Shape keys intermittently do not work when using more than 4 at once

[j-conrad]

An object with multiple shape keys (in this case, 10) intermittently doesn't respond as expected when mixing those morph shapes together. In the provided example, the cube has 9 shape keys where each of them only moves 1 vertex. The tenth shape twists the mesh, moving about 17 verts at once.
I am able to use the various glTF viewers to adjust the shape keys, however, when mixing beyond about 4 channels, the mesh begins to respond strangely. Sometimes there is no affect on the model until a slider reaches its limit. Other times, unrelated verts are affected when they shouldn't be. I assumed it was possibly related to an individual vertex being affected by multiple channels. But the first 9 are completely isolated verts.

To Reproduce
Steps to reproduce the behavior:

1. Drop the provided .glb into a glTF viewer that supports shape keys (morph targets, whatever).
2. Slide Keys 1, 3, 7, and 9 up to non-zero values. This should have four top corners sticking out.
3. Now attempt to slide Key 5 up.
4. Result: One of the other verts' position gets negated and the 5th vertex pops unexpectedly into position late in the slider's movement.

Expected behavior
I would expect each vertex to move individually as they do within Blender (with the obvious exception of the last twisty shape key).

Screenshots
https://gyazo.com/3f747db650c5f5bfbabc93312eaf67de

.blend file/ .gltf
MorphBug.zip

Version

• OS: Windows
• Blender Version 2.82.7

Additional context
Add any other context about the problem here.

[donmccurdy]

This is a common limitation in realtime engines — when shape keys are applied on the GPU, each active shape key requires 1-3 vertex attributes, and there are only 16 vertex attributes available on most devices. Other features (skinning, vertex colors, UVs, ...) also require vertex attributes.

In three.js, specifically, no more than 8 vertex attributes can be used by shape keys. You're hitting a limit at 4 shape keys because each shape key has both position and normal attribute. If you disable the "Shape Key Normals" option, you'll likely see the limit increase from 4 shape keys to 8 shape keys in most engines, because each shape key then requires 1 vertex attribute instead of 2.

Beyond that, an exporter can't increase these limits.

[donmccurdy]

Related: https://discourse.threejs.org/t/limit-8-blendshapes-on-the-fbxloader/3906/8

EDIT: Fixed incorrect link.

[j-conrad]

Thank you for the information and clarification. This makes a lot of sense now that I understand it.