How to make 4 different colour on four different quadrant?

Hi, i have a question regarding the line colour. It was not equally or orderly distributed. It supposed to be a blue color on -45 degree to 45 degree, a red color starting from 45 degree to 135 degree, a black color from 135 degree to 225 degree. Last, the green color from 225 degree to 315 degree. Below are my coding.

for x = 1:9

    for y = 1:9

          Q=atan(Mnew(x,y));
        if ((Q>=(-pi/4))&&(Q<(pi/4)))
            plot([X(x,y) Xnew(x,y)],[Y(x,y) Ynew(x,y)],'b','LineWidth',3);

        elseif ((Q>=(pi/4))&&(Q<(3*pi/4)))
            plot([X(x,y) Xnew(x,y)],[Y(x,y) Ynew(x,y)],'r','LineWidth',3);

        elseif ((Q>=(3*pi/4))&&(Q<(5*pi/4)))
            plot([X(x,y) Xnew(x,y)],[Y(x,y) Ynew(x,y)],'k','LineWidth',3);

        else
            plot([X(x,y) Xnew(x,y)],[Y(x,y) Ynew(x,y)],'g','LineWidth',3);
        end

        hold on;
    end
end

Mind to tell you that this is only half of my coding, and i hope anyone can check whether my coding for color distribution are correct or not.

Answer 1

[Geoff Hayes]

Nik - the documentation for atan indicates that calling this function returns values in the interval [-pi/2,pi/2]. You should probably use atan2 instead as it will return values in the interval of [-pi,pi]. With that in mind, you will need to adjust your checks for whether Q is less than 225 degrees and whether Q is less than 315 degrees (you will need to use the negative equivalents of -135 and -45 respectively).

[Nik Lutfi Jamal]

Thank you for your help Geoff. However i still cannot run my matlab project. It said that there are not enough input arguments. Here below are my coding, perhaps you can help me to figure out something.

function FocusPoint()

[x,y] = meshgrid(1:9,1:9);
m = zeros(9,9);
plotbargraph(m,x,y);
for k = 1:1 
[x1,y1,button] = ginput(1);
while button ~=3
m1 = (y-y1)./(x-x1); 
plotbargraph(m1,x,y);
[x1,y1,button] = ginput(1); 
end
end
end

function plotbargraph(M,X,Y)

   C = Y-M.*X;
   r = 0.8;
   a = 1+M.^2;
   b = 2*M.*C-2*X-2*M.*Y;
   c = X.^2+C.^2-2*C.*Y+Y.^2-r^2;

   Xnew = (-b+sqrt(b.^2-4*a.*c))./(2*a); 
   Ynew = M.*Xnew+C; 
   Mnew = (Y-Ynew)./(X-Xnew); 

for x = 1:9
    for y = 1:9

          Q=atan(Mnew(x,y));
          if ((Q>=(-pi/4))&&(Q<(pi/4)))
              plot([X(x,y) Xnew(x,y)],[Y(x,y) Ynew(x,y)],'b','LineWidth',3);

          elseif ((Q>=(pi/4))&&(Q<(3*pi/4)))
              plot([X(x,y) Xnew(x,y)],[Y(x,y) Ynew(x,y)],'r','LineWidth',3);

          elseif ((Q>=(3*pi/4))&&(Q<(5*pi/4)))
              plot([X(x,y) Xnew(x,y)],[Y(x,y) Ynew(x,y)],'k','LineWidth',3);

          else
              plot([X(x,y) Xnew(x,y)],[Y(x,y) Ynew(x,y)],'g','LineWidth',3);
          end

          hold on;
      end
  end
     hold off;  
  end

[Geoff Hayes]

Nik - I don't observe any errors when I run the above code, so perhaps you have copied and pasted something different? As well, you need to use atan2 which requires two inputs (perhaps that is where the error is being generated?).

Answer 2

[Thorsten]

Use atan2 to get the four-quadrant inverse tangent.