Understanding Mesh.MarkDynamic in Unity4

[cecarlsen]

Hi

So in Unity 4 there is a new Mesh method called “MarkDynamic()” that is supposed to create “better performance when continually updating mesh”. But what does it actually do?

Carl Emil

[lordofduct]

I’m not sure, but I’ll bet it flags it so the mesh doesn’t get cached or something. There’s no reason to cache a mesh that is constantly being altered… and caching is work, so not caching will up performance in the case of a mesh that updates frequently.

[cecarlsen]

Thanks lordofduct. I tested it a bit with a mesh compound system I’m building and found no significant performance boost. I’d like to know in more detail what it does. Perhaps a Unity developer could explain?

[Marrrk]

This is a part from the DirectX documentation which explains it very well, but must not be the most fitting in the case of how Unity does things:

Locking a static vertex buffer while the graphics processor is using the buffer can have a significant performance penalty. The lock call must wait until the graphics processor is finished reading vertex or index data from the buffer before it can return to the calling application, a significant delay. Locking and then rendering from a static buffer several times per frame also prevents the graphics processor from buffering rendering commands, since it must finish commands before returning the lock pointer. Without buffered commands, the graphics processor remains idle until after the application is finished filling the vertex buffer or index buffer and issues a rendering command.

Ideally the vertex or index data would never change, however this is not always possible. There are many situations where the application needs to change vertex or index data every frame, perhaps even multiple times per frame. For these situations, the vertex or index buffer should be created with D3DUSAGE_DYNAMIC. This usage flag causes Direct3D to optimize for frequent lock operations. D3DUSAGE_DYNAMIC is only useful when the buffer is locked frequently; data that remains constant should be placed in a static vertex or index buffer.

[Dreamora]

In case of Unity I think I recall a thread where it was mentioned that it also impacts how the mesh is stored to be more optimized and naturally it will not constantly resend the mesh as it does not change, making best use of VBOs etc.
though U4 in general has a much more opted mesh storage (distinct buffers and only reuploading the vertex buffer not the index buffer, uv buffer etc which already can have a major impact) than 3 so the direct impact will likely require a more sophisticated benchmarking when comparing it to U3

[Alesk]

Hi,

I would like to know when this function has to be executed : only once after the mesh creation, or before every update of its content ?

[numberkruncher]

@Alesk

I would like to know when this function has to be executed : only once after the mesh creation, or before every update of its content ?

The documentation for this totally sucks.

I call this function before setting my vertices since I have been led to understand that it simply sets an internal flag.