Crabs walking on wet beach, leaving traces

[ljilekor]

We need to set up crabs walking on a wet beach, leaving traces on the wet sand everywhere a paw hits the ground floor.

Because we need to render in Modo, I'd like to generate a displacement-map sequence of the traces on the ground-floor in Maya and use it into modo.

The obvious way to do this would be to paint (art3DPaintCtx) a dot (or other brush) on the floor mesh each time one of the paws hits the ground and export the resulting groundfloor texture on every frame.

I'm able to catch the the exact UV coordinate on the floor surface each time a paw hits the floor surface (nearestPointOnMesh etc...). The art3DPaintCtx provides everything I need to choose the right brush orientation/pressure/... and to generate (save) the displacement map sequence.
So everything is set up fine... except...

The only missing link is the 'actual painting' on the surface/mesh. To manage the 8 paws of a lot of crabs the 'actual painting` should be triggered through MEL. But I can't find any MEL function that does the 'actual painting'.

I was wondering if it is possible to paint onto an object using only MEL (no interactive painting in a viewport).
Is it possible to paint using 'only' mel? If yes, how?

Thanks in advance.

[joojaa]

Well the short aswer is no.

The long aswer is maybe. Its possible under some circumstances, but its a very long way around. Basically you can do it in mel, but your not doing it directly but rether indirectly trough a noniteractive render, that doesnt really have anyting to do with paint effects per see (even if you use pfx).

So now the only thing i need to know to answer your question simply and efficently where does the line of what mel does do and whats not done in mel end. You can do this in mel but is it ONLY mel? Insofar as i see it theres nothing you can do in only mel.

[ljilekor]

Ok, major thanks for looking into this.

Well... , stripping it down to the bare essentials I have:
- A floorplane (Floor_Mesh) set up for painting : place2dTexture & file nodes are connected.
- A locator (PawOnFloor_Locator) that gives me the exact UV coordinates of the locator on the floorplane on impact. (using nearestPointOnMesh Node)
- All 'triggering the paint, 'setting up the brush' andsaving the texture every frame' MEL code.

The actual code is not written yet but I see no problems concerning all the above.
The only missing link is the 'actual painting' MEL code:

pseudo code:

global proc paintADot (  string $art3dPaintCtx,
                          string $Floor_Mesh,
                          float $uv[];
                          string $pawOnFloor_Locator)
{
    // $art3dPaintCtx         -> the art3dPaintCtx (texture assigned / Paint Effect Brushes set / Paint Attribute set on "Color" / ...)
    // $Floor_Mesh                -> the actual mesh on which to paint (Shape)
    // $uv                    -> a float array with 2 elements representung U & V values on the $Floor_Mesh {.5 , .5}
    // $pawOnFloor_Locator    -> Optional : 3D transformations of the place of impact may be needed... ?

       ... stranded...

}

What we need now is the functionality to 'paint on surface at uv coordinates' or 'place mouspointer over pawOnFloor_Locator in viewport and click!'

or ...

I'm really lost on how to get this done...

[nathan1347]

Hi !
I see an alternate way of doing this, that might workout smooth. It needs a 3delight (www.3delight.com) but the good news is that one licence is FREE, even for commercial use.

So here is my idea: 3delight has a tool (tdlmake) that can convert a txt bake file to a 32bit tiff texture.
The bake file is just an ASCII list of (UV,color) pairs, so it's trivial to write this for every frame of animation. You should probably export several samples around a collision point within some radius, possibly modifying values to create nice bulging of sand.
Then you convert your .bake sequence to TIFF, and load TIFF sequence to comp, to create time  accumulation - so the dots can last for few seconds and fade out...
The output from tdlmake.exe is actually a mipmapped 32bit tiff, so you might have troubles opening it in comp, but photoshop loads them for sure.
One possible caveat is filtering at conversion syage, that can give unexpected results - I think ensuring enought resolution and samples will solve it.

I used similar technique in past, to generate acceleration maps on deformed meshes. I had particles goaled, and wrote .bake files reading acceleration and goalUV from particles. I then used those maps to drive emission of water droplets.

good luck with your crabs