EXT_stencil_two_side
GL_EXT_stencil_two_side
Mark J. Kilgard, NVIDIA Corporation (mjk 'at' nvidia.com)
Copyright NVIDIA Corporation, 2001-2002.
Implemented in CineFX (NV30) Emulation driver, August 2002.
Shipping in Release 40 NVIDIA driver for CineFX hardware, January 2003.
Last Modified Date: $Date: 2003/01/08 $
$Id: //sw/main/docs/OpenGL/specs/GL_EXT_stencil_two_side.txt#6 $
268
Written based on the OpenGL 1.3 specification.
NV_packed_depth_stencil affects the definition of this extension.
This extension provides two-sided stencil testing where the
stencil-related state (stencil operations, reference value, compare
mask, and write mask) may be different for front- and back-facing
polygons. Two-sided stencil testing may improve the performance
of stenciled shadow volume and Constructive Solid Geometry (CSG)
rendering algorithms.
Is this sufficient for shadow volume stencil update in a single pass?
RESOLUTION: Yes.
An application that wishes to increment the stencil value for
rasterized depth-test passing fragments of front-facing polygons and
decrement the stencil value for rasterized fragments of depth-test
passing back-facing polygons in a single pass can use the following
configuration:
glDepthMask(0);
glColorMask(0,0,0,0);
glDisable(GL_CULL_FACE);
glEnable(GL_STENCIL_TEST);
glEnable(GL_STENCIL_TEST_TWO_SIDE_EXT);
glActiveStencilFaceEXT(GL_BACK);
glStencilOp(GL_KEEP, // stencil test fail
GL_KEEP, // depth test fail
GL_DECR_WRAP_EXT); // depth test pass
glStencilMask(~0);
glStencilFunc(GL_ALWAYS, 0, ~0);
glActiveStencilFaceEXT(GL_FRONT);
glStencilOp(GL_KEEP, // stencil test fail
GL_KEEP, // depth test fail
GL_INCR_WRAP_EXT); // depth test pass
glStencilMask(~0);
glStencilFunc(GL_ALWAYS, 0, ~0);
renderShadowVolumePolygons();
Notice the use of EXT_stencil_wrap to avoid saturating decrements
losing the shadow volume count. An alternative, using the
conventional GL_INCR and GL_DECR operations, is to clear the stencil
buffer to one half the stencil buffer value range, say 128 for an
8-bit stencil buffer. In the case, a pixel is "in shadow" if the
final stencil value is greater than 128 and "out of shadow" if the
final stencil value is 128. This does still create a potential
for stencil value overflow if the stencil value saturates due
to an increment or decrement. However saturation is less likely
with two-sided stencil testing than the conventional two-pass
approach because front- and back-facing polygons are mixed together,
rather than processing batches of front-facing then back-facing
polygons.
Contrast the two-sided stencil testing approach with the more
or less equivalent approach using facingness-independent stencil
testing:
glDepthMask(0);
glColorMask(0,0,0,0);
glEnable(GL_CULL_FACE);
glEnable(GL_STENCIL_TEST);
glStencilMask(~0);
glStencilFunc(GL_ALWAYS, 0, ~0);
// Increment for front faces
glCullFace(GL_BACK);
glStencilOp(GL_KEEP, // stencil test fail
GL_KEEP, // depth test fail
GL_INCR); // depth test pass
renderShadowVolumePolygons();
// Decrement for back faces
glCullFace(GL_FRONT);
glStencilOp(GL_KEEP, // stencil test fail
GL_KEEP, // depth test fail
GL_DECR); // depth test pass
renderShadowVolumePolygons();
Notice that all the render work implicit
in renderShadowVolumePolygons is performed twice with the
conventional approach, but only once with the two-sided stencil
testing approach.
Should there be just front and back stencil test state, or should
the stencil write mask also have a front and back state?
RESOLUTION: Both the stencil test and stencil write mask state
should have front and back versions.
The shadow volume application for two-sided stencil testing does
not require differing front and back versions of the stencil write
mask, but we anticipate other applications where front and back
write masks may be useful.
For example, it may be useful to draw a convex polyhedra such that
(assuming the stencil bufer is cleared to the binary value 1010):
1) front-facing polygons that pass the depth test set stencil bit 0
2) front-facing polygons that fail the depth test zero stencil bit 1
3) back-facing polygons that pass the depth test set stencil bit 2
4) back-facing polygons that fail the depth test zero stencil bit 3
This could be accomplished in a single rendering pass using:
glStencilMask(~0);
glStencilClear(0xA);
glClear(GL_STENCIL_BUFFER_BIT);
glDepthMask(0);
glColorMask(0,0,0,0);
glDisable(GL_CULL_FACE);
glEnable(GL_STENCIL_TEST);
glEnable(GL_STENCIL_TEST_TWO_SIDE_EXT);
glActiveStencilFaceEXT(GL_BACK);
glStencilOp(GL_KEEP, // stencil test fail
GL_ZERO, // depth test fail
GL_REPLACE); // depth test pass
glStencilMask(0xC);
glStencilFunc(GL_ALWAYS, 0x4, ~0);
glActiveStencilFaceEXT(GL_FRONT);
glStencilOp(GL_KEEP, // stencil test fail
GL_ZERO, // depth test fail
GL_REPLACE); // depth test pass
glStencilMask(0x3);
glStencilFunc(GL_ALWAYS, 0x1, ~0);
renderConvexPolyhedra();
Is there a performance advantage to using two-sided stencil testing?
