About PARFOR using 2d array

Dear all,

I'm trying to use a parfor in the 2d array with the code below:

parfor j = 1:6000

    for k = 1:8100

        SR(k+8100,j+6000,1) = CAM04R(k,j,1);   

        SG(k+8100,j+6000,2) = CAM04G(k,j,1);   

        SB(k+8100,j+6000,3) = CAM04B(k,j,1);     

    end  

end

But the Matlab its showing the follow messages:

The parfor loop cannot run due to the way variable 'SR' is used

The parfor loop cannot run due to the way variable 'SG' is used

The parfor loop cannot run due to the way variable 'SB' is used

Any suggestion about how I can try to fix this?

Thanks!

Answer 1

[Edric Ellis]

To make SR, SG, and SB be "sliced" output variables from the parfor loop, you need to follow the rules described here. Basically, you need to remove the offset to the indexing expressions. For example, the following works just fine:

parfor i = 1:10
    for j = 1:10
        x(j, i, :) = rand(3,1);
    end
end

Answer 2

[Leonardo Filho]

Hello Edric,

Thanks by your prompt feedback!

Yes, I know that I can't use offset inside the PARFOR, however I need try to use this because I am processing sub-images to generate an synthetic image. Below you have the new entire code [modified]:

%CAM01[RGB]:

 parfor j = 1:8100

     for k = 1:6000

         S(j+8100,k,:) = CAM01(j,k,:);   

     end  
 end

%CAM02[RGB]:

 parfor j = 1:8100

     for k = 1:6000

         S(j,k,:) = CAM02(j,k,:); 

     end  
 end

%CAM03[RGB]:

 parfor j = 1:6000

     for k = 1:8100

         S(k,j+6000,:) = CAM03(k,j,:);   

     end  
 end

%CAM04[RGB]:

 parfor j = 1:6000

     for k = 1:8100

         S(k+8100,j+6000,:) = CAM04(k,j,:);   

     end  
 end

For CAM01, CAM02, CAM03 I am using offset inside the PARFOR but I dont have issues. Only for CAM04.

Any suggestion about?

Thanks,

Leonardo.

[Edric Ellis]

If that's all you are doing inside parfor, it's surely much quicker to use indexing expressions like so:

S(8101:16200, 1:6000, :)     = CAM01(1:8100, 1:6000, :);
S(1:8100, 1:6000, :)         = CAM02(1:8100, 1:6000, :);
S(1:8100, 6001:12000, :)     = CAM03(1:8100, 1:6000, :);
S(8101:16200, 6001:12000, :) = CAM04(1:8100, 1:6000, :);

(I'm not sure why parfor can handle one offset but not two...)

[Leonardo Filho]

Hi Edrick, Thanks again by your response. The issue is that is not a simply "indexing question". In the parloop I need to use i,j values of the synthetic image to calculate new values for i and j (named "c" and "l") to get RGB values in each camera (CAM01,02,03 and 04). Bellow you can see all code for one CAM01:

parfor j=8101:16200 for i=1:6000

        %projective equations DX/DZ e DY/DZ
        DX = ms1(1,1)*((res_pixel*((i-1)-Cx)+(s1(16,1)*res_pixel))-s1(10,1)) + ms1(1,2)*((-res_pixel*((j-1)-Cy)+(s1(17,1)*res_pixel))-s1(11,1)) + ms1(1,3)*(Zp-s1(12,1));
        DY = ms1(2,1)*((res_pixel*((i-1)-Cx)+(s1(16,1)*res_pixel))-s1(10,1)) + ms1(2,2)*((-res_pixel*((j-1)-Cy)+(s1(17,1)*res_pixel))-s1(11,1)) + ms1(2,3)*(Zp-s1(12,1));
        DZ = ms1(3,1)*((res_pixel*((i-1)-Cx)+(s1(16,1)*res_pixel))-s1(10,1)) + ms1(3,2)*((-res_pixel*((j-1)-Cy)+(s1(17,1)*res_pixel))-s1(11,1)) + ms1(3,3)*(Zp-s1(12,1));

        %image coordinates without distortion
        x = -s1(1,1)*(DX/DZ);
        y = -s1(1,1)*(DY/DZ);

        %Radial simmetric distortion k0,k1,k2,k3
        r = sqrt((x^2)+(y^2));
        drad_x = (s1(4,1)+(s1(5,1)*r^2)+(s1(6,1)*r^4)+(s1(7,1)*r^6))*x;
        drad_y = (s1(4,1)+(s1(5,1)*r^2)+(s1(6,1)*r^4)+(s1(7,1)*r^6))*y;

        %Decentering distortion p1,p2
        ddesc_x = s1(8,1)*(r^2+(2*x^2))+2*s1(9,1)*x*y;
        ddesc_y = s1(9,1)*(r^2+(2*y^2))+2*s1(8,1)*x*y;

        %Coordinates with distortion
        x_dist = x + s1(2,1) + drad_x + ddesc_x;
        y_dist = y + s1(3,1) + drad_y + ddesc_y;

        %double value (column and line)
        c1 = ((x_dist/res_pixel2)+Cx2)+1;
        l1 = ((-1*(y_dist/res_pixel2))+Cy2)+1;

        %int value (column and line)
        c2 = fix(((x_dist/res_pixel2)+Cx2)+1);
        l2 = fix(((-1*(y_dist/res_pixel2))+Cy2)+1);

        %Bilinear interpolation:
        dx1 = double(c1 - c2);
        dy1 = double(l1 - l2);

        a1 = double(CAM01(l2,c2,:));
        a2 = double(CAM01(l2,c2+1,:));
        a3 = double(CAM01(l2+1,c2,:));
        a4 = double(CAM01(l2+1,c2+1,:));        

        RGB = uint8(a1 + (dx1*(a2 - a1)) + (dy1*(a3 - a1)) + (dx1*dy1*(a1 - a2 - a3 + a4)));

        S(j,i,:) = RGB;                      

    end
end

The parloop is working fine but the main problem is the speed. Is too slow....

Any suggestion?

Thanks!