Struct bevy::render::render_resource::BufferSlice

pub struct BufferSlice<'a> { /* private fields */ }

Implementations

impl<'a> BufferSlice<'a>

pub fn id(&self) -> BufferId

pub fn offset(&self) -> u64

Methods from Deref<Target = BufferSlice<'a>>

pub fn map_async( &self, mode: MapMode, callback: impl FnOnce(Result<(), BufferAsyncError>) + WasmNotSend + 'static )

Map the buffer. Buffer is ready to map once the callback is called.

For the callback to complete, either queue.submit(..), instance.poll_all(..), or device.poll(..) must be called elsewhere in the runtime, possibly integrated into an event loop or run on a separate thread.

The callback will be called on the thread that first calls the above functions after the gpu work has completed. There are no restrictions on the code you can run in the callback, however on native the call to the function will not complete until the callback returns, so prefer keeping callbacks short and used to set flags, send messages, etc.

Examples found in repository

examples/shader/gpu_readback.rs (lines 219-223)

fn map_and_read_buffer(
    render_device: Res<RenderDevice>,
    buffers: Res<Buffers>,
    sender: Res<RenderWorldSender>,
) {
    // Finally time to get our data back from the gpu.
    // First we get a buffer slice which represents a chunk of the buffer (which we
    // can't access yet).
    // We want the whole thing so use unbounded range.
    let buffer_slice = buffers.cpu_buffer.slice(..);

    // Now things get complicated. WebGPU, for safety reasons, only allows either the GPU
    // or CPU to access a buffer's contents at a time. We need to "map" the buffer which means
    // flipping ownership of the buffer over to the CPU and making access legal. We do this
    // with `BufferSlice::map_async`.
    //
    // The problem is that map_async is not an async function so we can't await it. What
    // we need to do instead is pass in a closure that will be executed when the slice is
    // either mapped or the mapping has failed.
    //
    // The problem with this is that we don't have a reliable way to wait in the main
    // code for the buffer to be mapped and even worse, calling get_mapped_range or
    // get_mapped_range_mut prematurely will cause a panic, not return an error.
    //
    // Using channels solves this as awaiting the receiving of a message from
    // the passed closure will force the outside code to wait. It also doesn't hurt
    // if the closure finishes before the outside code catches up as the message is
    // buffered and receiving will just pick that up.
    //
    // It may also be worth noting that although on native, the usage of asynchronous
    // channels is wholly unnecessary, for the sake of portability to WASM
    // we'll use async channels that work on both native and WASM.

    let (s, r) = crossbeam_channel::unbounded::<()>();

    // Maps the buffer so it can be read on the cpu
    buffer_slice.map_async(MapMode::Read, move |r| match r {
        // This will execute once the gpu is ready, so after the call to poll()
        Ok(_) => s.send(()).expect("Failed to send map update"),
        Err(err) => panic!("Failed to map buffer {err}"),
    });

    // In order for the mapping to be completed, one of three things must happen.
    // One of those can be calling `Device::poll`. This isn't necessary on the web as devices
    // are polled automatically but natively, we need to make sure this happens manually.
    // `Maintain::Wait` will cause the thread to wait on native but not on WebGpu.

    // This blocks until the gpu is done executing everything
    render_device.poll(Maintain::wait()).panic_on_timeout();

    // This blocks until the buffer is mapped
    r.recv().expect("Failed to receive the map_async message");

    {
        let buffer_view = buffer_slice.get_mapped_range();
        let data = buffer_view
            .chunks(std::mem::size_of::<u32>())
            .map(|chunk| u32::from_ne_bytes(chunk.try_into().expect("should be a u32")))
            .collect::<Vec<u32>>();
        sender
            .send(data)
            .expect("Failed to send data to main world");
    }

    // We need to make sure all `BufferView`'s are dropped before we do what we're about
    // to do.
    // Unmap so that we can copy to the staging buffer in the next iteration.
    buffers.cpu_buffer.unmap();
}

examples/app/headless_renderer.rs (lines 441-445)

fn receive_image_from_buffer(
    image_copiers: Res<ImageCopiers>,
    render_device: Res<RenderDevice>,
    sender: Res<RenderWorldSender>,
) {
    for image_copier in image_copiers.0.iter() {
        if !image_copier.enabled() {
            continue;
        }

        // Finally time to get our data back from the gpu.
        // First we get a buffer slice which represents a chunk of the buffer (which we
        // can't access yet).
        // We want the whole thing so use unbounded range.
        let buffer_slice = image_copier.buffer.slice(..);