RESOLUTION: It depends.
In a fill-rate limited situation, rendering front-facing primitives,
then back-facing primitives in two passes will generate the same
number of rasterized fragments as rendering front- and back-facing
primitives in a single pass.
However, in other situations that are CPU-limited,
transform-limited, or setup-limited, two-sided stencil testing can
be faster than the conventional two-pass face culling rendering
approaches. For example, if a lengthy vertex program is executed
for every shadow volume vertex, rendering the shadow volume with
a single two-sided stencil testing pass is advantageous.
Often applications using stencil shadow volume techniques require
substantial CPU resources to determine potential silhouette
boundaries to project shadow volumes from. If the shadow volume
geometry generated by the CPU is only required to be sent to the GL
once per-frame (rather than twice with the conventional technique),
that can ease the CPU burden required to implement stenciled shadow
volumes.
Should GL_FRONT_AND_BACK be accepted by glActiveStencilFaceEXT?
RESOLUTION: No.
GL_FRONT_AND_BACK is useful when materials are being updated for
two-sided lighting because the front and back material are often
identical and may change frequently (glMaterial calls are allowed
within glBegin/glEnd pairs).
Two-sided stencil has no similiar performance justification.
It is also likely that forcing implementations to support this mode
would increase the amount of overhead required to set stencil
state, even for applications that don't use two-sided stencil.
How should the two-sided stencil enable operate?
RESOLUTION: It should be modeled after the way two-sided lighting
works. There is a GL_LIGHTING enable and then an additional
two-sided lighting mode. Unlike two-sided lighting which is a
light model boolean, the two-sided stencil testing is a standard
enable named GL_STENCIL_TEST_TWO_SIDE_EXT.
Here is the pseudo-code for the stencil testing enables:
if (glIsEnabled(GL_STENCIL_TEST)) {
if (glIsEnabled(GL_STENCIL_TEST_TWO_SIDE_EXT) && primitiveType == polygon) {
use two-sided stencil testing
} else {
use conventional stencil testing
}
} else {
no stencil testing
}
How should the two-sided stencil interact with glPolygonMode?
RESOLUTION: Primitive type is determined by the begin mode
so GL_TRIANGLES, GL_TRIANGLE_STRIP, GL_QUAD_STRIP, GL_QUADS,
GL_TRIANGLE_FAN, and GL_POLYGON generate polygon primitives. If the
polygon mode is set such that lines or points are rasterized,
two-sided stencil testing still operates based on the original
polygon facingness if stencil testing and two-sided stencil testing
are enabled.
This is consistent with how two-sided lighting and face culling
interact with glPolygonMode.
   New Procedures and Functions |
void ActiveStencilFaceEXT(enum face);
Accepted by the <cap> parameter of Enable, Disable, and IsEnabled,
and by the <pname> parameter of GetBooleanv, GetIntegerv,
GetFloatv, and GetDoublev:
STENCIL_TEST_TWO_SIDE_EXT 0x8910
Accepted by the <face> parameter of ActiveStencilFaceEXT:
FRONT
BACK
Accepted by the <pname> parameters of GetBooleanv, GetIntegerv,
GetFloatv, and GetDoublev:
ACTIVE_STENCIL_FACE_EXT 0x8911
| Additions to Chapter 2 of the GL Specification (OpenGL Operation) |
None
| Additions to Chapter 3 of the GL Specification (Rasterization) |
None
| Additions to Chapter 4 of the GL Specification (Per-Fragment Operations and the Framebuffer) |
-- Section 4.1.5 "Stencil test"
Replace the first paragraph in the section with:
"The stencil test conditionally discards a fragment based on the
outcome of a comparison between the value in the stencil buffer at
location (xw,yw) and a reference value.
The test is enabled or disabled with the Enable and Disable commands,
using the symbolic constant STENCIL_TEST. When disabled, the stencil
test and associated modifications are not made, and the fragment is
always passed.
Stencil testing may operate in a two-sided mode. Two-sided stencil
testing is enabled or disabled with the Enable and Disable commands,
using the symbolic constant STENCIL_TEST_TWO_SIDE_EXT. When stencil
testing is disabled, the state of two-sided stencil testing does
not affect fragment processing.
