Trying to plot a checkerboard on a graph using rectangles and for loop.

So for a 'background' of a graph I am working on must be in the style of a checkerboard, I have worked out how to do it for two squares but not sure what values to change to get it to the right size for the axis.

for k=1:2
  a=0;
  if(k==1)
rectangle('Position',[a 0  10 10], 'FaceColor' , 'white');
  else
      rectangle('Position',[a+10 0  10 10], 'FaceColor' , 'black');
  end
end

The axis is -

axis([0,160,-60, 60]);

Thank you in advance

Answer 1

[Kelly Kearney]

By using the rectangle command, you're going to end up with a lot of patch objects, which will work, but the patch command allows you to create multiple polygons with a single call, and is usually my preference when creating lots of shapes with the same number of vertices.

% Lower left corner of each box

x = 0:10:150;
y = -60:10:50;

[x,y] = meshgrid(x,y);

% Checkerboard values

if mod(size(x,1),2)
    val = ones(size(x));
    v(1:2:end) = 2;
else
    val = ones(size(x,1)+1,size(x,2));
    val(1:2:end) = 2;
    val = val(1:end-1,:);
end

% Coordinates of a box, relative to lower left corner

xref = [0 0 10 10 0];
yref = [0 10 10 0 0];

% Add to get coordinates of each box

xbox = bsxfun(@plus, x(:), xref);
ybox = bsxfun(@plus, y(:), yref);

% Plot using patch with RGB cdata

cmap = [0 0 0; 1 1 1];
col = permute(cmap(val(:), :), [3 1 2]);

h = patch(xbox', ybox', col);

Answer 2

[Geoff Hayes]

Lex - your code creates two neighbouring squares, each of size 10x10. Suppose you just wanted to draw one full row given your x-axis dimension which is from -60 to 60. You know that each square is of size ten, so you can just move from 0 to 160 in steps of 10, drawing the square in the correct colour

 figure;
 clr = [1 1 1];  % initial colour is 1 which will correspond to white

 for x=0:10:150 % we don't end at 160 because then we would draw a rectangle beyond our axis

     rectangle('Position',[x 0  10 10], 'FaceColor', clr);

     if x~=150
         clr = ~clr;
     end
 end

 axis([0,160,-60, 60]);

The above just takes exactly what you did and extends it for one complete row in your figure. Note how we make use of the fact that the checkerboard has white or black tiles. Rather than specifying 'white' or 'black' as the FaceColor, we can use an RGB vector (with values between 0 and 1) to do this instead: [0 0 0] corresponds to black, and [1 1 1] corresponds to white. On each iteration of the loop we just invert the clr vector to switch from white to black. And we only do that switching for every iteration except the last one...which will become important as we move to the next line whose initial colour should be the same colour as the last colour on the previous line.

So if one loop allows us to create a row, we would need a second loop to do the same for multiple rows along the y-axis. Using the above as a guide, how would you modify it to draw multiple rows?