Public access to canvas size

🚀 Feature

There does not seem to be a public way to access the canvas size (alternatively the shape of the displayed/rendered image). viewer._canvas_size is private and also viewer.window.qt_viewer.canvas.size now gives a FutureWarning and will break all such use cases as soon as 0.5.0 is released.

Motivation

The property viewer.camera.zoom is (still) public and connects the size of canvas pixels with the size of world pixels. Since we usually work a lot in the world and data spaces with world pixels, we would also need some information about the canvas space for being able to set a reasonable zoom factor.

Pitch

Provide a public API for reading the current rendered view’s size (width, height).

Additional context

Example use case (simplified):

from napari import Viewer
from napari.layers import Layer
import numpy as np

def zoom_layer_extents(viewer: Viewer, layer: Layer):
    dim_indices = [d - viewer.dims.ndim for d in viewer.dims.displayed]
    if np.max(np.abs(dim_indices)) <= layer.ndim:
        extent = layer.extent.world[:, dim_indices]
        canvas_size = np.asarray(viewer.window.qt_viewer.canvas.size)
        layers_size = np.flip(np.diff(extent, axis=0))
        size_ratios = canvas_size / layers_size
        zoom = np.min(size_ratios)
        viewer.camera.zoom = zoom
        viewer.camera.center = np.mean(extent, axis=0)

viewer = Viewer(show=True)
layer1 = viewer.add_image(np.random.random((200, 100)))
layer2 = viewer.add_image(np.random.random((100, 200)))
zoom_layer_extents(viewer, layer2)

FutureWarning: Public access to Window.qt_viewer is deprecated and will be removed in
v0.5.0. It is considered an "implementation detail" of the napari
application, not part of the napari viewer model. If your use case
requires access to qt_viewer, please open an issue to discuss.
  viewer.window.qt_viewer.canvas.size

[jni]

I think let’s just make viewer.canvases a list from the beginning, containing one canvas initially. But yes, I need canvas size also in my plugin 😂 So we are 100% on board @aeisenbarth!

[Czaki]

you may also would like to trace #3781, which, I have hope,should be implemented before 0.5.0 and which should allow to set viewing area instead of zoom calculation.