Filtering a region of Interest

Please I would like some help wih a question, I am trying to apply a filter to a region of interest using the roipoly and roifilt2 function, then show the effect on the original image. The problem is, I have to extract a portion of the image containing a range of pixels, which I managed to do with this:

image = imread('image8.jpg');
imshow(image)

%  imModified = blockproc(image,[20 20],@(blkStruct) dct2(blkStruct.data));
%  imshow(imModified)

%Select [56:281, 56:281]
Subimage = image(56:281, 221:412);
imshow(Subimage)

Then I was able to select the region of interest with the roipoly function

%Hexagon
c = [60 27 14 78 130 139];
r = [14 38 127 177 160 69];
Hex = roipoly(Subimage,c,r);
figure
imshow(Hex)

Then I applied an averaging filter

%Average filter
C = fspecial('average', [20,20]);
M = roifilt2(C,Subimage,Hex);

then showed the image

%Show images
figure
imshow(M)

I understand up to here, but now I want to show the effect of the filter on the original full image ('image8.jpg') and not the Subimage (from above). Please if anyone could help me I would appreciate it.

Answer 1

[Robert U]

Hi Oreoluwa ADESINA,

What you describe is part of the example shown in the roifilt2-documentation found here: https://de.mathworks.com/help/images/ref/roifilt2.html

You would have to redefine your "c" and "r".

Kind regards,

Robert

[Oreoluwa ADESINA]

Thank you for taking the time to answer the question. I understand I have to redefine c and r, but I dont know how to take the c, r from the cropped image (Subimage) and translate that c, r to the full image. Thats where I need help.

Thanks again!

[Robert U]

Hi Oreoluwa ADESINA,

Since all values defining vertices are pixel based you can shift them around by appropriately adding and subtracting the crop-pixel values.

You cropped your image by

Subimage = image(56:281, 221:412);

thus, all x-pixels are shifted by 56 and y-pixels by 221. In order to apply the same polygon as on the cropped Subimage you would have to add the "offset".

cFull = c + 221;
rFull = r + 56;
HexFull = roipoly(image,cFull,rFull);

MFull = roifilt2(C,image,HexFull);

In order to avoid confusion you probably want to rename your variable "image" due to the fact that image() is a built-in command.

Kind regards,

Robert

[Oreoluwa ADESINA]

Thank you, I understand it clearly now