on Dec 11, 2014 at 6:36:36 pm Last Edited By Adam Trachtenberg on Dec 11, 2014 at 6:45:07 pm
I don't think you can do it with a standard emitter because it isn't intended to allow animation of the birth rate. You'd probably have to use Thinking Particles.
To set the rate you have to determine the speed of the point. You can do that by feeding the point's position into an Xpresso memory node. The memory node allows you to output the value of a parameter at an earlier time. So if you input point position and set a history depth of 2 and Level 1, you'll get the point's position at the previous frame.
Now you can use the distance node to measure the distance that the point covered in a single frame. That in effect gives you what you need for speed, since speed is Distance/Time. So you can feed the memory node output directly into the PStorm's rate input, or you can pipe it through a range mapper for more control.