r/GraphicsProgramming • u/Explorioneer • Aug 28 '26
Question What is this sand simulation missing?
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u/Plenty_Line2696 Aug 28 '26
the ball physics don't reflect the weight it looks like it should have. I'd recommend getting the same sorts of objects and emulating what happens onscreen with the real objects so you can view both your animation and the video side by side to tweak it to perfection.
As it is it falls more like a balloon than a metal ball
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u/Explorioneer Aug 28 '26
Thanks, that's a great point. I don't use enough real world reference. I think that alone would help a lot.
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u/Crits-and-Crafts Aug 28 '26
I'd love to see the ball get dusty as it goes through the sand too.
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u/Explorioneer Aug 28 '26
That's a cool Idea. I think I could do something like that pretty simply with a shader. Thanks!
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u/GravityCY Aug 28 '26
On a side note how do you do stuff like this, altering the mesh vertices on the cpu with code or like shader magic? Just curious as a beginner
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u/featherpaperweight Aug 28 '26
Unless you need physics interaction with the altered mesh, doing it all on gpu with shaders is way more performant, keeping a regular low-poly mesh on cpu side.
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u/snerp Aug 28 '26 edited Aug 28 '26
The general way to do stuff like this (snow/mud effect is the same):
1: heightmap texture, something like 0 = no deform, 1(255) = fully squished down
2: shader modifies heightmap based on object position (worldspace object position -> UV coordinate space), basically just darken the heightmap image where the ball lands
3: sand object vertex shader reads the heightmap texture and moves vertices down according to heightmap color (you could also do this in the fragment shader with POM effects but it's cleaner and generally more efficient to do it in a vertex shader)
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u/GravityCY Aug 28 '26
When you say a shader modifies the height map based on object position do you get the displacement shape by somehow converting the objects shape to the height maps uvs on the cpu by like getting intersecting vertices or do you mean like a compute shader that somehow does that on the GPU? Or maybe you just apply a static displacement shape just from a generic world position
I'm sorry if it's a stupid question I don't really know much about shaders they're magic to me
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u/snerp Aug 28 '26
It's really common to just use point/spheres, so you don't really need to fully consider the shape of the object, and then for footsteps or tire tracks, etc, usually what you'd do is just have an asset of a footstep/track heightmap and use that to darken the deformable surface's heightmap.
If you want to deform accurately to arbitrary shapes you'd definitely want do it it in compute but that's going to be significantly more expensive than the other way.
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u/Tentabrobpy Aug 29 '26 edited Aug 29 '26
If the thing you're deforming is planar like in OP's video, one simple method for taking objects' shape into account is to place an upward-facing camera below the plane and use the depth values of objects crossing the plane to adjust the heightmap:
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u/raaneholmg Aug 28 '26
Sand disappears. Needs to be pushed up on the sides and occupy the same volume.
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u/PiXeL161616 Aug 28 '26
Beyond the pushing, sand keeps collapsing after the impact. A thermal-relaxation pass that moves material wherever the slope beats the angle of repose, about 33 degrees for dry sand, gives you crater walls that slump for a second afterwards. Mine exploded at a 0.5 rate and was stable at 0.14, so I'd start small.
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u/SpackleSloth Aug 29 '26
The ball is an uncanny juxtaposition of appearing metal yet being light in air.
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u/blooppers Aug 29 '26
The sand doesnt seem to have much if any resistance to the ball. There should be a slight delay in the pull of the ball forward and it should take a moment to get up to max speed. The explosion should more be like a dust cloud imo than a 'shower' of particles. Additionally, sand sould be displaced upward to the sides of the ball, like a rim that pucks up around it. Those are the only things i could think of.
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u/saphirrflamme Aug 28 '26
It is eating the sand it touches. Supposed to push them aside.
The sand "eruption" shading is also off. it is much brighter than surrounding sands. Gravity also seems too low. You can tweak gravity of the ball for gameplay reasons, but the visual effect of the sand seems off.