Variables

In order for the expression to refer to a pixel in the original image, a few variables are set:
xy (xy:2)
The cartesian coordinates of the pixel.
x (nil:1)
The first component of the cartesian coordinates of the pixel.
y (nil:1)
The second component of the cartesian coordinates of the pixel.
ra (ra:2)
The polar coordinates of the pixel.
r (nil:1)
The first component of the polar coordinates of the pixel (0 <= r < 2*pi).
a (nil:1)
The second component of the polar coordinates of the pixel (the distance from the center).
To make it easier to write expressions which depend on the image size, a few additional variables are set:
WH (xy:2)
The size of the image.
W (nil:1)
The width of the image.
H (nil:1)
The height of the image.
R (nil:1)
The biggest possible value for r for the image.
XY (xy:2)
The biggest possible value (in both components) for xy.
X (nil:1)
The biggest possible value for x for the image.
Y (nil:1)
The biggest possible value for y for the image.
For the purpose of animations two additional variable are set:
t (nil:1)
The time which is 0 <= t < 1. If animation is disabled, the value of t can be chosen in the Settings tab. If you want to make animations loop, set the 'Periodic' check-box in the Settings tab and make sure that the images at t == 0 and t == 1 are the same.
frame (nil:1)
The number of the current frame, beginning with 0 for the first frame.