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GPU Particles, Fixed Bounding Box, Other Assets


Recently decided to make a waterfall within UE4 for my project but have run into some major issues. I started with the template located in the "Effects Cave" map titled "P_Waterfall2" to see if I could reverse engineer it and learn something. However, after watching nearly every tutorial I could find as well as reading all of the documentation on "Fixed Bounding Boxes" I am at a loss.

Whenever I first come into viewing distance of the waterfall it flows decently, however, whenever I walk behind a tree it cuts the waterfall in half. What was beneath the cut continues to flow but whatever was above ceases. If I walk straight back from the waterfall enough of a distance I can still see it from a mile away but, the same issue, if I walk behind a tree it instantly cuts the waterfall in half again. I've checked the box for "Use Fixed Bounding Box" and I've played with all of the attributes at -1,-1,-1, and 1, 1, 1.. all 0's.. and even up to -500,000 and 500,000.. Put in solid 99999999999999 and maxed it out in each positive and negative direction. It does not matter what I set the distance to, the waterfall is immediately cut in half whenever I pass through another object.

Please help. =(

Also, is there no way to set an initial state for the particles like you can in Maya? I've set "warmup time" between 1 and 10 and nothing happens.

Sincerely hope someone can help me out with this issue. I've been struggling with it for at least 5 hours.

Thank you,


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asked Oct 31 '14 at 11:31 PM in Using UE4

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avatar image Lovecraft_K ♦♦ STAFF Nov 07 '14 at 03:28 PM

Hey Tarrtarus -

Can you post a video or series of screenshots showing exactly what your troubles look like? I have tried to reproduce what you are describing and haven't been successful as of yet.

Thank You

Eric Ketchum

avatar image Tarrtarus Nov 07 '14 at 07:14 PM

Eric! Thank you for taking the time to assist with this issue. It is much appreciated!

Video of issue: http://youtu.be/QOr-Td_GqGg

Fixed Bounding Box Screenshot: http://imgur.com/9eY155B

Thank you,

Christopher Bolyer

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Hey Tarrtarus -

Ok so I know what the issue is and its a little complicated to explain so stay with me. In the setup you have in your video you are rendering a GPU Particle Cascade Waterfall that collides with the rocks as it falls down in the scene. This happens because the Particle System has Collision based on Scene Depth. Scene Depth is actually a viewable Buffer Visualization in the engine (I would recommend looking at it in your scene as I explain this as well.) GPU particles only have collision based on that depth reading which is great because it offers a very performance friendly way of having an effect like this waterfall, but it does have a limitation. That limitation is that the scene depth is based on your scene itself. So when you move the camera to occlude the Particle System and render the tree in front of it, your scene depth no longer includes the rocks in the scene and your GPU Particles actually fall through the meshes. (A really good way to see this is to select the Particle System in the level and then go behind the tree, you should still see the selected highlights of the system but now it looks like it is flowing straight down instead of following your rocks.)

The ways around this would be to use a CPU particle system which can call actual collision with meshes or (and this is more prevalent in games) create a mesh that follows the shape of your waterfall and then animate a material to simulate the flowing of the water and highlight the mesh with smaller GPU effects like mist clouds or splashes.

Thank You

Eric Ketchum

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answered Nov 07 '14 at 08:04 PM

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Lovecraft_K ♦♦ STAFF
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avatar image Tarrtarus Nov 10 '14 at 05:18 AM

Thank you very much!

May not be a fix to the problem but I feel better understanding it!

avatar image Tarrtarus Nov 10 '14 at 05:19 AM

Thank you very much!

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