When using a program containing tessellation evaluation shaders, the fixed-function tessellation primitive generator consumes the input patch specified by an application and emits a new set of primitives. The following invariance rules are intended to provide repeatability guarantees. Additionally, they are intended to allow an application with a carefully crafted tessellation evaluation shader to ensure that the sets of triangles generated for two adjacent patches have identical vertices along shared patch edges, avoiding “cracks” caused by minor differences in the positions of vertices along shared edges.

**Rule 1** *When processing two patches with identical outer and inner
tessellation levels, the tessellation primitive generator will emit an
identical set of point, line, or triangle primitives as long as the active
program used to process the patch primitives has tessellation evaluation
shaders specifying the same tessellation mode, spacing, vertex order, and
point mode decorations. Two sets of primitives are considered identical if
and only if they contain the same number and type of primitives and the
generated tessellation coordinates for the vertex numbered m of the
primitive numbered n are identical for all values of m and n.*

**Rule 2** *The set of vertices generated along the outer edge of the
subdivided primitive in triangle and quad tessellation, and the tessellation
coordinates of each, depends only on the corresponding outer tessellation
level and the spacing decorations in the tessellation shaders of the
pipeline.*

**Rule 3** *The set of vertices generated when subdividing any outer primitive
edge is always symmetric. For triangle tessellation, if the subdivision
generates a vertex with tessellation coordinates of the form (0, x, 1-x),
(x, 0, 1-x), or (x, 1-x, 0), it will also generate a vertex with coordinates
of exactly (0, 1-x, x), (1-x, 0, x), or (1-x, x, 0), respectively. For quad
tessellation, if the subdivision generates a vertex with coordinates of (x,
0) or (0, x), it will also generate a vertex with coordinates of exactly
(1-x, 0) or (0, 1-x), respectively. For isoline tessellation, if it
generates vertices at (0, x) and (1, x) where x is not zero, it will also
generate vertices at exactly (0, 1-x) and (1, 1-x), respectively.*

**Rule 4** *The set of vertices generated when subdividing outer edges in
triangular and quad tessellation must be independent of the specific edge
subdivided, given identical outer tessellation levels and spacing. For
example, if vertices at (x, 1 - x, 0) and (1-x, x, 0) are generated when
subdividing the w = 0 edge in triangular tessellation, vertices must be
generated at (x, 0, 1-x) and (1-x, 0, x) when subdividing an otherwise
identical v = 0 edge. For quad tessellation, if vertices at (x, 0) and
(1-x, 0) are generated when subdividing the v = 0 edge, vertices must be
generated at (0, x) and (0, 1-x) when subdividing an otherwise identical
u = 0 edge.*

**Rule 5** *When processing two patches that are identical in all respects
enumerated in rule 1 except for vertex order, the set of triangles generated
for triangle and quad tessellation must be identical except for vertex and
triangle order. For each triangle n1 produced by processing the first patch,
there must be a triangle n2 produced when processing the second patch each
of whose vertices has the same tessellation coordinates as one of the
vertices in n1.*

**Rule 6** *When processing two patches that are identical in all respects
enumerated in rule 1 other than matching outer tessellation levels and/or
vertex order, the set of interior triangles generated for triangle and quad
tessellation must be identical in all respects except for vertex and
triangle order. For each interior triangle n1 produced by processing the
first patch, there must be a triangle n2 produced when processing the
second patch each of whose vertices has the same tessellation coordinates as
one of the vertices in n1. A triangle produced by the tessellator is
considered an interior triangle if none of its vertices lie on an outer edge
of the subdivided primitive.*

**Rule 7** *For quad and triangle tessellation, the set of triangles
connecting an inner and outer edge depends only on the inner and outer
tessellation levels corresponding to that edge and the spacing decorations.*

**Rule 8** *The value of all defined components of* `TessCoord`

*will be in the range [0, 1]. Additionally, for any defined component x of*
** TessCoord**,