These are a Barycentric Coordinate System, so the weights on the targets are normalized, must sum to one, and must be positive.

https://en.wikipedia.org/wiki/Barycentric_coordinate_system

Barycentric coordinates are the local coordinate system inside a simplex. A simplicial complex is what you get when you glue multiple simplices together along shared k-faces for k = 0 … n -- vertices (0-faces), edges (1-faces), triangles (2-faces), tetrahedra (3-faces), and higher-dimensional faces -- to form a larger state space.

https://en.wikipedia.org/wiki/Simplicial_complex

It's not possible to have negative eggs, but you can apply the same machinery to many other things, like facial animation mesh blend shapes (Apple ARKit, Blender Blend Shapes and the FaceIt plugin, Unity SkinnedMeshRenderer, etc), where weights are often allowed to be overdriven >1 or even underdriven <0 for exaggerated or monstrous effects.

Nouveau Art Pipeline Demo: Blend Shapes:

https://youtu.be/phM8Wnzs_-g?t=104

(None of Epstein's spiritually close friends and shameless guru confidants were harmed or embarrassed in this demo, alas.)

Eric Hedman - "Doppel" Character Modelling with Blendshapes for Animation (ARKit):

https://www.youtube.com/watch?v=6L7jtgRD5rs

(Eric "Irk" Hedman designed and created the character animations and objects in The Sims 1, and as you can see is extremely skilled and delightful to work with! Hire him if you need professional high quality creative artwork and animation, and can pay him in bananas: https://erichedman.artstation.com/projects/8wJDgw )

Faceit: Facial Expressions And Performance Capture (Blender):

https://superhivemarket.com/products/faceit

Unity SkinnedMeshRenderer:

https://docs.unity3d.com/6000.3/Documentation/ScriptReferenc...

Apple ARKit Tracking and visualizing faces:

https://developer.apple.com/documentation/ARKit/tracking-and...

ARFaceAnchor.BlendShapeLocation: Identifiers for specific facial features, for use with coefficients describing the relative movements of those features.

https://developer.apple.com/documentation/arkit/arfaceanchor...