        // Now things get complicated. WebGPU, for safety reasons, only allows either the GPU
        // or CPU to access a buffer's contents at a time. We need to "map" the buffer which means
        // flipping ownership of the buffer over to the CPU and making access legal. We do this
        // with `BufferSlice::map_async`.
        //
        // The problem is that map_async is not an async function so we can't await it. What
        // we need to do instead is pass in a closure that will be executed when the slice is
        // either mapped or the mapping has failed.
        //
        // The problem with this is that we don't have a reliable way to wait in the main
        // code for the buffer to be mapped and even worse, calling get_mapped_range or
        // get_mapped_range_mut prematurely will cause a panic, not return an error.
        //
        // Using channels solves this as awaiting the receiving of a message from
        // the passed closure will force the outside code to wait. It also doesn't hurt
        // if the closure finishes before the outside code catches up as the message is
        // buffered and receiving will just pick that up.
        //
        // It may also be worth noting that although on native, the usage of asynchronous
        // channels is wholly unnecessary, for the sake of portability to WASM
        // we'll use async channels that work on both native and WASM.

        let (s, r) = crossbeam_channel::bounded(1);

        // Maps the buffer so it can be read on the cpu
        buffer_slice.map_async(MapMode::Read, move |r| match r {
            // This will execute once the gpu is ready, so after the call to poll()
            Ok(r) => s.send(r).expect("Failed to send map update"),
            Err(err) => panic!("Failed to map buffer {err}"),
        });

        // In order for the mapping to be completed, one of three things must happen.
        // One of those can be calling `Device::poll`. This isn't necessary on the web as devices
        // are polled automatically but natively, we need to make sure this happens manually.
        // `Maintain::Wait` will cause the thread to wait on native but not on WebGpu.

        // This blocks until the gpu is done executing everything
        render_device.poll(Maintain::wait()).panic_on_timeout();

        // This blocks until the buffer is mapped
        r.recv().expect("Failed to receive the map_async message");

        // This could fail on app exit, if Main world clears resources (including receiver) while Render world still renders
        let _ = sender.send(buffer_slice.get_mapped_range().to_vec());

        // We need to make sure all `BufferView`'s are dropped before we do what we're about
        // to do.
        // Unmap so that we can copy to the staging buffer in the next iteration.
        image_copier.buffer.unmap();
    }
}

pub fn get_mapped_range(&self) -> BufferView<'a>

Synchronously and immediately map a buffer for reading. If the buffer is not immediately mappable through [BufferDescriptor::mapped_at_creation] or [BufferSlice::map_async], will panic.

Examples found in repository

examples/shader/gpu_readback.rs (line 237)

fn map_and_read_buffer(
    render_device: Res<RenderDevice>,
    buffers: Res<Buffers>,
    sender: Res<RenderWorldSender>,
) {
    // Finally time to get our data back from the gpu.
    // First we get a buffer slice which represents a chunk of the buffer (which we
    // can't access yet).
    // We want the whole thing so use unbounded range.
    let buffer_slice = buffers.cpu_buffer.slice(..);

    // Now things get complicated. WebGPU, for safety reasons, only allows either the GPU
    // or CPU to access a buffer's contents at a time. We need to "map" the buffer which means
    // flipping ownership of the buffer over to the CPU and making access legal. We do this
    // with `BufferSlice::map_async`.
    //
    // The problem is that map_async is not an async function so we can't await it. What
    // we need to do instead is pass in a closure that will be executed when the slice is
    // either mapped or the mapping has failed.
    //
    // The problem with this is that we don't have a reliable way to wait in the main
    // code for the buffer to be mapped and even worse, calling get_mapped_range or
    // get_mapped_range_mut prematurely will cause a panic, not return an error.
    //
    // Using channels solves this as awaiting the receiving of a message from
    // the passed closure will force the outside code to wait. It also doesn't hurt
    // if the closure finishes before the outside code catches up as the message is
    // buffered and receiving will just pick that up.
    //
    // It may also be worth noting that although on native, the usage of asynchronous
    // channels is wholly unnecessary, for the sake of portability to WASM
    // we'll use async channels that work on both native and WASM.

    let (s, r) = crossbeam_channel::unbounded::<()>();

    // Maps the buffer so it can be read on the cpu
    buffer_slice.map_async(MapMode::Read, move |r| match r {
        // This will execute once the gpu is ready, so after the call to poll()
        Ok(_) => s.send(()).expect("Failed to send map update"),
        Err(err) => panic!("Failed to map buffer {err}"),
    });

    // In order for the mapping to be completed, one of three things must happen.
    // One of those can be calling `Device::poll`. This isn't necessary on the web as devices
    // are polled automatically but natively, we need to make sure this happens manually.
    // `Maintain::Wait` will cause the thread to wait on native but not on WebGpu.

