| GL_NV_packed_depth_stencil |
NV_packed_depth_stencil
GL_NV_packed_depth_stencil
Matt Craighead, NVIDIA Corporation (mcraighead 'at' nvidia.com)
Copyright NVIDIA Corporation, 2000, 2001.
NVIDIA Proprietary.
Shipping (version 1.1)
NVIDIA Date: August 22, 2001 (version 1.1)
$Date$ $Revision$
$Id: //sw/main/docs/OpenGL/specs/GL_NV_packed_depth_stencil.txt#3 $
226
Written based on the wording of the OpenGL 1.2.1 specification.
SGIX_depth_texture affects the definition of this extension.
Many OpenGL implementations have chosen to interleave the depth and
stencil buffers into one buffer, often with 24 bits of depth
precision and 8 bits of stencil data. 32 bits is more than is needed
for the depth buffer much of the time; a 24-bit depth buffer, on the
other hand, requires that reads and writes of depth data be unaligned
with respect to power-of-two boundaries. On the other hand, 8 bits
of stencil data is more than sufficient for most applications, so it
is only natural to pack the two buffers into a single buffer with
both depth and stencil data. OpenGL never provides direct access to
the buffers, so the OpenGL implementation can provide an interface to
applications where it appears the one merged buffer is composed of
two logical buffers.
One disadvantage of this scheme is that OpenGL lacks any means by
which this packed data can be handled efficiently. For example, when
an application reads from the 24-bit depth buffer, using the type
GL_UNSIGNED_SHORT will lose 8 bits of data, while GL_UNSIGNED_INT has
8 too many. Both require expensive format conversion operations. A
24-bit format would be no more suitable, because it would also suffer
from the unaligned memory accesses that made the standalone 24-bit
depth buffer an unattractive proposition in the first place.
Many applications, such as parallel rendering applications, may also
wish to draw to or read back from both the depth and stencil buffers
at the same time. Currently this requires two separate operations,
reducing performance. Since the buffers are interleaved, drawing to
or reading from both should be no more expensive than using just one;
in some cases, it may even be cheaper.
This extension provides a new data format, GL_DEPTH_STENCIL_NV, that
can be used with the glDrawPixels, glReadPixels, and glCopyPixels
commands, as well as a packed data type, GL_UNSIGNED_INT_24_8_NV,
that is meant to be used with GL_DEPTH_STENCIL_NV. No other formats
are supported with GL_DEPTH_STENCIL_NV. If SGIX_depth_texture is
supported, GL_DEPTH_STENCIL_NV/GL_UNSIGNED_INT_24_8_NV data can also
be used for textures; this provides a more efficient way to supply
data for a 24-bit depth texture.
GL_DEPTH_STENCIL_NV data, when passed through the pixel path,
undergoes both depth and stencil operations. The depth data is
scaled and biased by the current GL_DEPTH_SCALE and GL_DEPTH_BIAS,
while the stencil data is shifted and offset by the current
GL_INDEX_SHIFT and GL_INDEX_OFFSET. The stencil data is also put
through the stencil-to-stencil pixel map.
glDrawPixels of GL_DEPTH_STENCIL_NV data operates similarly to that
of GL_STENCIL_INDEX data, bypassing the OpenGL fragment pipeline
entirely, unlike the treatment of GL_DEPTH_COMPONENT data. The
stencil and depth masks are applied, as are the pixel ownership and
scissor tests, but all other operations are skipped.
glReadPixels of GL_DEPTH_STENCIL_NV data reads back a rectangle from
both the depth and stencil buffers.
glCopyPixels of GL_DEPTH_STENCIL_NV data copies a rectangle from
both the depth and stencil buffers. Like glDrawPixels, it applies
both the stencil and depth masks but skips the remainder of the
OpenGL fragment pipeline.
glTex[Sub]Image[1,2,3]D of GL_DEPTH_STENCIL_NV data loads depth data
into a depth texture. glGetTexImage of GL_DEPTH_STENCIL_NV data can
be used to retrieve depth data from a depth texture.
