Some explanation:
The user defined function is something
which should return true or false
to indicate that something should
be drawn on this pixel.
The color user defined function should
return a number indicating the
red, blue, green or alpha value.
This value can
be anything, but it will
be sized down to the range 0-254 via modulo.
'x' and 'y' can be used within the text fields
and javascript methods are available.
Providing a user definition
and pressing
enter or the update
button will override
the current selected option.