The Fade alteration will cause the sprite to incrementally change its transparency.
This effects the alpha value of the sprite. Therefore, to fade, you must specify a negative value for the rate and to unfade, use a positive value.

create routine as Start launch horse end create sprite from horse12.png as horse where x=100 and y=100 having alt=(sub create fade as f where rate=-0.5 and until=0.1 end) end


a fade rate per second. Use a negative value to fade (increase transparency) and a positive value to make the sprite more opaque.
the value at which the fade should end. Also, the final alpha value for the sprite. Must be a value between 0.0 and 1.0. Be sure this number makes sense with respect to the initial alpha of the sprite and the rate being negative or positive.
specify a routine to run when the fade is complete.