* Depth data has a format of GL_DEPTH_COMPONENT, and stencil data
has a format of GL_STENCIL_INDEX. So shouldn't the enumerant be
called GL_DEPTH_COMPONENT_STENCIL_INDEX_NV?
RESOLVED: No, this is fairly clumsy.
* Should we support CopyPixels?
RESOLVED: Yes. Right now copying stencil data means masking off
just the stencil bits, while copying depth data has strange
unintended consequences (fragment operations) and is difficult to
implement. It is easy and useful to add CopyPixels support.
* Should we support textures?
RESOLVED: Yes. 24-bit depth textures have no good format without
this extension.
* Should the depth/stencil format support other standard types,
like GL_FLOAT or GL_UNSIGNED_INT?
RESOLVED: No, this extension is designed to be minimalist.
Supporting more types gains little because the new types will
just require data format conversions. Our goal is to match the
native format of the buffer, not add broad new classes of
functionality.
* Should the 24/8 format be supported for other formats, such as
LUMINANCE_ALPHA? Should we support an 8/24 reversed format as
well?
RESOLVED: No and no, this adds implementation burden and gains us
little, if anything.
* Does anything need to be written in the spec on the topic of
using GL_DEPTH_STENCIL_NV formats for glTexImage* or
glGetTexImage?
RESOLVED: No. Since the SGIX_depth_texture extension spec was
never actually written (the additions to Section 3 are "XXX -
lots" and a few brief notes on how it's intended to work), it's
impossible to write what would essentially be amendments to that
spec.
However, it is worthwhile to mention here the intended behavior.
When downloading into a depth component texture, the stencil
indices are ignored, and when retrieving a depth component
texture, the stencil indices obtained from the texture are
undefined.
* Should anything be said about performance?
RESOLVED: No, not in the spec. However, common sense should
apply. Apps should probably check that GL_DEPTH_BITS is 24 and
that GL_STENCIL_BITS is 8 before using either the new DrawPixels
or ReadPixels formats. CopyPixels is probably safe regardless of
the size of either buffer. The 24/8 format should probably only
be used with 24-bit depth textures.
   New Procedures and Functions |
None.
Accepted by the <format> parameter of DrawPixels, ReadPixels,
TexImage1D, TexImage2D, TexImage3D, TexSubImage1D, TexSubImage2D,
TexSubImage3D, and GetTexImage, and by the <type> parameter of
CopyPixels:
DEPTH_STENCIL_NV 0x84F9
Accepted by the <type> parameter of DrawPixels, ReadPixels,
TexImage1D, TexImage2D, TexImage3D, TexSubImage1D, TexSubImage2D,
TexSubImage3D, and GetTexImage:
UNSIGNED_INT_24_8_NV 0x84FA
| Additions to Chapter 2 of the OpenGL 1.2.1 Specification (OpenGL Operation) |
None.
| Additions to Chapter 3 of the OpenGL 1.2.1 Specification (Rasterization) |
Update the first paragraph on page 90 to say:
"... If the GL is in color index mode and <format> is not one of
COLOR_INDEX, STENCIL_INDEX, DEPTH_COMPONENT, or DEPTH_STENCIL_NV,
then the error INVALID_OPERATION occurs. If <type> is BITMAP and
<format> is not COLOR_INDEX or STENCIL_INDEX then the error
INVALID_ENUM occurs. If <format> is DEPTH_STENCIL_NV and <type> is
not UNSIGNED_INT_24_8_NV then the error INVALID_ENUM occurs. Some
additional constraints on the combinations of <format> and <type>
values that are accepted is discussed below."
Add a row to Table 3.5 (page 91):
type Parameter GL Type Special
------------------------------------------------
... ... ...
UNSIGNED_INT_2_10_10_10_REV uint Yes
UNSIGNED_INT_24_8_NV uint Yes
Add a row to Table 3.6 (page 92):
Format Name Element Meaning and Order Target Buffer
------------------------------------------------------------------
... ... ...
DEPTH_COMPONENT Depth Depth
DEPTH_STENCIL_NV Depth and Stencil Index Depth and Stencil
... ... ...
