In this tutorial, we will continue to customize our snowfall particle system. So far we have set the Shape parameter for our snowfall to the Line option. We will continue working with the Emitter Controls in the Motion Inspector and HUD. At this point in the Inspector, the Emission Range slider control is set to the default 360 degrees and the Emission Angle is set to the default of zero degrees. In the HUD, the Emission Range is also set to 360 degrees. Notice the appearance of the circular display under the Emission Range slider control in the HUD.

1. To make the snowflake particles move only downward from the Line Emitter, let's move the slider control in the Inspector until the Emission Range value is 180 degrees.
   
   Notice the changes in the HUD. The Emission Range control is set to 180 degrees and the display has changed in that the arrows are only pointing outward from one half of the circle. Finally, you will notice on the Canvas that the snowflake particles above the Line Emitter have disappeared.
   
   
2. One next step is to move the Line Emitter to the top of the screen, so that it will appear that the snowfall begins in the upper sky.
   
   Play the animation for another 2 seconds and you will notice that the particles are all moving downward. But not all are moving downward in a straight line. Some are falling within the range of 180 degrees from the line. That is to say, some snowflakes are falling downward and to the right or left and some fall straight downward.
   
   What would happen if we set the Emission Range to 0 degrees?
   
   
3. Set the Emission Range to 0 degrees and play the animation again for a few seconds.
   
   You will notice that now all the particles are falling straight downward, that is perpendicular to the Line Emitter. You will also notice that all the blue arrows in the HUD are pointing directly perpendicular to the white control handles.
   
   

So what have we accomplished? We now have a way to make our snowflakes fall downward on the Canvas, by setting the Emission Range to 180 degrees. We can set the Emission Range to 0 for a heavy snowfall, as the flakes will fall straight down. For a more delicate snowfall, keep the Emission Range to 180, so that some of the flakes move slightly to the left or right when falling.

Tutorial Example:


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