SGI Developer Toolbox 6.1/SGI Developer Toolbox 6.1 - Disc 4.iso/documents/OpenGL/analysisPEX

Text File | 1994-08-02 | 35.8 KB | 782 lines

             Analysis of PEX 5.1 and OpenGL 1.0



                 Allen Akin

              Silicon Graphics Computer Systems
                   August 3, 1992





1.  Introduction

PEX and OpenGL are two 3D graphics systems contending for the position
of de facto standard in the workstation market. While they are similar
in many ways, they also differ in important aspects of their
application programming interfaces, functionality, performance,
portability, openness, and responsiveness to changes in markets and in
technology.

To our knowledge, no detailed comparisons of PEX and OpenGL have been
published. Perhaps this is because the two systems are evolving and few
individuals have experience with both. With the release of PEX 5.1 and
OpenGL 1.0, we believe the systems are relatively stable and it is now
time to undertake a comparison.

This paper analyzes some of the significant differences between PEX and
OpenGL, with attention to issues faced by users as well as
implementors. We hope it will be a positive contribution to the
debate.



2.  Programming Interfaces

There is an essential difference in scope between PEX and OpenGL:

    OpenGL is an application programming interface. The OpenGL
    specification defines the data structures and procedure calls
    application programs must use to produce graphical output, as
    well as the semantics of the drawing process.

    PEX is a wire protocol. That is, PEX specifies how graphics
    requests are to be encoded (as if for transmission over a
    network), and what the semantics of those encoded requests must
    be.

PEX does not specify an application programming interface. One of the
primary goals of PEX is efficient support for PHIGS. The original PEX
designers planned to use the ANSI and ISO programming language bindings
for PHIGS, and they explicitly refused to define a competing API. Their
decision was reflected in the name PEX, which is often said to be an
acronym for PHIGS Extensions to X.

This choice generated considerable controversy. Although PHIGS has the
advantages of international standardization and an existing software
base, it is quite complex and some software developers have claimed
that using it would force them to restructure their applications. (For
example, standard PHIGS lacks support for immediate-mode graphics.) As
a result, many PEX proponents favor the use of a "PEXlib" which would
expose the PEX protocol more completely. The X Consortium has proposed
such an interface. It offers greater device-specific control and in
many cases better performance than PHIGS. However, it also exposes PEX
implementation dependencies (such as subsets) that must be handled by
special-case code in applications rather than in the API.

The deficiencies of PHIGS and PEXlib have led to proposals for
additional PEX-compatible APIs. While supporting a variety of APIs
improves the chance that at least one will be appropriate for any given
application, the proliferation of interfaces has slowed the acceptance
of PEX. Many hardware vendors and software developers have chosen to
wait for one API to dominate, rather than risk investing in one that
fails. The possibility that incompatible protocol changes in PEX 6.0
will force incompatible API changes in PEXlib has also caused concern.

The situation with OpenGL is much simpler. There is a single
well-defined OpenGL API. To guarantee interoperability in heterogeneous
networks, there is an OpenGL extension (GLX) and wire protocol for use
with the X Window System. In other environments (e.g. Microsoft
Windows) where currently there is no need for a wire protocol, OpenGL
has none.

In recent months some in the PEX community have advanced the view that
the PEX protocol is a general-purpose foundation for supporting all 3D
graphics APIs. IRIS GL is arguably the market-leading API for 3D, but
PEX 5.1 is missing functionality that is crucial for supporting it.
(See below.) In contrast, it has been demonstrated that GL has
sufficient functionality to support PHIGS because there are commercial
PHIGS implementations built on IRIS GL.  Furthermore, IRIS GL has also
been used to support higher-level APIs including IRIS Inventor, IRIS
Explorer, and Ithaca Software's HOOPS. These examples lead us to
believe that the GLX protocol is actually a better candidate than PEX
to support a wide variety of APIs on an equal footing.



3.  Functionality

It is difficult to compare PEX and OpenGL functionality because of a
major difference in design philosophy:

    Nearly all PEX functionality is optional: it is not present in
    all PEX subsets, or some aspect of it is
    "implementation-dependent." The PHIGS API or PEXlib-based
    applications must query the PEX extension before using any
    implementation-dependent feature, and adjust their behavior
    accordingly. For example, a PEX implementation might include
    just the "PHIGS Workstation" subset, and thus be incapable of
    immediate-mode rendering.  Alternatively, it might include just
    the "Immediate Rendering" subset, and thus be incapable of
    anything else!  As another example, PEX provides an interface
    to select Phong shading, but there is no requirement that Phong
    shading actually be supported (and to our knowledge, no current
    implementations of PEX support it).  In short, it is not easy
    to say precisely what functionality PEX includes, because it
    varies so much from implementation to implementation.

