Custom allocator for a uniform buffer of SVector/SMatrix

[JMLX42]

Hello @sebcrozet !

Incredible work on this project. I hope you're doing well 😄

I am working on a new version of the open source SmartShape Engine written 100% in Rust using wgpu.

WebGPU has uniform buffers: the goal is to avoid many calls to glUniform() with a single call to set a uniform buffer. I would like to take it one step further.

I would like my Rust code to do math on Vector and Matrices that are allocated in a uniform buffer. Presumably a Vec<u8> or [u8]. The goal is to make sure the underlying memory for a specific set of Vector/Matrices is a contiguous buffer than can directly be used as a uniform buffer without copy.

I have read the Computer-graphics recipes already. But the difference is it relies on copying the Vector/Matrix data to the uniform buffer.

My first take would be that I would need to implement a custom allocator for static SVector and SMatrix. Am I going in the right direction? Does it even make sense?

Thanks!

[sebcrozet]

Hey!

I think using a custom implementation of the Allocator traits would be too complex (if at all possible) since there is no way to override the DefaultAllocator used for the output of most operations in nalgebra. So there is no easy way to force any operation of nalgebra to output the result of any operation (that doesn’t operate in-place) into your own buffer. The Allocator system in nalgebra is mainly a way for us to deal with dimension-dependent storage, rather that something users can customize.

If all the elements of your uniform buffer are supposed to have the same type, you can enable the bytemuck feature of nalgebra, and use the bytemuck crate to convert your &mut [u8] to, for example, a &mut [Vector3<f32>], and store the result of your math ops on that slice (or apply in-place math operations on these vectors).

[JMLX42]

@sebcrozet thank you for your answer!

If all the elements of your uniform buffer are supposed to have the same type

IMHO that's unlikely. The primary driver to place a specific uniform into a specific buffer is most likely semantics. For example, group all the material properties in a single buffer, so I can set that specific uniform buffer once and do all the corresponding draw calls.

store the result of your math ops on that slice (or apply in-place math operations on these vectors).

@sebcrozet what do you mean exactly by "apply in-place math operations"? Do you have an example with nalgebra?