Add a row to Table 3.8 (page 94):
type Parameter GL Type Components Pixel Formats
------------------------------------------------------------------
... ... ... ...
UNSIGNED_INT_2_10_10_10_REV uint 4 RGBA,BGRA
UNSIGNED_INT_24_8_NV uint 2 DEPTH_STENCIL_NV
Update the last paragraph on page 93 to say:
"Calling DrawPixels with a <type> of UNSIGNED_BYTE_3_3_2, ...,
UNSIGNED_INT_2_10_10_10_REV, or UNSIGNED_INT_24_8_NV is a special
case in which all the components of each group are packed into a
single unsigned byte, unsigned short, or unsigned int, depending on
the type."
Add the following diagram to Table 3.11 (page 97):
UNSIGNED_INT_24_8_NV
31 30 29 28 27 26 ... 12 11 10 9 8 7 6 5 4 3 2 1 0
+----------------------------------+---------------+
| 1st Component | 2nd Component |
+----------------------------------+---------------+
Add a row to Table 3.12 (page 98):
Format | 1st 2nd 3rd 4th
-----------------+-------------------------------
... | ... ... ... ...
BGRA | blue green red alpha
DEPTH_STENCIL_NV | depth stencil N/A N/A
Add the following paragraph to the end of the section "Conversion to
floating-point" (page 99):
"For groups of components that contain both standard components and
index elements, such as DEPTH_STENCIL_NV, the index elements are not
converted."
Update the last paragraph in the section "Conversion to Fragments"
(page 100) to say:
"... Groups arising from DrawPixels with a <format> of STENCIL_INDEX
or DEPTH_STENCIL_NV are treated specially and are described in
section 4.3.1."
Update the first paragraph of section 3.6.5 "Pixel Transfer
Operations" (pages 100-101) to say:
"The GL defines five kinds of pixel groups:
1. RGBA component: Each group comprises four color components:
red, green, blue, and alpha.
2. Depth component: Each group comprises a single depth component.
3. Color index: Each group comprises a single color index.
4. Stencil index: Each group comprises a single stencil index.
5. Depth/stencil: Each group comprises a depth component and a
stencil index."
Update the first paragraph in the section "Arithmetic on Components"
(page 101) to say:
"This step applies only to RGBA component and depth component groups
and the depth components in depth/stencil groups. ..."
Update the first paragraph in the section "Arithmetic on Indices"
(page 101) to say:
"This step applies only to color index and stencil index groups and
the stencil indices in depth/stencil groups. ..."
Update the first paragraph in the section "Stencil Index Lookup"
(page 102) to say:
"This step applies only to stencil index groups and the stencil
indices in depth/stencil groups. ..."
| Additions to Chapter 4 of the OpenGL 1.2.1 Specification (Per-Fragment Operations and the Frame Buffer) |
Replace section 4.3.1 "Writing to the Stencil Buffer" (page 156) with
the following:
"4.3.1 Writing to the Stencil Buffer or to the Depth and Stencil
Buffers
The operation of DrawPixels was described in section 3.6.4, except if
the <format> argument was STENCIL_INDEX or DEPTH_STENCIL_NV. In this
case, all operations described for DrawPixels take place, but window
(x,y) coordinates, each with the corresponding stencil index or depth
value and stencil index, are produced in lieu of fragments. Each
coordinate-data pair is sent directly to the per-fragment operations,
bypassing the texture, fog, and antialiasing application stages of
rasterization. Each pair is then treated as a fragment for purposes
of the pixel ownership and scissor tests; all other per-fragment
operations are bypassed. Finally, each stencil index is written to
its indicated location in the framebuffer, subject to the current
setting of 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.
The error INVALID_OPERATION results if there is no stencil buffer, or
if the <format> argument was DEPTH_STENCIL_NV, if there is no depth
buffer."
