how to inverse the output image to show image like sketch

clc;

clear all;

close all;

Im = imread('sketch.jpg');

Im =double(rgb2gray(Im));

X = double([-1 0 1;-2 0 2;-1 0 1]);

X=rot90(X,2);

Y = double([-1 -2 -1; 0 0 0; 1 2 1]);

Y=rot90(Y,2);

I= Im;

[r,c,z]=size(I);

Fx = zeros(r,c);

Fy = zeros(r,c);

I = padarray(I,[1 1]);

for i=2:r-1

for j=2:c-1

Fx(i,j)=sum(sum(X.*I(i-1:i+1,j-1:j+1)));

Fy(i,j)=sum(sum(Y.*I(i-1:i+1,j-1:j+1)));

end

end

Im=uint8(Im);

FMag=mat2gray(sqrt(Fx.^2+Fy.^2));

im1=FMag;

imshow(im1)

Answer 1

[Image Analyst]

To invert the image subtract from 1:

FMag = 1 - FMag;