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GL_OML_subsample
OML_subsample
Name Strings
GL_OML_subsample
Contact
Jon Leech, Silicon Graphics (ljp 'at' sgi.com)
Status
Complete. Approved by the Khronos SIG on July 19, 2001.
Version
Last Modified Date: 07/23/2001
Author Revision: $Header: //depot/main/doc/registry/extensions/OML/subsample.spec#10 $
Number
240
Dependencies
This extension is written against the OpenGL 1.2.1 Specification,
Overview
Many video image formats and compression techniques utilize various
component subsamplings, so it is necessary to provide a mechanism to
specify the up- and down-sampling of components as pixel data is
drawn from and read back to the client. Though subsampled components
are normally associated with the video color space, YCrCb, use of
subsampling in OpenGL does not imply a specific color space. Color
space conversion may be performed using other extensions or core
capabilities such as the color matrix.
This extension defines two new pixel storage formats representing
subsampled data on the client. It is loosely based on the
SGIX_subsample extension, but specifies subsampling with the data
format parameter rather than pixel packing parameters. It also
adds support for CYA subsampled data.
When pixel data is received from the client and an unpacking
upsampling mode other than PIXEL_SUBSAMPLE_NONE_OML is specified,
upsampling is performed via replication, unless otherwise specified
by UNPACK_RESAMPLE_OML.
Similarly, when pixel data is read back to the client and a packing
downsampling mode other than PIXEL_SUBSAMPLE_NONE_OML is specified,
downsampling is performed via simple component decimation (point
sampling), unless otherwise specified by PACK_RESAMPLE_OML.
Issues
* Which subsampled component orderings should be supported?
Only CY and CYA component ordering, since this matches contemporary
video hardware. YC and YCA ordering will require a separate
extension defining new formats.
* The new enumerant naming scheme gives the component frequencies in
the same order as the components themselves; that is,
FORMAT_SUBSAMPLE_24_24_OML corresponds to CY 4:2:2, and
FORMAT_SUBSAMPLE_244_244_OML corresponds to CYA 4:2:2:4. This makes
naming YC and YCA orderings easier.
* Should subsampling be specified with new pixel storage parameters,
like the SGIX_subsample extension, or with new formats, like the
EXT_422 extension?
With new formats. There are many invalid format/type combinations
when specifying subsampling with a pixel storage parameter. Also,
there's an ambiguity when doing this because the <format> parameter
represents the after-upsampling data format, not the host format.
* Because subsampled data is inherently pixel / texture oriented, this
extension only supports the new formats for pixel and texture
operations; it does not support them for convolution filters,
histograms, minmax, or color tables.
* The only packed pixel type supported is 10_10_10_2, since this is
needed for video data interoperability. It would be possible to
support many other packed pixel formats, but most are unused in
practice.
Is support for other packed pixels types, particularly
2_10_10_10_REV, required?
* Should readbacks of non-even widths be allowed when downsampling?
No. This is not consistent with draw operations, where this
constraint already exists. It also makes OML_resample more complex
when using an AVERAGE filter, since the edge cases may also apply to
even pixel coordinates. The spec may need to be more explicit about
this restriction.
IP Status
No known issues.
None.
Accepted by the <format> parameter of DrawPixels, ReadPixels,
TexImage1D, TexImage2D, TexImage3D, TexSubImage1D, TexSubImage2D,
TexSubImage3D, and GetTexImage
FORMAT_SUBSAMPLE_24_24_OML 0x8982
FORMAT_SUBSAMPLE_244_244_OML 0x8983
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)
- (3.6.4, p. 88 "Rasterization of Pixel Rectangles")
Add prior to the "Unpacking" subsection on page 90:
If <format> is FORMAT_SUBSAMPLE_24_24_OML or
FORMAT_SUBSAMPLE_244_244_OML, and <type> is one of the packed pixel
formats in table 3.8 other than UNSIGNED_INT_10_10_10_2, then the
error INVALID_OPERATION occurs; if <width> is not a multiple of 2
pixels, or if the value of the UNPACK_SKIP_PIXELS or
UNPACK_ROW_LENGTH parameters is not a multiple of 2 pixels, then the
error INVALID_OPERATION occurs.
