How to divide an image into 8 equal sectors?

I have an image. I need to divide the image into 8 equal sectors as shown below.

I have used the following code:

clear all;
close all;
im=imread('apple.jpg');
r=100;
out1 = ones(max(size(im,1),r*2)+2,max(size(im,2),r*2)+2,3).*255;

xoffset = floor((size(out1,2)-size(im,2))/2);
yoffset = floor((size(out1,1)-size(im,1))/2);
out1(yoffset:yoffset+size(im,1)-1,xoffset:xoffset+size(im,2)-1,:) = im(:,:,:);
im = out1;

cy = floor(size(im,1)/2);
cx = floor(size(im,2)/2);

figure;
imshow(uint8(im));
hold on
pos = [cx-r+1 cy-r+1 r*2 r*2];
rectangle('Position',pos,'Curvature',[1 1]);
x1 = [-r, 0, -r*cosd(45), -r*cosd(45); r, 0, r*cosd(45), r*cosd(45)]+cx+1;
y1 = [0, -r, -r*sind(45), r*sind(45); 0, r, r*sind(45), -r*sind(45)]+cy+1;
plot(x1,y1);
hold off

figure;
for i = 0:45:315
    t = linspace(-i,-i-45,128);
    x = [cx, cx+r*cosd(t), cx];
    y = [cy, cy+r*sind(t), cy];
    bw = poly2mask( x, y, size(im,1),size(im,2));
    bw = repmat(bw,1,1,3);
    out = ones(size(im,1),size(im,2),size(im,3)).*155;
    out(bw) = im(bw);
    subplot(2,4,(i/45)+1); imshow(uint8(out));
end;

I need to do this for a dataset of over 100 images with different sizes. My problem is that, I have to manually assign the value for r for every image. How can I automatically assign the value for r? Please help. I also need to minimize the background (white parts) in the sectors as much as possible. How do I go about this? Please help. Thank you.

Answer 1

[Image Analyst]

Just mask the image with sector shapes. See the FAQ:

http://matlab.wikia.com/wiki/FAQ#How_do_I_mask_an_image.3F

http://matlab.wikia.com/wiki/FAQ#How_do_I_create_an_arc.3F

[anastasia]

But even in those links, the radius has to be pre-specified. My question was how to automatically decide the radius value instead of pre-specifying every time for my images which are of different sizes

[anastasia]

I am getting the background white pixels into my sectors. How do I eliminate those?

Is it possible to split ONLY the apple image into equal parts using the centroid and the major and minor axis?

I do not want the background pixels in my split parts, just the ROI pixels.

Please guide.

[Image Analyst]

Find the apple mask by converting to hsv colorspace with rgb2hsv() then thresholding the S channel. The apple will have S values more than about 0.2. Now you have a mask and you can use it to blacken outside the apple, crop the image, or both.