NAP-4: Asynchronous slicing#


Andy Sweet <>, Jun Xi Ni, Eric Perlman, Kim Pevey










Standards Track

Version effective:



Slicing a layer in napari generates a partial view of the layer’s data based on the dimension slider positions and the current field of view.

This project has two major goals.

  1. Slice layers asynchronously to keep napari responsive while allowing for slow slicing.

  2. Improve the logic of slicing layers to help developers better understand how it works and unlock new feature development.

We propose a lock-free asynchronous solution that uses Python’s built-in concurrent.futures module and exploits napari’s main event loop. For each slice, an immutable shallow copy of all the required input to slicing is collected on the main thread and is then processed on a separate single-threaded executor. When the slice task is finished, we notify vispy and Qt objects back on the main thread so they can be safely updated. This approach allows us to gradually introduce asynchronous slicing to napari layer by layer without breaking other functionality that relies on existing layer slice state.

Motivation and scope#

Currently, all slicing in napari is performed synchronously. For example, if a dimension slider is moved, napari waits to slice a layer before updating the canvas. When slicing layers is slow, this blocking behavior makes interacting with data difficult and napari may be reported as not responding by the host operating system.

The napari viewer displaying a 2D slice of a 3D multi-resolution electron microscopy image stored remotely. Dragging the slider changes the 2D slice, but the slider position and canvas updates are slow and napari stops responding.

There are two main reasons why slicing can be slow.

  1. Some layer specific slicing operations perform non-trivial calculations (e.g. points).

  2. The layer data is read lazily (i.e. it is not in RAM) and latency from the source may be non-negligible (e.g. stored remotely, napari-omero plugin).

By slicing asynchronously, we can keep napari responsive while allowing for slow slicing operations. We could also consider optimizing napari to make (1) less of a problem, but that is outside the scope of this project.

Introducing asynchrony to software often complicates program flow and logic. However, the existing technical design of slicing is already complicated and often causes issues for developers.

  • Layers have too much state [1] [2] [3].

  • The logic is hard to understand and debug [4].

  • The names of the classes are not self-explanatory [5].

Some of these issues were caused by a previous effort around asynchronous slicing in an effort to keep it isolated from the core code base. By contrast, our approach in this project is to redesign slicing in napari to provide a solid foundation for asychronous slicing and related future work like multi-canvas and multi-scale slicing.


To summarize the scope of this project, we define a few high level goals. Each goal has many prioritized features where P0 is a must-have, P1 is a should-have, and P2 is a could-have. Some of these goals may already be achieved by napari in its current form, but are captured here to prevent any regression caused by this work.

1. Keep responding when slicing slow data#

  • P0. When moving a dimension slider, the slider can still be controlled so that I can navigate to the desired location.

    • Slider can be moved when data is in the middle of loading.

    • Slider location does not return to position of last loaded slice after it was moved to a different position.

  • P0. When the slider is dragged, only slice at some positions so that I don’t wait for unwanted intermediate slicing positions.

    • Once slider is moved, wait before performing slicing operation, and cancel any prior pending slices (i.e. be lazy).

    • If we can reliably infer that slicing will be fast (e.g. data is a numpy array), consider skipping this delay.

  • P0. When slicing fails, I am notified so that I can understand what went wrong.

    • May want to limit the number of notifications (e.g. lost network connection for remote data).

  • P1. When moving a dimension slider and the slice doesn’t immediately load, I am notified that it is being generated, so that I am aware that my action is being handled.

    • Need a visual cue that a slice is loading.

    • Show visual cue to identify the specific layer(s) that are loading in the case where one layer loads faster than another.

2. Clean up slice state and logic in layers#

  • P0. Encapsulate the slice input and output state for each layer type, so that I can quickly and clearly identify those.

    • Minimize number of (nested) classes per layer-type (e.g. ImageSlice, ImageSliceData, ImageView, ImageLoader).

  • P0. Simplify the program flow of slicing, so that developing and debugging against allows for faster implementation.

    • Reduce the complexity of the call stack associated with slicing a layer.