There are two sets of stencil-related state, the front stencil
state set and the back stencil state set. When two-sided stencil
testing is enabled, stencil tests and writes use the front set of
stencil state when processing fragments rasterized from non-polygon
primitives (points, lines, bitmaps, image rectangles) and front-facing
polygon primitives while the back set of stencil state is used when
processing fragments rasterized from back-facing polygon primitives.
For the purposes of two-sided stencil testing, a primitive is still
considered a polygon even if the polygon is to be rasterized as
points or lines due to the current polygon mode. Whether a polygon
is front- or back-facing is determined in the same manner used for
two-sided lighting and face culling (see sections 2.13.1 and 3.5.1).
When two-sided stencil testing is disabled, the front set of stencil
state is always used when stencil testing fragments.
The active stencil face determines whether stencil-related commands
update the front or back stencil state. The active stencil face is
set with:
void ActiveStencilFace(enum face);
where face is either FRONT or BACK. Stencil commands (StencilFunc,
StencilOp, and StencilMask) that update the stencil state update the
front stencil state if the active stencil face is FRONT and the back
stencil state if the active stencil face is BACK. Additionally,
queries of stencil state return the front or back stencil state
depending on the current active stencil face.
The stencil test state is controlled with
void StencilFunc(enum func, int ref, uint mask);
void StencilOp(enum sfail, enum dpfail, enum dppass);"
Replace the third and second to the last sentence in the last
paragraph in section 4.1.5 with:
"In the initial state, stencil testing and two-sided stencil testing
are both disabled, the front and back stencil reference values are
both zero, the front and back stencil comparison functions are ALWAYS,
and the front and back stencil mask are both all ones. Initially,
both the three front and the three back stencil operations are KEEP."
-- Section 4.2.2 "Fine Control of Buffer Updates"
Replace the last sentence of the third paragraph with:
"The initial state is for both the front and back stencil plane mask
to be all ones. The clear operation always uses the front stencil
write mask when clearing the stencil buffer."
-- Section 4.3.1 "Writing to the Stencil Buffer or to the Depth and
Stencil Buffers"
Replace the final sentence in the first paragraph with:
"Finally, each stencil index is written to its indicated location
in the framebuffer, subject to the current front stencil mask state
(set with StencilMask), and if a depth component is present, if the
setting of DepthMask is not FALSE, it is also written to the
framebuffer; the setting of DepthTest is ignored."
| Additions to Chapter 5 of the GL Specification (Special Functions) |
None
| Additions to Chapter 6 of the GL Specification (State and State Requests) |
None
| Additions to the GLX, WGL, and AGL Specification |
None
A new GL rendering command is added. The following command is sent to the
server as part of a glXRender request:
ActiveStencilFaceEXT
2 8 rendering command length
2 4220 rendering command opcode
4 ENUM face
None
(table 6.15, page 205) amend the following entries:
Get Value Type Get Command Initial Value Description Sec Attribute
------------------------- ---- ----------- ------------- ------------------- ----- --------------
STENCIL_FUNC 2xZ8 GetIntegerv ALWAYS Stencil function 4.1.4 stencil-buffer
STENCIL_VALUE_MASK 2xZ+ GetIntegerv 1's Stencil mask 4.1.4 stencil-buffer
STENCIL_REF 2xZ+ GetIntegerv 0 Stencil reference 4.1.4 stencil-buffer
value
STENCIL_FAIL 2xZ6 GetIntegerv KEEP Stencil fail action 4.1.4 stencil-buffer
STENCIL_PASS_DEPTH_FAIL 2xZ6 GetIntegerv KEEP Stencil depth 4.1.4 stencil-buffer
buffer fail action
STENCIL_PASS_DEPTH_PASS 2xZ6 GetIntegerv KEEP Stencil depth 4.1.4 stencil-buffer
buffer pass action
[Type field is amended with "2x" prefix.]
(table 6.15, page 205) add the following entries:
Get Value Type Get Command Initial Value Description Sec Attribute
------------------------- ---- ----------- ------------- ----------------- ------ ---------------------
STENCIL_TEST_TWO_SIDE_EXT B IsEnabled False Two-sided stencil 4.1.4 stencil-buffer/enable
test enable
ACTIVE_STENCIL_FACE_EXT Z2 GetIntegerv FRONT Active stencil 4.1.4 stencil-buffer
face selector
(table 6.16, page 205) ammend the following entry:
Get Value Type Get Command Initial Value Description Sec Attribute
------------------------- ---- ----------- ------------- ----------------- ------ --------------
STENCIL_WRITE_MASK 2xZ+ GetIntegerv 1's Stencil buffer 4.2.2 stencil-buffer
writemask
[Type field is amended with "2x" prefix.]
None
None
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