Model: GPU Styling

[ptrgags]

Though CustomShaders is the new way to do GPU styling, we still are supporting the existing styling language. some parts of that must be handled on the GPU (e.g. per-point styling).

Some notes from @lilleyse excerpted from This branch describe when to use which type of styling:

Styles may be evaluated on the CPU or GPU depending on the style and the
types of properties involved. On the CPU styles are evaluated per-feature
and the color/show results are stored in a batch texture, which is later
applied on the GPU, either in the vertex shader or fragment shader.
For GPU styling the style is converted to a shader and executed in the
vertex shader or fragment shader directly. CPU styling is preferred.

In some cases a style may require both CPU styling and GPU styling, in which
case the style can't be applied and an error is thrown.

Situations where CPU styling is required:

• Style uses language features not supported in GLSL like strings or regex
• Style uses properties that are not GPU compatible like strings, variable-size arrays, or fixed sized arrays with more than 4 components
• Style uses properties in JsonMetadataTable or BatchTableHierarchy
• Style uses custom evaluate functions, see Cesium3DTileStyle#color

Situations where GPU styling is required:

• Style uses per-point properties. Large point clouds are generally impractical to style on the CPU.
• Style uses per-vertex properties. Per-vertex properties need to be interpolated before being styled.
• Style uses feature texture properties
• Style uses vertex attributes like position, color, etc
• Style uses uniforms
• Style references features in different feature tables
• Point size style is used

Situations where the style must be applied in the fragment shader:

• Primitive uses a feature ID texture
• Style uses feature texture properties
• Style uses per-vertex properties
• Style uses interpolated vertex attributes
• Style is evaluated on the CPU and vertex texture fetch is not supported

Situations where the style must be applied in the vertex shader:

• Point size style is used

Since style variables aren't scoped there can be name collisions. They are
resolved in the following order of precedence:

1. Input semantics (e.g. POSITION)
2. Attributes (e.g. _TEMPERATURE)
3. Uniforms
4. Properties (for each item below, search by semantic first, then by property ID)
   a. Feature
   b. Tile
   c. Group
   d. Tileset