    • The implementation details for some layer/data types might be complex (e.g. multi-scale image), but the high level logic should be simple.

  • P1. Move the slice state off the layer, so that its attributes only represent the whole data.

    • Layer may still have a function to get a slice.

    • May need alternatives to access currently private state, though doesn’t necessarily need to be in the Layer (e.g. a plugin with an ND layer, that gets interaction data from 3D visualization , needs some way to get that data back to ND).

  • P2. Store multiple slices associated with each layer, so that I can cache previously generated slices.

    • Pick a default cache size that should not strain most machines (e.g. 0-1GB).

    • Make cache size a user defined preference.

3. Measure slicing latencies on representative examples#

  • P0. Define representative examples that currently cause desirable behavior in napari, so that I can check that async slicing does not degrade those.

    • E.g. 2D slice of a 3D image layer where all data fits in RAM, but not VRAM.

  • P0. Define representative examples that currently cause undesirable behavior in napari, so that I can check that async slicing improves those.

    • E.g. 2D slice of a 3D points layer where all data fits in RAM, but not VRAM.

    • E.g. 2D slice of a 3D image layer where all data is not on local storage.

  • P0. Define slicing benchmarks, so that I can understand if my changes impact overall timing or memory usage.

    • E.g. Do not increase the latency of generating a single slice more than 10%.

    • E.g. Decrease the latency of dealing with 25 slice requests over 1 second by 50%.

  • P1. Log specific slicing latencies, so that I can summarize important measurements beyond granular profile timings.

    • Latency logs are local only (i.e. not sent/stored remotely).

    • Add an easy way for users to enable writing these latency measurements.


To help clarify the scope, we also define some things that were are not explicit goals of this project and briefly explain why they were omitted.

  • Make a single slicing operation faster.

    • Useful, but can be done independently of this work.

  • Improve slicing functionality.

    • Useful, but can be done independently of this work.

  • Make async a toggleable setting (i.e. async is always on).

    • May complicate the program flow of slicing.

    • May still automatically infer that slicing can be synchronously.

  • When a slice doesn’t immediately load, show a low level of detail version of it, so that I can preview what is upcoming.

    • Requires a low level of detail version to exist.

    • Should be part of a to-be-defined multi-scale project.

  • Store multiple slices associated with each layer, so that I can easily implement a multi-canvas mode for napari.

    • Should be part of a to-be-defined multi-canvas project.

    • Solutions for goal (2) should not block this in the future.

  • Open, save, or process layers asynchronously.

    • More related to plugin execution.

  • Lazily load parts of data based on the canvas’ current field of view.

    • An optimization that is dependent on specific data formats.

  • Identify and assign dimensions to layers and transforms.

    • Should be part of a to-be-defined dimensions project.

    • Solutions for goal (2) should not block this in the future.

  • Thick slices of non-visualized dimensions.

    • Currently being prototyped in [6].

    • Solutions for goal (2) should not block this in the future.

  • Keep the experimental async fork working.

    • Nice to have, but should not put too much effort into this.

    • May want to remove it to avoid confusion.

Detailed description#

The following diagram shows the new proposed approach to slicing layers asynchronously.

As with the existing synchronous slicing design, the event is the shared starting point. In the new approach, we pass ViewerModel.layers through to the newly defined LayerSlicer, which makes a slice request for each layer on the main thread. This request is processed asynchronously on a dedicated thread for slicing, while the main thread returns quickly to the Qt main event loop, allowing napari to keep responding to other updates and interactions. When all the layers have generated slice responses on the slicing thread, the slice_ready event is emitted. That triggers QtViewer.on_slice_ready to be executed on the main thread, so that the underlying QWidgets can be safely updated.

The rest of this section defines some new types to encapsulate state that is critical to slicing and some new methods that redefine the core logic of slicing.

Request and response#

First, we introduce a request to encapsulate the required input to slicing.

class LayerSliceRequest:
    data: ArrayLike
    world_to_data: Transform
    point: Tuple[float, ...]
    dims_displayed: Tuple[int, ...]
    dims_not_displayed: Tuple[int, ...]

