How to enable MAP_READ on a buffer produces by the AsBindGroup macro?

[davids91]

e.g. for the following type:

#[derive(Resource, Clone, AsBindGroup, TypePath, ExtractResource)]
#[type_path = "..."]
pub struct MyData {
    #[storage(5, visibility(compute))]
    pub(crate) some_array: Vec<u32>,
}

Basically if I try to read from it I get the following error:

wgpu_core::device::global: Buffer::map_async error: Usage flags BufferUsages(COPY_DST | STORAGE) for buffer Id(16,1,vk) do not contain required usage flags BufferUsages(MAP_READ)    

[JMS55]

You can't.

[davids91]

I believe the relevant part (found here) is :
https://gpuweb.github.io/gpuweb/#dom-gpudevice-createbuffer

If descriptor.usage contains MAP_READ:

descriptor.usage must contain no other flags except COPY_DST.

But then the only way to read the contents of a buffer is to copy it to another buffer, with map_read enabled, if I understand correctly.

[JMS55]

Yes, basically. Without mappable primary buffers, you need to issue a copy to the buffer, then map the buffer for reading (iirc, I rarely ever touch buffer mapping).

But AsBindGroup doesn't expose any of that anyways.

[davids91]

Thanks for the info! How about CPY_DST ? is that possible?

[JMS55]

No it is not. You can see the available options here: https://docs.rs/bevy/latest/bevy/render/render_resource/trait.AsBindGroup.html#deriving

[davids91]

Thank you!