Add the following paragraph after the second paragraph of the
section "Obtaining Pixels from the Framebuffer" (page 158):
"If the <format> is DEPTH_STENCIL_NV, then values are taken from both
the depth buffer and the stencil buffer. If there is no depth buffer
or if there is no stencil buffer, the error INVALID_OPERATION
occurs. If the <type> parameter is not UNSIGNED_INT_24_8_NV, the
error INVALID_ENUM occurs."
Update the third paragraph on page 159 to say:
"If the GL is in RGBA mode, and <format> is one of RED, GREEN, BLUE,
ALPHA, RGB, RGBA, BGR, BGRA, LUMINANCE, or LUMINANCE_ALPHA, then red,
green, blue, and alpha values are obtained from the framebuffer
Update the first sentence of the section "Conversion of RGBA values"
(page 159) to say:
"This step applies only if the GL is in RGBA mode, and then only if
<format> is neither STENCIL_INDEX, DEPTH_COMPONENT, nor
DEPTH_STENCIL_NV."
Update the section "Conversion of Depth values" (page 159) to say:
"This step applies only if <format> is DEPTH_COMPONENT or
DEPTH_STENCIL_NV. Each element taken from the depth buffer is taken
to be a fixed-point value in [0,1] with m bits, where m is the number
of bits in the depth buffer (see section 2.10.1)."
Add a row to Table 4.6 (page 160):
type Parameter Index Mask
---------------------------------
... ...
INT 2^31-1
UNSIGNED_INT_24_8_NV 2^8-1
Add the following paragraph to the end of the section "Final
Conversion" (page 160):
"For a depth/stencil pair, first the depth component is clamped to
[0,1]. Then the appropriate conversion formula from Table 4.7 is
applied to the depth component, while the index is masked by the
value given in Table 4.6 or converted to a GL float data type if the
<type> is FLOAT."
Add a row to Table 4.7 (page 161):
type Parameter GL Type Component Conversion ...
------------------------------------------------------------------
... ... ...
UNSIGNED_INT_2_10_10_10_REV uint c = (2^N - 1)f
UNSIGNED_INT_24_8_NV uint c = (2^N - 1)f (depth only)
Update the second and third paragraphs of section 4.3.3 (page 162) to
say:
"<type> is a symbolic constant that must be one of COLOR, STENCIL,
DEPTH, or DEPTH_STENCIL_NV, indicating that the values to be
transfered are colors, stencil values, depth values, or depth/stencil
pairs, respectively. The first four arguments have the same
interpretation as the corresponding arguments to ReadPixels.
Values are obtained from the framebuffer, converted (if appropriate),
then subjected to the pixel transfer operations described in section
3.6.5, just as if ReadPixels were called with the corresponding
arguments. If the <type> is STENCIL, DEPTH, or DEPTH_STENCIL_NV,
then it is as if the <format> for ReadPixels were STENCIL_INDEX,
DEPTH_COMPONENT, or DEPTH_STENCIL_NV, respectively. If the <type> is
COLOR, then if the GL is in RGBA mode, it is as if the <format> were
RGBA, while if the GL is in color index mode, it is as if the
<format> were COLOR_INDEX."
| Additions to Chapter 5 of the OpenGL 1.2.1 Specification (Special Functions) |
None.
| Additions to Chapter 6 of the OpenGL 1.2.1 Specification (State and State Requests) |
None.
None.
The error INVALID_ENUM is generated if DrawPixels or ReadPixels is
called where format is DEPTH_STENCIL_NV and type is not
UNSIGNED_INT_24_8_NV.
The error INVALID_OPERATION is generated if DrawPixels or ReadPixels
is called where type is UNSIGNED_INT_24_8_NV and format is not
DEPTH_STENCIL_NV.
The error INVALID_OPERATION is generated if DrawPixels or ReadPixels
is called where format is DEPTH_STENCIL_NV and there is not both a
depth buffer and a stencil buffer.
The error INVALID_OPERATION is generated if CopyPixels is called
where type is DEPTH_STENCIL_NV and there is not both a depth buffer
and a stencil buffer.
None.
August 22, 2001 - Fixed a small typo in the updates to Section 4.3.3.
August 22, 2001 - Fixed a small typo in the updates to Section 4.3.3.
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