At a minimum, this request must contain the data to be sliced, the point at which we are slicing in napari’s shared world coordinate system, and a way to transform world coordinates to a layer’s data coordinate system. In the future, the point may become a bounding box that includes the current field of view. Given our understanding about the existing slice input state, each layer type should extend this request type to capture the input it needs for slicing, such as Points.face_color.

Second, we introduce a response to encapsulate the output of slicing.

class LayerSliceResponse:
    data: ArrayLike
    data_to_world: Transform

At a minimum, this response must contain the sliced data and a transform that tells us where that data lives in the currently displayed scene of napari’s shared world coordinate system. At this point data should be fast to access either from RAM (e.g. as a numpy array) or VRAM (e.g. as a CuPy array), so that it can be consumed on the main thread without blocking other operations for long.

For each layer type, we need to extend the request and response types to include state that is specific to slicing those types of layers. For example, the image request type should look something like the following.

class ImageSliceRequest(LayerSliceRequest):
    rgb: bool
    multiscale: bool
    corner_pixels: np.ndarray
    data_level: int

class ImageSliceResponse(LayerSliceResponse)
    thumbnail: np.ndarray

As napari supports RGB and multi-scale images, we need a few more inputs to know what data should be accessed to generate the desired slice for the canvas. And the thumbnail needs to access the layer data as well, so should do so while we are slicing asynchronous to avoid any blocking behavior.

The points request and response types should look something like the following.

class PointsSliceRequest(LayerSliceRequest):
    out_of_slice_display: bool
    size: np.ndarray
    shown: np.ndarray
    face_color: np.ndarray
    edge_color: np.ndarray
    edge_width: np.ndarray

class PointsSliceResponse(LayerSliceResponse):
    indices: np.ndarray
    size: np.ndarray
    view_size_scale: Union[int, np.ndarray]
    face_color: np.ndarray
    edge_color: np.ndarray
    edge_width: np.ndarray

While the names and typing of some of the points request and response attributes are the same (e.g. face_color) they differ semantically because the response represent the sliced values only. Defining two distinct and explicit request and response types gives us more flexibility later in terms of how slicing works. For example, we may want to generate point face colors lazily, which could mean that PointsSliceRequest.face_color would become a callable instead of a materialized array as in the response.

Layer methods#

We require that each Layer type implements two methods related to slicing.

class Layer:

    def _make_slice_request(dims: Dims) -> LayerSliceRequest:
        raise NotImplementedError()

    def _get_slice(request: LayerSliceRequest) -> LayerSliceResponse:
        raise NotImplementedError()

The first, _make_slice_request, combines the state of the layer with the current state of the viewer’s instance of Dims passed in as a parameter to create an immutable slice request that the slicing operation will use on another thread. This method should be called from the main thread, so that nothing else should be mutating the Layer or Dims. Therefore, we should try to ensure that this method returns quickly, so as not to block the main thread.

Most of the request’s fields, like point and dims_displayed are small, and can be quickly copied in this new instance. Other fields, like data, are too large to copy in general, so instead we store a reference. If that reference is mutated in-place on the main thread while slicing is being performed on another thread, this may create an inconsistent slice output depending on when the values in data are accessed, but should be safe because the shape and dtype cannot change. If is reassigned on the main thread, then we can safely slice using the reference to the old data, though we may not want to consume the now stale output.

The second, _get_slice, takes the slice request and generates a response using layer-type specific logic. The method is static to prevent it from using any layer state directly and instead can only use the state in the immutable slice request. This allows us to execute this method on another thread without worrying about mutations to the layer that might occur on the main thread.

The main consumer of a layer slice response is the corresponding vispy layer. We require that a vispy layer type implement _set_slice to handle how it consumes the slice output.

class VispyBaseLayer:

    def _set_slice(self, response: LayerSliceResponse) -> None:
        raise NotImplementedError()

Layer slicer#

We define a dedicated class to handle execution of slicing tasks to avoid the associated state and logic leaking into the already complex ViewerModel.

