Graphics Reference
In-Depth Information
type are the degenerate triangles. The element indices in boldface type
represent the new indices added to the combined element index list.
For triangle strips in Figure 7-5 with the same vertex order for the last and
first triangles of the two strips being connected, the element indices for
each triangle strip are (0, 1, 2, 3, 4) and (8, 9, 10, 11), respectively. The
combined element index list if we were to draw both strips using one call
to
glDrawElements
would be (0, 1, 2, 3, 4,
4
,
4
,
8
, 8, 9, 10, 11). This new
element index results in the following triangles drawn: (0, 1, 2), (2, 1, 3),
(2, 3, 4),
(4, 3, 4)
,
(4, 4, 4)
,
(4, 4, 8)
,
(4, 8, 8)
,
(8, 8, 9)
, (8, 9, 10),
(10, 9, 11). The triangles in boldface type are the degenerate triangles. The
element indices in boldface type represent the new indices added to the
combined element index list.
Note that the number of additional element indices required and the
number of degenerate triangles generated vary depending on the number
of vertices in the first strip. This is required to preserve the
winding order
of
the next strip being connected.
It might also be worth investigating techniques that take the size of the
post-transform vertex cache
into consideration in determining how to
arrange element indices of a primitive. Most GPUs implement a post-
transform vertex cache. Before a vertex (given by its element index) is
executed by the vertex shader, a check is performed to determine whether
the vertex already exists in the post-transform cache. If the vertex exists in
the post-transform cache, the vertex is not executed by the vertex shader.
If it is not in the cache, the vertex will need to be executed by the vertex
shader. Using the post-transform cache size to determine how element
indices are created should help overall performance, as it will reduce the
number of times a vertex that is reused gets executed by the vertex shader.
Figure 7-6 shows the primitive assembly stage. Vertices that are supplied
through
glDraw***
are executed by the vertex shader. Each vertex
transformed by the vertex shader includes the vertex position that
describes the (
x
,
y
,
z
,
w
) value of the vertex. The primitive type and vertex
indices determine the individual primitives that will be rendered. For each
individual primitive (triangle, line, and point) and its corresponding vertices,
the primitive assembly stage performs the operations shown in Figure 7-6.
Before we discuss how primitives are rasterized in OpenGL ES, we need to
understand the various coordinate systems used within OpenGL ES 3.0. This
