Asynchronous rendering#

As discussed in the explanations document on rendering, asynchronous rendering is a feature that allows napari to stay usable and responsive even when data is loading slowly. There are two experimental asynchronous rendering features, they can be enabled using the environment variables NAPARI_ASYNC and NAPARI_OCTREE.


Running napari with NAPARI_ASYNC=1 enables asynchronous rendering using the existing Image class. The Image class will no longer call np.asarray() in the GUI thread. We do this so that if np.asarray() blocks on IO or a computation, the GUI thread will not block and the framerate will not suffer.

To avoid blocking the GUI thread the Image class will load chunks using the new ChunkLoader class. The ChunkLoader will call np.asarray() in a worker thread. When the worker thread finishes it will call on_chunk_loaded() with the loaded data. The next frame Image can display the new data.

Without NAPARI_ASYNC napari will block when switching slices. Napari will hang until the new slice has loaded. If the slice loads slowly enough you might see the “spinning wheel of death” on a Mac indicating the process is hung.

Asynchronous rendering allows the user to interrupt the loading of a slice at any time. The user can freely move the slice slider. This is especially nice for remote or slow-loading data.

Multi-scale images#

With today’s Image class there are no tiles or chunks. Instead, whenever the camera is panned or zoomed napari fetches all the data needed to draw the entire current canvas. This actually works amazingly well with local data. Fetching the whole canvas of data each time can be quite fast.

With remote or other high latency data, however, this method can be very slow. Even if you pan only a tiny amount, napari has to fetch the whole canvas worth of data, and you cannot interrupt the load to further adjust the camera.

With NAPARI_ASYNC overall performance is the same, but the advantage is you can interrupt the load by moving the camera at any time. This is a nice improvement, but working with slow-loading data is still slow. Most large image viewers improve on this experience with chunks or tiles. With chunks or tiles when the image is panned the existing tiles are translated and re-used. Then the viewer only needs to fetch tiles which newly slid onto the screen. This style of rendering is what the NAPARI_OCTREE flag enables.


Set NAPARI_OCTREE=1 to use the experimental OctreeImage class instead of the normal Image class. The new OctreeImage class will use the same ChunkLoader that NAPARI_ASYNC enables. In addition, NAPARI_OCTREE will use the new TiledImageVisual instead of the Vispy ImageVisual class that napari’s Image class uses.


The current OCTREE implementation only fully supports a single 2D image and may not function with 3D images or multiple images. Improving support for 3D and multiple images is part of future work on the OCTREE.

See Octree configuration file for Octree configuration options.

Octree visuals#

The visual portion of Octree rendering is implemented by three classes: VispyTiledImageLayer, TiledImageVisual, and TextureAtlas2D.

The first two classes are named “tiled image” rather than “octree” because currently they do not know that they are rendering out of an octree. We did this intentionally to keep the visuals simpler and more general. However, the approach has some limitations, and we might later need to create a subclass of TiledImageVisual which is Octree-specific, see Future work: Extending TextureAtlas2D.

The TextureAtlas2D class is a subclass of the generic Vispy Texture2D class. Like Texture2D the TextureAtlas2D class owns one texture. However TextureAtlas2D uses this one texture as an “atlas” which can hold multiple tiles.

For example, by default TextureAtlas2D uses a (4096, 4096) texture that stores 256 different (256, 256) pixel tiles. Adding or remove a single tile from the full atlas texture is very fast. Under the hood adding one tile calls glTexSubImage2D() which only updates the data in that specific (256, 256) portion of the full texture.

Aside from the data transfer cost, TextureAtlas2D is also fast because we do not have to modify the scene graph or rebuild any shaders when a tile is added or removed. In an early version of tiled rendering we created a new ImageVisual for every tile. This resulted in scene graph changes and shader rebuilds. At the time the scene graph changes were causing crashes with PyQt5, but the atlas approach is better for multiple reasons, so even if that crash were fixed the atlas is a better solution.

Octree rendering#

The interface between the visuals and the Octree is the OctreeImage method get_drawable_chunks(). The method is called by the VispyTiledImageLayer method _update_drawn_chunks() every frame so it can update which tiles are drawn. OctreeImage calls the get_intersection() method on its OctreeSlice to get an OctreeIntersection object which contains the “ideal chunks” that should be drawn for the current camera position.

The ideal chunks are the chunks at the preferred level of detail, the level of detail that best matches the current canvas resolution. Drawing chunks which are more detailed that this will look fine, the graphics card will downsample them to the screen resolution, but it’s not efficient to use higher resolution chunks than are needed. Meanwhile drawing chunks that are coarser than the ideal level will look blurry, but it’s much better than drawing nothing.