ViewerSliceRequest = dict[Layer, LayerSliceRequest]
ViewerSliceResponse = dict[Layer, LayerSliceResponse]

class LayerSlicer:

    _executor: Executor = ThreadPoolExecutor(max_workers=1)
    _task: Optional[Future[ViewerSliceResponse]] = None

    def __init__(self, ...): = EmitterGroup(source=self, slice_ready=Event)

    def slice_layers_async(self, layers: LayerList, dims: Dims) -> Future[ViewerSliceResponse]:
        if self._task is not None:
        requests = {
            layer: layer._make_slice_request(dims)
            for layer in layers
        self._task = self._executor.submit(self._slice_layers, request)
        return self._task

    def slice_layers(self, requests: ViewerSliceRequest) -> ViewerSliceResponse:
        return {layer: layer._get_slice(request) for layer, request in requests.items()}

    def _on_slice_done(self, task: Future[ViewerSliceResponse]) -> None:
        if task.cancelled():

For this class to be useful, there should be at least one connection to the slice_ready event. In napari, we expect the QtViewer to marshall the slice response that this signal carries to the vispy layers so that the canvas can be updated.

Hooking up the viewer#

Using Python’s standard library threads in the ViewerModel means that we have a portable way to perform asynchronous slicing in napari without an explicit dependency on Qt.

class ViewerModel:

    dims: Dims
    _slicer: LayerSlicer = LayerSlicer()

    def __init__(self, ...):


    def _slice_layers_async(self) -> None:
        self._slicer.slice_layers_async(self.layers, self.dims)

Initially we intend for each instance of ViewerModel to own a single instance of a LayerSlicer for the lifetime of that ViewerModel instance. That allows the LayerSlicer to coordinate slicing of any or all layers for a particular viewer instance, while allowing multiple viewer instances to slice independently.

The main response to the slice being ready occurs on the QtViewer because QtViewer.layer_to_visual provides a way to map from a layer to its corresponding vispy layer.

class QtViewer:

    viewer: ViewerModel
    layer_to_visual: Dict[Layer, VispyBaseLayer]

    def __init__(self, ...):

    def _on_slice_ready(self, responses: ViewerSliceResponse):
        for layer, response in responses.items():
            if visual := self.layer_to_visual[layer]:

The other reason is because QtViewer is a QObject that lives in the main thread, which makes it easier to ensure that the slice response is handled on the main thread. That’s useful because Qt widgets and vispy nodes can only be safely updated on the main thread [8], both of which occur when consuming slice output.


A tracking issue [9] serves as a way to communicate high level progress and discussion.

A prototype [10] has been created that implements the approach described above to show its basic technical feasibility in napari’s existing code base. It only implements it for some layer types, some slicing operations, and may not work when layer data is mutated. There are some other rough edges and plenty of ways to break it, but basic functionality should work.

The rough implementation plan is as follows.

  • Implement pure refactors related to slicing to support this work (no public API or behavior changes).

  • Implement high-level asynchronous slicing model described here (i.e. add LayerSlicer, changes to ViewerModel), but with existing layer slicing implementation (i.e. Layer._set_view_slice).

  • Incrementally add layer slicing implementation described here (i.e. Layer._get_slice) for each layer separately.

This allows us to merge small pieces of work into main, testing and fixing them as we go. The alternative is to maintain a large branch/fork of napari that may be less tested and harder to merge in the end.

Backward compatibility#

Breaks synchronous slicing behavior#

The main goal of this project is to perform slicing asynchronously, so it’s natural that we might break anyone that was depending on slicing being synchronous. In particular, users or plugins that rely on Viewer.screenshot to capture the contents of the canvas after updating things like Dims.current_step will be affected.

Our proposal will break that usage in general, but we plan to offer some public way to force synchronous slicing. For example, one promising idea is to define a context manager that would temporarily force LayerSlicer to always wait for asynchronous tasks to finish and could be used as follows.

with viewer.sync_slicing():
    viewer.dims.current_step = (10, 0, 0)

Having such a mechanism in LayerSlicer also allows us to implement support for async slicing incrementally for each layer type.

