Custom linear congruential generator in JavaScript

I am trying to create a custom linear congruential generator (LCQ) in JavaScript (the one used in glibc).

Its properties as it’s stated on Wikipedia are: m=2^31 , a=1103515245 , c=12345.

Now I am getting next seed value with

x = (1103515245 * x + 12345) % 0x80000000 ; // (The same as &0x7fffffff)

Although the generator seems to work, but when the numbers are tested on canvas:

cx = (x & 0x3fffffff) % canvasWidth; // Coordinate x (the same for cy)

They seem to be horribly biased: http://jsfiddle.net/7VmR9/3/show/

Why does this happen? By choosing a different modulo, the result of a visual test looks much better.

The testing JSFiddle is here: http://jsfiddle.net/7VmR9/3/

Update

At last I fixed the transformation to canvas coordinates as in this formula:

var cx = ((x & 0x3fffffff)/0x3fffffff*canvasWidth)|0

Now the pixel coordinates are not so much malformed as when used the modulo operation.

Updated fiddle: http://jsfiddle.net/7VmR9/14/

Answer

For the generator the formula is (you forgot a modulus in the first part):

current = (multiplier * current * modul + addend) % modulus) / modulus

I realize that you try to optimize it so I updated the fiddle with this so you can use it as a basis for the optimizations:

http://jsfiddle.net/AbdiasSoftware/7VmR9/12/