下雪效果js

/*
   DHTML PNG Snowstorm! OO-style Jascript-based Snow effect
   --------------------------------------------------------
   Version 1.0.20110101
   Dependencies: GIF/PNG images (0 through 4.gif/png)
   Code by STAGE - http://www.stage.my
   --------------------------------------------------------
   Description:
 
   Initializes after body onload() by default (via addEventHandler() call at bottom.)
 
   Properties:
 
   usePNG
   ---------------
   Enables PNG images if supported ("false" falls back to GIF)
 
   flakeTypes
   ---------------
   Sets the range of flake images to use (eg. a value of 5
   will use images ranging from 0.png to 4.png.)
 
   flakesMax
   ---------------
   Sets the maximum number of snowflakes that can exist on
   the screen at any given time.
  
   flakesMaxActive
   ---------------
   Sets the limit of "falling" snowflakes (ie. moving, thus
   considered to be "active".)
 
   vMax
   ---------------
   Defines the maximum X and Y velocities for the storm.
   A range up to this value is selected at random.
 
   flakeWidth
   ---------------
   The width (in pixels) of each snowflake image.
 
   flakeHeight
   ---------------
   Height (pixels) of each snowflake image.
  
   flakeBottom
   ---------------
   Limits the "bottom" coordinate of the snow.

   snowStick
   ---------------
   Allows the snow to "stick" to the bottom of the window.
   When off, snow will never sit at the bottom.
 
   snowCollect
   ---------------
   Enables snow to pile up (slowly) at bottom of window.
   Can be very CPU/resource-intensive over time.
   Also requires snowStick = true.

   followMouse
   ---------------
   Allow the mouse to affect the "wind", left-to-right.

*/