Bug report: Incorrect drawing order of Isosurfaces with opacity

• Reported by Łukasz as a post on the forum
• Plotly version: 6.2.0
• Python version: 3.11.13
• Browser: chrome

Hi, i have found quite annoying bug which can be seen only when the isosurface components are opaque. I observed that it doesn’t only occur for multiple trace isosurfaces, but also single multisurface isosurfaces and volume plots.

Here is an example code how to replicate it:
(The fun part is that when you change the opacity from 0.99 to 1 the plot gets visualised properly.)

import plotly.graph_objects as go
import numpy as np
import nrrd
from skimage.transform import resize


volume = np.zeros((50, 50, 50))
volume[1:10, 5:45, 5:45] =1
volume[10:20, 5:45, 5:45] = 1.5
volume[20:30, 5:45, 5:45] = 2
volume[30:40, 5:45, 5:45] = 2.5
volume[40:49, 5:45, 5:45] = 3
Z, X, Y = np.mgrid[0:volume.shape[0]:1, 0:volume.shape[1]:1, 0:volume.shape[2]:1]


fig = go.Figure(data=[go.Isosurface(
    x=X.flatten(),
    y=Y.flatten(),
    z=Z.flatten(),
    opacity=0.99,
    value=np.where(volume == 1, 1, 0).flatten(),
    surface_fill=1,
    surface_count=1,
    colorscale=[[0, 'rgb(0,255,0)'], [1, 'rgb(0,255,0)']],
    caps=dict(x_show=False, y_show=False, z_show=False),
    #slices_z=dict(show=True, locations=list(range(volume.shape[0]))),
    ),
    go.Isosurface(
        x=X.flatten(),
        y=Y.flatten(),
        z=Z.flatten(),
        opacity=0.99,
        value=np.where(volume == 3, 1, 0).flatten(),
        surface_fill=1,
        surface_count=1,
        colorscale=[[0, 'rgb(255,0,0)'], [1, 'rgb(255,0,0)']],
        caps=dict(x_show=False, y_show=False, z_show=False),
        # slices_z=dict(show=True, locations=list(range(volume.shape[0]))),
    )])

The bug can be seen looking form under the plot:

まず、提供された Python の再現コードを Chrome で実行し、opacity 0.99 と 1 の場合の isosurface と volume rendering を比較します。不透明およびほぼ不透明な Isosurfaces、multisurfaces、volume plots のレンダリングパスを調査します。提供されたケースが正しい順序で描画され、完全に不透明なレンダリングにリグレッションが発生しなければ完了です。