Store existing slice state on layer#

Many existing napari behaviors depend on the existing slice input and output state on the layer instances. In this proposal, we decide not to remove this state from the layer yet to prevent breaking other functionality that relies on it. As slice output is now generated asynchronously, we must ensure that this state is read and written atomically to mutually exclude the main and slicing thread from reading and writing inconsistent parts of that state.

In order to do this, we plan to encapsulate the input and output state of each state into private dataclasses. There are no API changes, but this forces any read/write access of this state to acquire an associated lock.

Future work#

Render each slice as soon as it is ready#

In this proposal, the slicing thread waits for slices of all layers to be ready before it emits the slice_ready event. There are a few reasons for that.

  1. We only use one slicing thread to keep behavior simple and to avoid GIL contention.

  2. It’s closer to the existing behavior of napari

  3. Shouldn’t introduce any new potential bugs, such as [11].

  4. It needs less UX design work to decide what should be shown while we are waiting for slices to be ready.

In some cases, rendering slices as soon as possible will provide a better user experience, especially when some layers are substantially slower than others. Therefore, this should be high priority future work.

To implement this behavior as a small extension to this proposal, we could do something like the following.

  • Use multiple workers in the LayerSlicer._executor.

  • Submit each call to Layer._get_slice separately.

  • When each call is done, emit a separate slice_ready event that only contains each layer’s slice response, so that the viewer/vispy can update as soon as possible.

This would cause a more complex call sequence and could make cancellation behavior more complex, but may be worth it regardless.


Extend the existing experimental async code#

  • Already being used somewhat successfully by some napari users in the wild.

  • Can still reuse some of the code, tooling, and learnings from this project.

  • The technical design of this approach was not well received (i.e. there is a reason it is experimental).

  • Does not address goal 2.

Just call set_view_slice or refresh asynchronously#

  • Simple to implement with few code changes needed.

  • Needs at least one lock to provide sensible access to layer slice state.

  • This will need to be acquired to access this state and at the beginning of many methods that access any of that state.

  • How to emit events on the main thread?

  • Does not address goal 2.

Just access data asynchronously#

  • Targets main cause of unresponsiveness (i.e. reading data).

  • No events are emitted on the non-main thread.

  • Less lazy when cancelling is possible (i.e. we do more work on the main thread before submitting the async task).

  • Splits up slicing logic into pre/post data reading, making program flow harder to follow.

  • Does not address goal 2.

Use QThread and similar utilities instead of concurrent.futures#

  • Standard way for plugins to support long running operations.

  • Can track progress and allow more opportunity for cancellation with yielded signal.

  • Can easily process done callback (which might update Qt widgets) on main thread.

  • Need to define our own task queue to achieve lazy slicing.

  • Need to connect a QObject, which ties our core to Qt, unless the code that controls threads does not live in core.

Use asyncio package instead of concurrent.futures#

  • May improve general readability of async code for some.

  • Mostly syntactic sugar on top of concurrent.futures.

  • Likely need an asyncio event loop distinct from Qt’s main event loop, which could be confusing and cause issues.

  • As discussed on Zulip, qt-async-threads is a possible solution, but it is still quite new.

Slice from vispy layer instead of viewer model#

Instead of making the ViewerModel the driver of slicing, we could instead drive it from the vispy layers. A rough implementation of this approach could look like the following.

class QtViewer:
    def __init__(self):

    def slice_layers(self):
        for layer in self.viewer.layers:

class Image:
    def _make_slice_request(self, dims: Dims) -> ImageSliceRequest:
    def _get_slice(self, request: ImageSliceRequest) -> ImageSliceResponse:

class VispyImageLayer:
    def _slice(self, dims: Dims) -> None:
        request = self.layer._make_slice_request(dims)
        task = self.slice_executor.submit(self.layer._get_slice, request)

    def _on_slice_done(self, task: Future[ImageSliceResponse]) -> None:

    def _set_slice(self, response: ImageSliceResponse) -> None:

Most of the internal guts of slicing, such as the implementation of _get_slice and _make_slice_request, are unchanged from the approach proposed here.

