[julia-users] Grid pattern in colorplot

[Matthias Redies]

I'm running this program to generate a plot:
using Images, Colors, FixedPointNumbers, Gadfly


function cond(color)
   mean = (color.r + color.g + color.b) / 3.
   var = (color.r - mean)^2 + (color.g - mean)^2 + (color.b - mean)^2
   if sqrt(var) < 0.25
       return 1
   else
       return 0
   end
end




function x_array(dim)
    buf = zeros(Int, dim)
    for i = 1:dim[1]
        for j = 1:dim[2]
            buf[i,j] = i
        end
    end
    return buf
end


function y_array(dim)
    buf = zeros(Int, dim)
    for i = 1:dim[1]
        for j = 1:dim[2]
            buf[i,j] = j
        end
    end
    return buf
end




function on(img)
    buf = zeros(UInt8, size(img))
    for i = 1:size(img)[1]
        for j = 1:size(img)[2]
            buf[i,j] = cond(img[i,j])
        end
    end
    return buf
end


img = imread("test.png") 
pap = on(img)
lX = x_array(size(img))
lY = y_array(size(img))
draw(PNG("output.png", 18cm, 11cm), plot(x=lX, y=lY, color=pap, Geom.rectbin
, Theme(background_color=color("white"))))

This results in the following plot:

<https://lh3.googleusercontent.com/-8C3SfWCfS0k/Vg8Qt_JiMJI/AAAAAAAAHp4/vJfiJJ48UPw/s1600/output.png>
Since the cond-function returns only 1 or 0, the plot should have only 2 
colors, but there seems to be a grid pattern across the whole picture. 
Where does this come from and how do I remove it?