In the field of Image Processing, **Ideal Highpass Filter (IHPF)** is used for image sharpening in the frequency domain. Image Sharpening is a technique to enhance the fine details and highlight the edges in a digital image. It removes low-frequency components from an image and preserves high-frequency components.

This ideal highpass filter is the reverse operation of the ideal lowpass filter. It can be determined using the following relation-

where, is the transfer function of the highpass filter and is the transfer function of the corresponding lowpass filter.

The transfer function of the IHPF can be specified by the function-

Where,

*is a positive constant. IHPF passes all the frequencies outside of a circle of radius**from the origin without attenuation and cuts off all the frequencies within the circle.*- This
*is the transition point between H(u, v) = 1 and H(u, v) = 0, so this is termed as*.*cutoff frequency* **is the Euclidean Distance from any point (u, v) to the origin of the frequency plane, i.e,**

Approach:Step 1: Input – Read an image

Step 2: Saving the size of the input image in pixels

Step 3: Get the Fourier Transform of the input_image

Step 4: Assign the Cut-off Frequency

Step 5: Designing filter: Ideal High Pass Filter

Step 6: Convolution between the Fourier Transformed input image and the filtering mask

Step 7: Take Inverse Fourier Transform of the convoluted image

Step 8: Display the resultant image as output

**Implementation in MATLAB:**

`% MATLAB Code | Ideal High Pass Filter` ` ` `% Reading input image : input_image ` `input_image = imread(` `'[name of input image file].[file format]'` `);` ` ` `% Saving the size of the input_image in pixels-` `% M : no of rows (height of the image)` `% N : no of columns (width of the image)` `[M, N] = size(input_image);` ` ` `% Getting Fourier Transform of the input_image` `% using MATLAB library function fft2 (2D fast fourier transform) ` `FT_img = fft2(double(input_image));` ` ` `% Assign Cut-off Frequency ` `D0 = 10; ` `% one can change this value accordingly` ` ` `% Designing filter` `u = 0:(M-1);` `idx = find(u>M/2);` `u(idx) = u(idx)-M;` `v = 0:(N-1);` `idy = find(v>N/2);` `v(idy) = v(idy)-N;` ` ` `% MATLAB library function meshgrid(v, u) returns 2D grid` `% which contains the coordinates of vectors v and u. ` `% Matrix V with each row is a copy of v, and matrix U ` `% with each column is a copy of u` `[V, U] = meshgrid(v, u);` ` ` `% Calculating Euclidean Distance` `D = sqrt(U.^2+V.^2);` ` ` `% Comparing with the cut-off frequency and ` `% determining the filtering mask` `H = double(D > D0);` ` ` `% Convolution between the Fourier Transformed image and the mask` `G = H.*FT_img;` ` ` `% Getting the resultant image by Inverse Fourier Transform` `% of the convoluted image using MATLAB library function` `% ifft2 (2D inverse fast fourier transform) ` `output_image = real(ifft2(double(G)));` ` ` `% Displaying Input Image and Output Image` `subplot(2, 1, 1), imshow(input_image),` `subplot(2, 1, 2), imshow(output_image, [ ]);` |

**Input Image –**

**Output:**

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