    // This blocks until the gpu is done executing everything
    render_device.poll(Maintain::wait()).panic_on_timeout();

    // This blocks until the buffer is mapped
    r.recv().expect("Failed to receive the map_async message");

    {
        let buffer_view = buffer_slice.get_mapped_range();
        let data = buffer_view
            .chunks(std::mem::size_of::<u32>())
            .map(|chunk| u32::from_ne_bytes(chunk.try_into().expect("should be a u32")))
            .collect::<Vec<u32>>();
        sender
            .send(data)
            .expect("Failed to send data to main world");
    }

    // We need to make sure all `BufferView`'s are dropped before we do what we're about
    // to do.
    // Unmap so that we can copy to the staging buffer in the next iteration.
    buffers.cpu_buffer.unmap();
}

examples/app/headless_renderer.rs (line 459)

fn receive_image_from_buffer(
    image_copiers: Res<ImageCopiers>,
    render_device: Res<RenderDevice>,
    sender: Res<RenderWorldSender>,
) {
    for image_copier in image_copiers.0.iter() {
        if !image_copier.enabled() {
            continue;
        }

        // Finally time to get our data back from the gpu.
        // First we get a buffer slice which represents a chunk of the buffer (which we
        // can't access yet).
        // We want the whole thing so use unbounded range.
        let buffer_slice = image_copier.buffer.slice(..);

        // Now things get complicated. WebGPU, for safety reasons, only allows either the GPU
        // or CPU to access a buffer's contents at a time. We need to "map" the buffer which means
        // flipping ownership of the buffer over to the CPU and making access legal. We do this
        // with `BufferSlice::map_async`.
        //
        // The problem is that map_async is not an async function so we can't await it. What
        // we need to do instead is pass in a closure that will be executed when the slice is
        // either mapped or the mapping has failed.
        //
        // The problem with this is that we don't have a reliable way to wait in the main
        // code for the buffer to be mapped and even worse, calling get_mapped_range or
        // get_mapped_range_mut prematurely will cause a panic, not return an error.
        //
        // Using channels solves this as awaiting the receiving of a message from
        // the passed closure will force the outside code to wait. It also doesn't hurt
        // if the closure finishes before the outside code catches up as the message is
        // buffered and receiving will just pick that up.
        //
        // It may also be worth noting that although on native, the usage of asynchronous
        // channels is wholly unnecessary, for the sake of portability to WASM
        // we'll use async channels that work on both native and WASM.

        let (s, r) = crossbeam_channel::bounded(1);

        // Maps the buffer so it can be read on the cpu
        buffer_slice.map_async(MapMode::Read, move |r| match r {
            // This will execute once the gpu is ready, so after the call to poll()
            Ok(r) => s.send(r).expect("Failed to send map update"),
            Err(err) => panic!("Failed to map buffer {err}"),
        });

        // In order for the mapping to be completed, one of three things must happen.
        // One of those can be calling `Device::poll`. This isn't necessary on the web as devices
        // are polled automatically but natively, we need to make sure this happens manually.
        // `Maintain::Wait` will cause the thread to wait on native but not on WebGpu.

        // This blocks until the gpu is done executing everything
        render_device.poll(Maintain::wait()).panic_on_timeout();

        // This blocks until the buffer is mapped
        r.recv().expect("Failed to receive the map_async message");

        // This could fail on app exit, if Main world clears resources (including receiver) while Render world still renders
        let _ = sender.send(buffer_slice.get_mapped_range().to_vec());

        // We need to make sure all `BufferView`'s are dropped before we do what we're about
        // to do.
        // Unmap so that we can copy to the staging buffer in the next iteration.
        image_copier.buffer.unmap();
    }
}

pub fn get_mapped_range_mut(&self) -> BufferViewMut<'a>

Synchronously and immediately map a buffer for writing. If the buffer is not immediately mappable through [BufferDescriptor::mapped_at_creation] or [BufferSlice::map_async], will panic.

Trait Implementations

impl<'a> Clone for BufferSlice<'a>

fn clone(&self) -> BufferSlice<'a>

Returns a copy of the value.

fn clone_from(&mut self, source: &Self)

Performs copy-assignment from source.

impl<'a> Debug for BufferSlice<'a>

fn fmt(&self, f: &mut Formatter<'_>) -> Result<(), Error>

Formats the value using the given formatter.

impl<'a> Deref for BufferSlice<'a>

type Target = BufferSlice<'a>

The resulting type after dereferencing.

fn deref(&self) -> &<BufferSlice<'a> as Deref>::Target

Dereferences the value.