OpenGL extension EXT.stencil_clear_tag
This module customises the behaviour of the
OpenGL.raw.GL.EXT.stencil_clear_tag to provide a more
Overview (from the spec)
Stencil-only framebuffer clears are increasingly common as 3D
applications are now using rendering algorithms such as stenciled
shadow volume rendering for multiple light sources in a single frame,
recent "soft" stenciled shadow volume techniques, and stencil-based
constructive solid geometry techniques. In such algorithms there
are multiple stencil buffer clears for each depth buffer clear.
Additionally in most cases, these algorithms do not require all
of the 8 typical stencil bitplanes for their stencil requirements.
In such cases, there is the potential for unused stencil bitplanes
to encode a "stencil clear tag" in such a way to reduce the number
of actual stencil clears. The idea is that switching to an unused
stencil clear tag logically corresponds to when an application would
otherwise perform a framebuffer-wide stencil clear.
This extension exposes an inexpensive hardware mechanism for
amortizing the cost of multiple stencil-only clears by using a
client-specified number of upper bits of the stencil buffer to
maintain a per-pixel stencil tag.
The upper bits of each stencil value is treated as a tag that
indicates the state of the upper bits of the "stencil clear tag" state
when the stencil value was last written. If a stencil value is read
and its upper bits containing its tag do NOT match the current upper
bits of the stencil clear tag state, the stencil value is substituted
with the lower bits of the stencil clear tag (the reset value).
Either way, the upper tag bits of the stencil value are ignored by
subsequent stencil function and operation processing of the stencil
When a stencil value is written to the stencil buffer, its upper bits
are overridden with the upper bits of the current stencil clear tag
state so subsequent reads, prior to any subsequent stencil clear
tag state change, properly return the updated lower bits.
In this way, the stencil clear tag functionality provides a way to
replace multiple bandwidth-intensive stencil clears with very
inexpensive update of the stencil clear tag state.
If used as expected with the client specifying 3 bits for the stencil
tag, every 7 of 8 stencil-only clears of the entire stencil buffer can
be substituted for an update of the current stencil clear tag rather
than an actual update of all the framebuffer's stencil values. Still,
every 8th clear must be an actual stencil clear. The net effect is
that the aggregate cost of stencil clears is reduced by a factor of
1/(2^n) where n is the number of bits devoted to the stencil tag.
The application specifies two new pieces of state: 1) the number of
upper stencil bits, n, assigned to maintain the tag bits for each
stencil value within the stencil buffer, and 2) a stencil clear tag
value that packs the current tag and a reset value into a single
integer values. The upper n bits of the stencil clear tag value
specify the current tag while the lower s-min(n,s) bits specify
the current reset value, where s is the number of bitplanes in the
stencil buffer and n is the current number of stencil tag bits.
If zero stencil clear tag bits are assigned to the stencil tag
encoding, then the stencil buffer operates in the conventional
The official definition of this extension is available here:
|EXTENSION_NAME = 'GL_EXT_stencil_clear_tag'|
GL_STENCIL_CLEAR_TAG_VALUE_EXT = GL_STENCIL_CLEAR_TAG_VALUE_EXT
GL_STENCIL_TAG_BITS_EXT = GL_STENCIL_TAG_BITS_EXT
__file__ = '/home/mcfletch/pylive/OpenGL/GL/EXT/stencil_clear_tag.pyc'
__name__ = 'OpenGL.GL.EXT.stencil_clear_tag'
__package__ = 'OpenGL.GL.EXT'
glStencilClearTagEXT = <OpenGL.platform.baseplatform.glStencilClearTagEXT object at 0x76ea7d0>