- Add new entries to table 3.6:
Format Name Element Meaning and Order Target Buffer
----------- ------------------------- -------------
FORMAT_SUBSAMPLE_24_24_OML CbY / CrY Color
FORMAT_SUBSAMPLE_244_244_OML CbYA / CrYA Color
- Append to the caption of table 3.6:
Subsampled formats yield components that are further modified during
conversion to uniform sampling. The subsampled components are
denoted as Cb, Y, Cr, and A, although subsampled data is not defined
to be in any specific color space.
- Modify table 3.8:
<type> Parameter GL Data Number of Matching
Token Name Type Components Pixel Formats
----------------------- ------- ---------- -------------
UNSIGNED_INT_10_10_10_2 uint 2, 3, 4 RGBA, BGRA,
FORMAT_SUBSAMPLE_24_24_OML,
FORMAT_SUBSAMPLE_244_244_OML
- Append to the caption of table 3.8:
Subsampled formats may pack components from multiple groups into a
single uint.
- Modify table 3.11's UNSIGNED_INT_10_10_10_2 entry:
UNSIGNED_INT_10_10_10_2, <format>s RGBA and BGRA:
(use existing 4-component diagram)
UNSIGNED_INT_10_10_10_2, <format> SUBSAMPLE_24_24_OML:
31 22 21 12 11 2 1 0
| pixel 0/comp. 0 | pixel 0/comp. 1 | pixel 1/comp. 0 | xx |
| pixel 1/comp. 1 | pixel 2/comp. 0 | pixel 2/comp. 1 | xx |
UNSIGNED_INT_10_10_10_2, <format> SUBSAMPLE_244_244_OML:
31 22 21 12 11 2 1 0
| 1st comp. | 2nd comp. | 3rd comp. | xx |
- Change caption of table 3.11:
Table 3.11: UNSIGNED_INT formats. Subsampled formats are packed into
words, so components from a group may lie in different words. ``xx''
fields are unused.
- Add new subsection before "Conversion to RGB" on page 99:
Conversion to Uniform Sampling
This step is applied only to subsampled data. If <format> is
FORMAT_SUBSAMPLE_24_24_OML, then the number of components per pixel
is increased from two to three. If <format> is
FORMAT_SUBSAMPLE_244_244_OML. then the number of components per
pixel is increased from three to four.
After conversion to uniform sampling (see figure 3.9). pixels are
thereafter treated as though they were RGB (three component) or RGBA
(four component) format.
In the remainder of this section, the j'th component of the i'th
pixel in a row is denoted by S_i,j (for source pixels in client
memory) and D_i,j (for destination pixels in the color buffer).
Destination component values are defined as:
For even pixels ((i mod 2) == 0):
D_i,0 = S_i,0
D_i,1 = S_i,1
D_i,2 = S_i+1,0
D_i,3 = S_i,2
For odd pixels ((i mod 2) == 1):
D_i,0 = S_i-1,0
D_i,1 = S_i,1
D_i,2 = S_i,0
D_i,3 = S_i,2
- Add new figure 3.9 (renumber following figures):
FORMAT_SUBSAMPLE_24_24_OML:
<Cb0,Y0> <Cr0,Y1> <Cb2,Y2> <Cr2, Y3>
| | | | | | | |
| | ____/ \__|__ | | ____/ \__|__
| | / | \ | | / | \
|\_|__|_____ | | |\__|__|_____ | |
| | | \ | | | | | \ | |
V V V V V V V V V V V V
<Cb0,Y0,Cr0> <Cb0,Y1,Cr0> <Cb2,Y2,Cr2> <Cb2,Y3,Cr2>
< R0,G0,B1 > < R0,G1,B1 > < R2,G2,B3 > < R2,G3,B3 >
FORMAT_SUBSAMPLE_244_244_OML:
<Cb0,Y0,A0> <Cr0,Y1,A1> <Cb2,Y2,A2> <Cr2,Y3,A3>
| | \ | | \ | | \ | | \
| | _\_____/ \__|__ \ | | _\_____/ \__|__ \
| | / \ | \ \ | | / \ | \ \
|\_|__|___\____ | | \ |\__|__|___\____ | | \
| | | | \ | | | | | | | \ | | |
V V V V V V V V V V V V V V V V
<Cb0,Y0,Cr0,A0> <Cb0,Y1,Cr0,A1> <Cb2,Y2,Cr2,A2> <Cb2,Y3,Cr2,A3>
< R0,G0,B1,A0 > < R0,G1,B1,A1 > < R2,G2,B3,A2 > < R2,G3,B3,A3 >
Figure 3.9: Upsampling with component replication of subsampled data
from client memory to form RGB or RGBA pixels.
