# Ambient Dust gravity?

I have been messing around with the Starter Content P_Ambient_Dust and I can’t seem to figure out how to get it to spawn at that top and fall instead of float around. Is this even possible? I really like the way the dust looks but I want to make it fall at a constant rate.

Thanks for any help.

Check if there already is a Initial Velocity Module. If not add one (with right click on the emitter).

Just set the Start Velocity to something that has negative Z and you should be done.

Thank you, DennyR. I’ll give that a try. But why Z? From my years of computer gaming Y has always been the up and down.

In 3D it is just a matter of definition really and Unreal has choosen Z-Up somewhere along its path. 3Ds Max uses Z-up as well.

Here is good read on the reasons.

This confuses matters more… In that link you just gave, the title is “Why is Y up in many Games?” Not Z.

Easiest way to understand Z up is to picture an architect drawing his plans on paper. The paper lies flat on the table. He draws X line moving his hand left and right, Y moving his hand forward and backward. If he wanted to go to the third dimension he would draw Z by moving his arm up and down perpendicular to the table. Hence Z up.

That is a fairly good example since it also applies to tangent space normal maps.

Say you have a normal map for a sphere. the X channel will be the “side to side” gradient, Y channel will be “top to bottom gradient” and the Z channel will be “Front to back” gradient. But you lay down that normal map on the ground, and suddenly the Z channel is actually pointing up into the air, off the texture sheet while X and Y are planar to the mesh surface normals.

so in the case of a mesh laying on the ground like that, a transform tangent to world would not actually do anything (assuming the X,Y directions were right), since the mesh already happened to by laying with its local tangent vectors aligned to the world space axes.