notes

bitset.txt (4354B)

---

 ===============================================================================
 bitset
 ===============================================================================
 
 Example:
 
 Each flag owns a different bit position, so they never overlap. That lets you
 store any subset of them in one number:
 
 INDEX         = 1 << 4 = 00_0001_0000 = 16
 VERTEX        = 1 << 5 = 00_0010_0000 = 32
 UNIFORM       = 1 << 6 = 00_0100_0000 = 64
 STORAGE       = 1 << 7 = 00_1000_0000 = 128
 INDIRECT      = 1 << 8 = 01_0000_0000 = 256
 QUERY_RESOLVE = 1 << 9 = 10_0000_0000 = 512
 
 Each of those constants is one switch in a row of switches, and a buffer's usage
 value records which switches are on.
 
 1. Each usage gets its own bit. INDEX is switch 4, VERTEX is switch 5, UNIFORM
 is switch 6, and so on. 1 << n means "a number with only switch n turned on."
 
 2. Combining usages means turning on several switches. If you create a buffer
 with VERTEX | INDEX | UNIFORM, you get:
 
    00_0001_0000   INDEX
    00_0010_0000   VERTEX
    00_0100_0000   UNIFORM
    ────────────   OR them together
    00_0111_0000 = 112
 
 The single number 112 means "this buffer may be used as an index buffer,
 a vertex buffer, and a uniform buffer."
 
 
 
 wgpu-types/src/buffer.rs
 
 bitflags::bitflags! {
     /// Different ways that you can use a buffer.
     ///
     /// The usages determine what kind of memory the buffer is allocated from and what
     /// actions the buffer can partake in.
     ///
     /// Specifying only usages the application will actually perform may increase performance.
     /// Additionally, on the WebGL backend, there are restrictions on [`BufferUsages::INDEX`];
     /// see [`DownlevelFlags::UNRESTRICTED_INDEX_BUFFER`] for more information.
     ///
     /// Corresponds to [WebGPU `GPUBufferUsageFlags`](
     /// https://gpuweb.github.io/gpuweb/#typedefdef-gpubufferusageflags).
     #[repr(transparent)]
     #[cfg_attr(feature = "serde", derive(Serialize, Deserialize))]
     #[cfg_attr(feature = "serde", serde(transparent))]
     #[derive(Debug, Copy, Clone, PartialEq, Eq, Hash)]
     pub struct BufferUsages: u32 {
         /// Allow a buffer to be mapped for reading using [`Buffer::map_async`] + [`Buffer::get_mapped_range`].
         /// This does not include creating a buffer with [`BufferDescriptor::mapped_at_creation`] set.
         ///
         /// If [`Features::MAPPABLE_PRIMARY_BUFFERS`] isn't enabled, the only other usage a buffer
         /// may have is COPY_DST.
         const MAP_READ = 1 << 0;
         /// Allow a buffer to be mapped for writing using [`Buffer::map_async`] + [`Buffer::get_mapped_range_mut`].
         /// This does not include creating a buffer with [`BufferDescriptor::mapped_at_creation`] set.
         ///
         /// If [`Features::MAPPABLE_PRIMARY_BUFFERS`] feature isn't enabled, the only other usage a buffer
         /// may have is COPY_SRC.
         const MAP_WRITE = 1 << 1;
         /// Allow a buffer to be the source buffer for a [`CommandEncoder::copy_buffer_to_buffer`] or [`CommandEncoder::copy_buffer_to_texture`]
         /// operation.
         const COPY_SRC = 1 << 2;
         /// Allow a buffer to be the destination buffer for a [`CommandEncoder::copy_buffer_to_buffer`], [`CommandEncoder::copy_texture_to_buffer`],
         /// [`CommandEncoder::clear_buffer`] or [`Queue::write_buffer`] operation.
         const COPY_DST = 1 << 3;
         /// Allow a buffer to be the index buffer in a draw operation.
         const INDEX = 1 << 4;
         /// Allow a buffer to be the vertex buffer in a draw operation.
         const VERTEX = 1 << 5;
         /// Allow a buffer to be a [`BufferBindingType::Uniform`] inside a bind group.
         const UNIFORM = 1 << 6;
         /// Allow a buffer to be a [`BufferBindingType::Storage`] inside a bind group.
         const STORAGE = 1 << 7;
         /// Allow a buffer to be the indirect buffer in an indirect draw call.
         const INDIRECT = 1 << 8;
         /// Allow a buffer to be the destination buffer for a [`CommandEncoder::resolve_query_set`] operation.
         const QUERY_RESOLVE = 1 << 9;
         /// Allows a buffer to be used as input for a bottom level acceleration structure build
         const BLAS_INPUT = 1 << 10;
         /// Allows a buffer to be used as input for a top level acceleration structure build
         const TLAS_INPUT = 1 << 11;
     }
 }