Additions to Chapter 4 of the OpenGL 1.2.1 Specification (Per-Fragment
Operations and the Frame Buffer)
4.3.2 Reading Pixels
- Add new subsection before "Final Conversion" on page 160:
Conversion to Subsampled Form
This step is applied only if <format> is FORMAT_SUBSAMPLE_24_24_OML
or FORMAT_SUBSAMPLE_244_244_OML.
In the remainder of this section, the j'th component of the i'th
pixel in a row is denoted by S_i,j (for source pixels in the color
buffer) and D_i,j (for destination pixels in client memory).
If <format> is FORMAT_SUBSAMPLE_24_24_OML, then the resulting pixels
have 2 components; if <format> is FORMAT_SUBSAMPLE_244_244_OML, then
the resulting pixels have 3 components (see figure 4.3). Destination
component values are defined as:
For even pixels ((i mod 2) == 0):
D_i,0 = S_i,0
D_i,1 = S_i,1
D_i,2 = S_i,3 (only for FORMAT_SUBSAMPLE_244_244_OML)
For odd pixels ((i mod 2) == 1):
D_i,0 = S_i-1,2
D_i,1 = S_i,1
D_i,2 = S_i,3 (only for FORMAT_SUBSAMPLE_244_244_OML)
- Add new figure 4.3 (renumber following figures):
FORMAT_SUBSAMPLE_24_24_OML:
<R0,G0,B0,A0> <R1,G1,B1,A1> <R2,G2,B2,A2> <R3,G3,B3,A3>
| | | | | | | | | | | | | | | |
| | \ * * | * * | | \ * * | * *
| | | | | | | |
V V V V V V V V
<Cb0,Y0><Cr0, Y1> <Cb2,Y2><Cr2, Y3>
<--- pixel pair ----> <--- pixel pair ---->
FORMAT_SUBSAMPLE_244_244_OML:
<R0,G0,B0,A0> <R1,G1,B1,A1> <R2,G2,B2,A2> <R3,G3,B3,A3>
| | | | | | | | | | | | | | | |
| | \__|__ * | * | | | \__|__ * | * |
| | | \ | | | | | \ | |
| | / | | / | | / | | /
V V V V V V V V V V V V
<Cb0,Y0,A0> <Cr0,Y1,A1> <Cb2,Y2,A2> <Cr2,Y3,A3>
<--- pixel pair ----> <--- pixel pair ---->
Figure 4.3: Downsampling of RGB or RGBA pixels to form subsampled
data in host memory.
- Add prior to the last sentence of subsection "Placement in Client
Memory" on page 162:
If <format> is FORMAT_SUBSAMPLE_24_24_OML, then only the
corresponding two elements (first two components of each group) are
written. If <format> is FORMAT_SUBSAMPLE_244_244_OML, then only the
corresponding three elements (first three components of each group)
are written.
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.
Additions to the GLX 1.3 Specification
TBD. Discussion of image formats in the GLX Protocol Specification
may need to be expanded.
Errors
See above.
New State
None.
New Implementation Dependent State
None.
Revision History
* Revision 10, 07/24/2001 - Finalized Status for OpenML 1.0.
* Revision 9, 07/16/2001 - Fix label in 24_24 packed pixel diagram.
* Revisions 7-8, 07/11/2001 - Assign enum values and extension number
for the registry.
* Revision 6 - Correct errors in the equations describing subsampling.
* Revision 5 - formatting changes for OpenML Specification
* Revision 4 - Rewrite to use the <format> parameter, rather than a
pixel storage mode, to specify subsampled data. Specify which
format/type combinations are allowed for subsampled data, and define
the representation of 10-bit component subsampled packed pixel data.
* Revision 3 - Removed support for YC component orders. Renamed CY and
CYA enumerants more sensibly. Changed text descriptions of sampling
to equations. Made enum values undefined until we've determined if
this extension is backwards compatible with SGIX_subsample.
* Revision 2 - corrected 4224 upsampling and downsampling figures.
Moved discussion of errors for non-even image widths from the
OML_resample specification.
* Revision 1 - derived from SGIX_subsample.
Implementation Support
List of OpenGL implementations supporting the GL_OML_subsample extension
Original File
Original text file for the GL_OML_subsample extension
Page generated on Sun Nov 20 18:40:37 2005