The decision about what level of detail to use is made by the OctreeLoader class and its method get_drawable_chunks(). There are many different approaches one could take here as far as what to draw when. Today we are doing something reasonable but it could potentially be improved. In addition to deciding what level of detail to draw for each ideal chunk, the class initiates asynchronous loads with the ChunkLoader for chunks it wants to draw in the future.

The loader will only use chunks from a higher resolution if they are already being drawn. For example when zooming out. However, it will never initiate loads on higher resolution chunks, since it’s better off loading and drawing the ideal chunks.

The loader will load lower resolution chunks in some cases. Although this can slightly delay when the ideal chunks are loaded, it’s a very quick way to get reasonable looking “coverage” of the area of interest. Often data from one or two levels up isn’t even that noticeably degraded. This table shows how many ideal chunks are “covered” by a chunk at a higher level:

Levels Above Ideal








Although data 3 levels above will be quite blurry, it’s pretty amazing you can load one chunk and it will cover 64 ideal chunks. This is the heart of the power of Octrees, Quadtrees or multiscale images.

Octree configuration file#

Setting NAPARI_OCTREE=1 enables Octree rendering with the default configuration. To customize the configuration set NAPARI_OCTREE to be the path of a JSON config file, such as NAPARI_OCTREE=/tmp/octree.json.

See DEFAULT_OCTREE_CONFIG for the current config file format:

    "loader_defaults": {
        "log_path": None,
        "force_synchronous": False,
        "num_workers": 10,
        "use_processes": False,
        "auto_sync_ms": 30,
        "delay_queue_ms": 100,
    "octree": {
        "enabled": True,
        "tile_size": 256,
        "log_path": None,
        "loaders": {
            0: {"num_workers": 10, "delay_queue_ms": 100},
            2: {"num_workers": 10, "delay_queue_ms": 0},

The loader_defaults key contains settings that will be used by the ChunkLoader.




Write ChunkLoader log file to this path. For debugging.


If true the ChunkLoader loads synchronously.


The number of worker threads or processes.


If true use worker processes instead of threads.


Switch to synchronous if loads are faster than this.


Delay loads by this much.


The number of worker threads or processes.

The octree key contains these settings:




If false then use the old Image class.


Size of render tiles to use for rending.


Octree specific log file for debugging.


Optional custom loaders, see below.

The loaders key lets you define and configure multiple LoaderPool pools. The key of each loader is the level relative to the ideal level. In the above example configuration we define two loaders. The first with key 0 is for loading chunks at the ideal level or one above. While the second with key 2 will load chunks two above the ideal level or higher.

Each loader uses the loader_defaults but you can override the num_workers, auto_sync_ms and delay_queue_ms values in each loader defined in loaders.

Multiple loaders#

We allow multiple loaders to improve loading performance. There are a lot of different strategies one could use when loading chunks. For example, we tend to load chunks at a higher level prior to loading the chunks at the ideal level. This gets “coverage” on the screen quickly, and then the data can be refined by loading the ideal chunks.

One consideration is during rapid movement of the camera it’s easy to clog up the loader pool with workers loading chunks that have already moved out of view. The DelayQueue was created to help with this problem.

While we can’t cancel a load if a worker has started working on it, we can trivially cancel loads that are still in our delay queue. If the chunk goes out of view, we cancel the load. If the user pauses for a bit, we initiate the loads.

With multiple loaders we can delay the ideal chunks, but we can configure zero delay for the higher levels. A single chunk from two levels up will cover 16 ideal chunks. So immediately loading them is a good way to get data on the screen quickly. When the camera stops moving the LoaderPool for the ideal layer will often be empty. So all of those workers can immediately start loading the ideal chunks.

The ability to have multiple loaders was only recently added. We still need to experiment to figure out the best configuration. And figure out how that configuration needs to vary based on the latency of the data or other considerations.

Future work: Compatibility with the existing Image class#

The focus for initial Octree development was Octree-specific behaviors and infrastructure. Loading chunks asynchronously and rendering them as individual tiles. One question we wanted to answer was will a Python/Vispy implementation of Octree rendering be performant enough? Because if not, we might need a totally different approach. It’s not been fully proven out, but it seems like the performance will be good enough, so the next step is full compatibility with the existing Image class.

The OctreeImage class is derived from Image, while VispyTiledImageLayer is derived from VispyImageLayer, and TiledImageVisual is derived from the regular Vispy ImageVisual class. To bring full Image capability to OctreeImage in most cases we just need to duplicate what those base classes are doing, but do it on a per-tile bases. Since there is no full image for them to operate on. This might involve chaining to the base class or it could mean duplicating that functionality somehow in the derived class.

Some Image functionality that needs to be duplicated in Octree code:

Contrast limits and color transforms#

The contrast limit code in Vispy’s ImageVisual needs to be moved into the tiled visual’s _build_texture(). Instead operating on it needs to transform tile’s which are newly being added to the visual. The color transform similarly needs to be per-tile.

