how to confusion image by sin-cos map?

I try to implement algorithm in this paper:An efficient color image encryption scheme based on a matrix scrambling method and a new hybrid chaotic map.

This is my cod to implement confusion phase for the algorithm an i write cod for gray image only, please corret it.

in my cod i have two problem, firstly :Error in sc (line 26)

b(j+1)=rem(a(j+1),s);Error using rem :Arguments must be real.

secondly:Pos array have negative no, so should i use abs function to covert all values to positive value ?

close all;

clear all;

clc;

tic

timg = imread('lena_gray.jpg');

[row,col]=size(timg);

s=row*col;

tencr=reshape(timg,s,1);% 1 means 1 col to make matrix in one dim

x=nan(1,s+1); %<------------PREALLOCATE!!!

y=nan(1,s+1); %<------------PREALLOCATE!!!

Pos=nan(1,s+1); %<------------PREALLOCATE!!!

a=nan(1,s+1);

x(1)=.75;

z=.95;% initial value for cofusion

E=.78;% initial value for cofusion

u=1.58;% initial value for cofusion

D(1) = 0.98; % initial value for diffusion

G(1) = 0.96 ; % % initial value for diffusion

y(1)=u*x(1)*(1-x(1));% initial value

for j=1:s

r=(z/y(j))^(3/2);

x(j+1)=sin(r);

w=E*acos(x(j));

y(j+1)=cos(w);

a(j+1)=x(j+1)*(10^14);

b(j+1)=rem(a(j+1),126000);

Pos(j+1) = round (b(j+1));

%Pos(j+1) = round (rem(a(j+1),s));

disp(Pos);

end

%a=x(1,2:s+1)*(10^14);

%z=uint8( rem(a,256) );

%Pos=rem(a,s);

% Make all values in Pos array positive

%g=abs(Pos);

%disp(g);

%Start of Encryption

encrimg=[];

for m = 1:s

encrimg(m)=tencr(Pos(m));

end

ImageEncr=reshape(encrimg,row,col);% convert matrix to 2D

figure

imshow(ImageEncr)

%Start of Decryption

tdecr=reshape(ImageEncr,s,1);

decrimg=[];

for m = 1:s

decrimg(Pos(m))=tdecr(m);

end

ImageDecr=reshape(decrimg,row,col);

figure

imshow(ImageDecr);

Answer 1

[Voss]

At some point in your for j=1:s loop, y(j+1) becomes negative, so in the next iteration r is complex. This is the source of the "Arguments must be real" error. I don't know how you might fix it.

Regarding the negative Pos question, you may consider using mod instead of rem so that a negative argument returns a positive remainder. I don't think doing abs is what you want, but it's hard to say for sure.

[Arshub]

Thank you, yes in algorithm used mod operation, I was wrong about this, but there is still a problem "Arguments must be real" error.

[Voss]

At some point in your for j=1:s loop, y(j+1) becomes negative, so in the next iteration r is complex. This is the source of the "Arguments must be real" error. I don't know how you might fix it.