[beginner] point detection

Hello people!

im a new using matlab for my image processing course and trying to do a point detection algorithm using laplacian kernel

and i have a problem understanding some parts of this simple code:

1: f = imread('pointDetection.jpg');

2: w=[-1,-1,-1;-1,8,-1;-1,-1,-1];

3: g=abs(imfilter(double(f),w));

4: T=max(g(:));

5: g=g>=T;

6: subplot(121),imshow(f),title('original image');

7: subplot(122),imshow(g),title('Result of point detection');

if the sum of product of applying the mask to a pixel larger than a particular threshold then this should be the point

my questions are (1 what does line 5 do

(2 when i run this code thses errors appear:

Error using images.internal.imageDisplayValidateParams>validateCData (line 119)

If input is logical (binary), it must be two-dimensional.

Error in images.internal.imageDisplayValidateParams (line 27)

common_args.CData = validateCData(common_args.CData,image_type);

Error in images.internal.imageDisplayParseInputs (line 78)

common_args = images.internal.imageDisplayValidateParams(common_args);

Error in imshow (line 246)

images.internal.imageDisplayParseInputs({'Parent','Border','Reduce'},preparsed_varargin{:});

Error in pointdetectionassignment (line 9)

subplot(122),imshow(g),title('Result of point detection');

Answer 1

[Vidhi Agarwal]

Hi @motaz yusuf

I understand that you have some queries regarding the errors you are encountering. Let’s break them down one by one:

g = g >= T;

This line is performing a thresholding operation on the image “g”. This operation creates a logical array where each element is true if the corresponding element in “g” is greater than or equal to “T”, and false otherwise, thus resulting in a binary image.

The errors you are encountering are related to the display of the image “g” using “imshow”. The error message indicates that “g” must be a two-dimensional logical array for “imshow” to display it correctly. You can try out the following solution for resolving the error:

1. If “f” is a color image, “imfilter” might return a 3D array. Convert “f” to grayscale before processing.
2. Verify that “f” is read correctly and is two-dimensional if it's supposed to be a grayscale image.

Here is the slightly revised version of your code considering the above points:

f = imread('pointDetection.jpg');

% Convert to grayscale if the image is not already
if size(f, 3) == 3
   f = rgb2gray(f);
end

w = [-1, -1, -1; -1, 8, -1; -1, -1, -1];
g = abs(imfilter(double(f), w));

T = max(g(:));
g = logical(g >= T);

subplot(121), imshow(f), title('Original Image');
subplot(122), imshow(g), title('Result of Point Detection');

For detailed understanding of “imshow”, “imfilter” and “subplot” refer to the following documentation:

• https://www.mathworks.com/help/releases/R2019a/matlab/ref/imshow.html
• https://www.mathworks.com/help/releases/R2019a/images/ref/imfilter.html
• https://www.mathworks.com/help/releases/R2019a/matlab/ref/subplot.html

Hope that Helps!.