Blending and opacity#

It might be hard to get opacity working correctly for tiles where loads are in progress. The way TiledImageVisual works today is the OctreeLoader potentially passes the visual tiles of various sizes, from different levels of the Octree. The tiles are rendered on top of each other from largest (coarsest level) to smallest (finest level). This is a nice trick so that bigger tiles provide “coverage” for an area, while the smaller tiles add detail only where that data has been loaded.

However, this breaks blending and opacity. We draw multiple tiles on top of each other, so the image is blending with itself. One solution which is kind of a big change is keep TiledImageVisual for the generic “tiled” case, but introduce a new OctreeVisual that knows about the Octree. It can walk up and down the Octree chopping up larger tiles to make sure we do not render anything on top of anything else.

Until we do that, we could punt on making things look correct while loads are in progress. We could even highlight the fact that a tile has not been fully loaded (purposely making it look different until the data is fully loaded). Aside from blending, this would address a common complaint with tiled image viewers: you often can’t tell if the data is still being loaded. This could be a big issue for scientific uses, you don’t want people drawing the wrong conclusions from the data.

Time-series multiscale#

To make time-series multiscale work should not be too hard. We just need to correctly create a new OctreeSlice every time the slice changes.

The challenge will probably be performance. For starters we probably need to stop creating the “extra” downsampled levels, as described in Future work: Extending TextureAtlas2D. We need to make sure constructing and tearing down the Octree is fast enough, and make sure loads for the previous slices are canceled and everything is cleaned up.

Future work: Extending TextureAtlas2D#

We could improve our TextureAtlas2D class in a number of ways:

  1. Support setting the atlas’s full texture size on the fly.

  2. Support setting the atlas’s tile size on the fly.

  3. Support a mix of tiles sizes in one atlas.

  4. Allow an atlas to have more than one backing texture.

One reason to consider these changes is so we could support “large tiles” in certain cases. Often the coarsest level of multi-scale data “in the wild” is much bigger than one of our (256, 256) tiles. Today we solve that by creating additional Octree levels, downsampling the data until the coarsest level fits within a single tile.

If we could support multiple tiles sizes and multiple backing textures, we could potentially have “interior tiles” which were small, but then allow large root tiles. Graphics cards can handle pretty big textures. A layer that’s (100000, 100000) obviously needs to be broken into tiles, b¡ut a layer that’s (4096, 4096) really does not need to be broken into tiles. That could be a single large tile.

Long term it would be nice if we did not have to support two image classes: Image and OctreeImage. Maintaining two code paths and two sets of visuals will become tiresome and lead to discrepancies and bugs.

Instead, it would be nice if OctreeImage became the only image class. One image class to rule them all. For that to happen, though, we need to render small images just as efficiently as the Image class does today. We do not want Octree rendering to worsen cases which work well today. To keep today’s performance for smaller images we probably need to add support for variable size tiles.

Future work: Level-zero-only Octrees#

In issue #1300 it takes 1500ms to switch slices. There we are rendering a (16384, 16384) image that is entirely in RAM. The delay is not from loading into RAM, it’s already in RAM, the delay is from transferring all that data to VRAM in one big gulp.

The image is not a multi-scale image. So can we turn it into a muli-scale image? Generally we’ve found downsampling to create multi-scale image layers is slow. So the question is how can we draw this large image without hanging? One idea is we could create an Octree that only has a level zero and no downsampled levels.

This is an option because chopping up a NumPy array into tiles is very fast. This chopping up phase is really just creating a bunch of “views” into the single existing array. So creating a level zero Octree should be very fast. For there we can use our existing Octree code and our existing TiledImageVisual to transfer over one tile at a time without hurting the frame rate.

The insight here is our Octree code is really two things, one is an Octree but two is a tiled or chunked image, basically a flat image chopped into a grid of tiles. How would this look to the user? With this approach switching slices would be similar to panning and zooming a multiscale Octree image, you’d see the new tiles loading in over time, but the framerate would not tank, and you could switch slices at any time.

Future work: Caching#

Basically no work has gone into caching or memory management for Octree data. It’s very likely there are leaks and extended usage will run out of memory. This hasn’t been addressed because using Octree for long periods of time is just now becoming possible.

One caching issue is figuring out how to combine the ChunkCache with Dasks’s built-in caching. We probably want to keep the ChunkCache for rendering non-Dask arrays? But when using Dask, we defer to its cache? We certainly don’t want to cache the data in both places.

Another issue is whether to cache OctreeChunks or tiles in the visual, beyond just caching the raw data. If re-creating both is fast enough, the simpler thing is evict them fully when a chunk falls out of view. And re-create them if it comes back in view. It’s simplest to keep nothing but what we are currently drawing.

However if that’s not fast enough, we could have a MRU cache of OctreeChunks and tiles in VRAM, so that reviewing the same data is nearly instant. This is adding complexity, but the performance might be worth it.