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Clarify the restriction on accessor counts within a primitive #1848

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Aug 12, 2020
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Clarify the restriction on accessor counts within a primitive #1848

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2 changes: 2 additions & 0 deletions specification/2.0/README.md
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Expand Up @@ -778,6 +778,8 @@ Valid accessor type and component type for each attribute semantic property are

All indices for indexed attribute semantics must start with 0 and be continuous positive integers: `TEXCOORD_0`, `TEXCOORD_1`, etc. Indices must not use leading zeroes to pad the number of digits, and clients are not required to support more indexed semantics than described above.

All attribute accessors for a given primitive must have the same `count`; when `indices` property is not defined, it indicates the number of vertices to render; when `indices` property is defined, it indicates the upper (exclusive) bound on the index values in the `indices` accessor.

> **Implementation note:** Each primitive corresponds to one WebGL draw call (engines are, of course, free to batch draw calls). When a primitive's `indices` property is defined, it references the accessor to use for index data, and GL's `drawElements` function should be used. When the `indices` property is not defined, GL's `drawArrays` function should be used with a count equal to the count property of any of the accessors referenced by the `attributes` property (they are all equal for a given primitive).

> **Implementation note:** When positions are not specified, client implementations should skip primitive's rendering unless its positions are provided by other means (e.g., by extension). This applies to both indexed and non-indexed geometry.
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