Dam Environment - Problem


Hi guys.

So. I’ve been modeling this scene of a dam in Max for the last month or so, in my spare time. I’m going to composite it in to some footage I’ve shot as a background. Everything in the lower portion of the image is still very much WIP.

But I’ve run into a problem. I’m trying to use Particle Flow to place small rocks over certain areas of the ground. I’m using a Position Object operator, and ticking the “density by material” option, using the green channel, with mat ID set to 02 (the ground object has a multi/sub-object material, and the geometry uses ID 03).
I also have a vertex paint modifier applied to the ground object, and I’ve painted the vertices near the bottom of the cliff cyan. With the multi/sub-object material applied to the ground, I assign a standard material with a vertex colour for the diffuse for it’s material ID02 input.

So far everything seems to be working just so. In the viewport, I get loads of rocks along the bottom of the cliff. But then when I render, the rocks are scattered all over the ground.
So just as a test, I changed the ground geometry to material ID02. It should render the ground with the vertex colour as the diffuse colour - but the rendered result is totally different from the viewport!

Here’s a viewport screenshot:

Here’s a render, with the ground material applied:

And here’s another render, with the ground using the same material ID (with vertex colour diffuse) as the position object operator.

So would anyone have any ideas why the vertex colour is different in the render compared to the viewport?



Hey man,

To be honest, that sounds like a really complicated way to place a few rocks. Are there any particular reasons you chose that method? Does the camera move?

I know it might sound demeaning, but have you tried just placing the rocks by hand, or using a sculpting tool on a single, high density mesh?



I don’t use Max, so please excuse me if I’m way off here…

Are you rendering from the starting frame? or are you possibly rending a frame of animation shortly after the water scatters those rocks around? I say that because I recognize a singularly distinct rock shape in your viewscreen, which has relocated to the central area.

I don’t know if you’ve noticed, but in the “red sea” looking version, there’s actually a triangular visual artifact going from the rock’s origin point, towards its new location… (wider at the start of the movement, and the rock ended up just beyond the peak of that triangle… like an arrow) This reinforces the thought that something in the physics may be causing it to slide down away from the wall… Or perhaps there is a vector assigned to the emitter which imparts a starting velocity, so it “throws” the rock?

A test for this, assuming you can’t find an unintended animation frame advance culprit, would be to render an area that includes the outer boundaries of the space you’ve used, and possibly also make that “ground” transparent. to get a better idea of where those rocks are actually going, so you can figure-out why they’re going there. The next step might be to remove the ground… or drop it down, to see how that affects the rocks.

Also, in some systems if you overlap materials, they can pass through each other when the “gravity” is active on the item because they’ve already breached the collider box of the object they’re penetrating and are therefore not stopped by it. Sometimes the object will actually fall all the way through, completely cancelling the collider boxes and fall all the way through (usually through the ground and drop into infinity). Other systems will merely trap the item as it tries to pass out through the other side of it and runs into the colliders on that side.

Most emitter systems I’m aware of will impart gravity settings to the particles automatically, especially if you’ve told it that it’s making rocks, rather than dust motes.


AJ1 - I know it’s a complicated way of doing it. The reason I’m using vertex painting is because I intend to have a lot more rocks placed on the ground. But I want to have some control over where they go. For instance, I’m going to have loads of smaller ones near the edge of the water. Placing thousands of rocks by hand would be impractical!
And I also really want to get to the bottom of this vertex paint mystery…

Deadguy71 - The only thing that’s animated in the scene is the camera. The particles are totally static. They are created at frame 000 and they stay where they are. There’s no collisions going on at all.

Just as I was writing this I stumbled upon the solution - I had a turbosmooth underneath the vertex paint in the modifier stack. It seems I accidentally set the render iterations to 0. That’s why all those triangular artifacts were appearing.

Boom. Case closed.
Cheers for the suggestions guys, much appreciated.


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