Is there a way to assign mesh.vertices[i] directly

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Hi I actually simulate a deformable surface (using mass-spring model as this used to simulate clothes for instance) and draw the mesh every frame.

To draw it i follow what's explain on this link (http://docs.unity3d.com/Documentation/ScriptReference/Mesh.html) at section '3. Continously changing the mesh triangles and vertices'.

However, the framerate decrease a lot, and when profiling I aware it was the garbage collector (gc.collect() call) in my drawing method which takes the most of the time by frame.

Is there a way to directly modify mesh.vertices[i] = new Vector3(....) for instance instead to pass an array which is going to be copy in the mesh and so garbage collected after ?

Thanks a lot

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Answer by Fattie · Mar 15, 2013 at 12:57 PM

are you asking, can you modify JUST ONE of the verts? no, you can only write the whole .vertices at the same time

I don't understand the graphics pipeline, but I believe the reason is possibly because it writes the whole thing to the graphics chips at once

yes you should certainly keep your own array ("yourArray")

and modify it as you wish.

then simply write that same array to the vertices (.vertices = yourArray)

never GET the vertices like this ... some other array of yours == mmm.vertices

never do that. it makes a copy each time.

(OK you could do it once when you launch, but generally don't it in play.)

http://answers.unity3d.com/questions/352162/optimized-vertex-manipulation.html
http://answers.unity3d.com/questions/352513/ways-of-modificating-mesh-triangles.html
http://answers.unity3d.com/questions/334473/can-i-move-just-some-of-the-verts-on-a-mesh.html

it's a bit of a gotchya in Unity, it only mentions it subtly in the doco. But you have to be super-aware of the few functions that do make a copy each time. It only took me like 5 years to figure that out :)

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Bunny83 · Mar 15, 2013 at 01:42 PM 0

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Yes, it's a pity they don't offer a custom "Lock" function to write only a part of the vertex / index array.

Anyway, good explanation. +1

Fattie · Mar 15, 2013 at 01:45 PM 0

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dude I just copied all your previous text on the subject ;-)

yes, it would be totally incredible - a huge change in the world - if you could one way or another write faster to the mesh. everything I work on in R&D relates to this

what would be even more incredible is if I understood WTF actually happens in the pipeline after you go .vertices = ;-)

Owen-Reynolds · Mar 15, 2013 at 03:36 PM 1

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"it writes the whole thing to the graphics chips at once"

Yes. In the old days, only texture data was stored in the GPU. Each drawcall still sent all the vertex data, each frame.

Now, VBO's (vertex buffer objects) also store all the vertex data in the graphics card, so the drawcall just says "model 17." The final format is (probably) not the same as in the CPU, so it hates to find&change just one.

In theory, you'd turn off using VBOs if you intend to change a model each frame (but I've never tried and don't know if it's possible, how it would affect batching... .)

mlechene · Mar 15, 2013 at 03:52 PM 0

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Thanks for quick replies. First yes, it seems mesh.vertices = myArrays copied all array in GPU, it's why it is not an efficient way when we attempt to modify the mesh each frame. I'm gonna look at the links send above to search other possible solutions to achieve it without decrease my framerate. As I already said, the issue is not to keep my own vertices coordinates, but the gc.collect called after assigning vertices and triangles !!

Fattie · Mar 15, 2013 at 04:02 PM 0

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Hi @mlechene, yes if I was not clear sorry. You must never do this: yourArray = mesh.vertices. Never do that.

@Owen - sounds amazing. NOw I will use the term "VBO" everywhere to try to sound intelligent. Thanks for that! Awesome. It's incredible you used to have to send it every drawcall. NOw I want to know how to turn off the VBOs.

Is there a way to assign mesh.vertices[i] directly

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