The main advantage of this approach is that the main consumer of the slice response (i.e. vispy) is the one driving slicing, which allows us to better define input and output types and avoid unnecessary type inheritance.

In the example code above, we avoid a tight dependency on Qt using @ensure_main_thread. But another advantage of this general approach is that we could rely on tighter dependencies between vispy and Qt, which may simplify the implementation and offer some performance benefits.

One disadvantage is that it becomes harder to control the execution of multiple layers being sliced. For example, with the above implementation we can no longer wait for all layers to be sliced before updating all the vispy layers. Another disadvantage is that this implies that slice state should not live in the model in the future, which might cause issues with things like selection that may depend on that state.


  • Initial announcement on Zulip.

    • Consider (re)sampling instead of slicing as the name for the operation discussed here.

  • Problems with NAPARI_ASYNC=1

    • The existing experimental async code doesn’t handle some seemingly simple usage.

  • Remove slice state from layer

    • Feature request to remove slice/render state from the layer types.

    • Motivated by creating multiple slices of the same layers for multiple canvases.

    • Decision: postpone.

      • There are too many behaviors that depend on slice state (e.g. Layer._get_value) and it’s easy enough to set it after an async slicing task is done, so leave it for now.

      • This work should simplify removing it in the future.

  • Draft PR on andy-sweet’s fork

    • Initial feedback before sharing more widely.

  • Define a class that encapsulates the slicing bounding box in world coordinates.

  • Replace layer slice request and response types with a single slice type

    • Motivation is make it easier to add a new layer type by reducing the number of class types needed.

    • Some disagreement here as one type for two purposes (input vs. output) seems confusing.

    • Keeping the request and response types separate may also allow us to better target alternatives to vispy.

    • Decision: keep request and response types separate for now.

      • Adding a new layer type is not a frequent occurrence.

      • Having one class may reduce the number of classes, but implementing a new layer type is already quite complex.

  • Slicing in a shader

    • How to handle future cases where layer data has been pre-loaded onto the GPU and we want to slice in the shader?

    • Less about large/slow data, and more about not blocking approaches to handling small/fast data efficiently.

    • May be particularly useful for multi-canvas.

    • Decision: postpone.

      • Keep related types private or vague to not blocking this in the future.

  • Support existing usage and plugins that depends on synchronous slicing

    • Main example is the napari-animation plugin, but there may be others.

    • This was explored [12] with the following main findings.

      • Forcing synchronous slicing when combining the prototype and napari-animation seems to work for the basic examples.

      • We only need to ensure synchronous slicing before using Viewer.screenshot, as keyframes just capture the value of current_step and similar.

      • screenshot is only called a few times in napari-animation, so wouldn’t require large changes if asynchronous slicing was the default.

    • Decision: always have some way to force synchronous slicing.

      • But no need to support synchronous slicing by default, which is harder to implement.

  • What should Viewer.screenshot do when the canvas is not fully rendered?

    • Initial consensus is that it should wait for pending slicing tasks and for the associated pushes to vispy.

      • There are some ideas of how to implement that, though it is not straightforward.

    • In the API, may want a keyword argument to control behavior. In the GUI, could have a dialog if there are pending tasks. = Decision: want to support both, with blocking behavior as default.

      • No need for a design or prototype before acceptance.

  • Should Dims.current_step (and corner_pixels) represent the last slice position request or the last slice response?

    • With sync slicing, there is no distinction.

    • With async slicing, current is ambiguous.

    • Initial small consensus around last request

    • Decision: last request.

      • The implementation is simpler if this represents the last request.

      • Can introduce an additional public attribute later if last response is needed.

References and footnotes#

All NAPs should be declared as dedicated to the public domain with the CC0 license [13], as in Copyright, below, with attribution encouraged with CC0+BY [14].