetect,img);
BB2 = BB;
%% Scale the rectangle to 3 times its original size
scale = 3;
%% Resize image
ImgResized = imresize(img,scale);
%% Resize bound box using the function named bboxresize in Matlab
BBResized = bboxresize(BB,scale);
figure(2),imshow(ImgResized);
%% Draw Bounding Box
for i=1:size(BBResized,1)
rectangle('position',BBResized(i,:),'lineWidth',2,'LineStyle','- ','EdgeColor','y');
end
end