    In contrast, all OpenGL implementations are required to support
    all the functionality described in the specification. Even the
    advanced rendering features are guaranteed to be present.

Despite this fundamental difficulty, some functionality comparisons can
be made, and in such areas it is important to understand the
differences between the two systems. Some examples follow.

3.1.  Text

PEX has extensive support for text, including a PEX font mechanism that
is independent of X Window System fonts. (Access to X11 fonts is
optional.) Ordinary PEX stroke-precision text is derived from a vector
font, and is fully 3D-transformable.

OpenGL provides routines that convert fonts from its environment (the X
Window System or Microsoft Windows) into OpenGL display lists. The only
font mechanism is that provided by the environment. If the original
fonts are in outline form (e.g. TrueType), then the display lists will
contain fully transformable geometry. If the original fonts are in
bitmap form, then the display lists will contain bitmaps.

PEX has the advantage that a fully transformable vector font is always
available. OpenGL has the advantage that it guarantees all of the
environment's existing fonts may be used, and there is no need to
create, license, or distribute new fonts specifically for 3D.

3.2.  Display List Editing

Both PEX and OpenGL are used in networked environments, where it may be
expensive to transport drawing requests from the client application to
the graphics server. Both provide a server-side storage mechanism
(called "display lists" in OpenGL, "structure store" in PEX) that
allows applications to cache drawing requests on the server and avoid
transport costs for requests that are reused.

PEX supports nearly all of the PHIGS structure store editing
functionality, including the ability to label sections of structures;
insert, replace, or delete elements of structures; copy structures; and
search through structures according to labels, hierarchical
organization, or spatial position of the primitives in the structures.

OpenGL display lists may only be created, replaced, and deleted.
However, these operations can be performed on structures with very fine
granularity. For example, in OpenGL it is possible to create display
lists containing the data for independent vertices of a polygon, and to
define the polygon as a set of calls to those display lists.  A given
vertex of the polygon may then be "edited" by replacing its display
list. This is not possible in PEX; individual vertices cannot be
edited, and primitives must be replaced in their entirety. (See
Performance below for some consequences of this difference.)

3.3.  Complex Polygons

Both OpenGL and PEX support single- and multi-contour polygons with
convex and nonconvex shapes. PEX supports self-intersecting contours
and polygon lists with shared geometry, while OpenGL does not.

3.4.  Feedback

OpenGL has provisions for tapping the graphics pipeline, so that the
results of geometric transformations and lighting computations can be
"fed back" to the application. PEX has no analogous capability.

3.5.  Alpha Blending

OpenGL supports an alpha (coverage) buffer and an explicit alpha
component in colors. These are used for antialiasing and image
composition. PEX does not support alpha blending.

3.6.  Image Operations

OpenGL includes routines to read, write, and copy pixels. These
routines may be directed to the framebuffer or to bitmap data
structures in the application. A number of operations including
scaling, luminance transformations, and type conversion may be applied
to the pixel data as it is moved. This functionality can be used to
implement ordinary 2D raster operations, but is also used to render
text and is needed to support image manipulation for texture mapping.

PEX has no support for image operations.

3.7.  Texture and Environment Mapping

OpenGL includes a complete implementation of texture mapping. The
application controls the exchange of rendering time for image quality,
and it may use its own texture prefiltering if that provided by OpenGL
is inadequate. Texture coordinates may be specified explicitly or
generated automatically, and they may be transformed by an arbitrary
4x4 matrix. Textures may be applied to a surface as if they were
decals, or they may be blended with the surface in a number of ways.

PEX has no equivalent capability.

3.8.  Antialiasing

PEX has no official support for antialiasing. Some implementations have
provided it as a nonstandard extension.

OpenGL has explicit support for antialiasing points, lines, and
polygons. In addition, the accumulation buffer may be used to implement
full-scene antialiasing.

3.9.  Progressive Refinement

The OpenGL accumulation buffer permits straightforward implementation
of progressive refinement, the gradual improvement of image quality
over a period of time. PEX has no equivalent.

3.10.  Capping Planes

OpenGL supports a multipass capping-plane algorithm using its stencil
buffer and other auxiliary buffers.  PEX as currently defined cannot
support this functionality.

3.11.  Constructive Solid Geometry

The OpenGL stencil, accumulation, and auxiliary buffers can be used to
render objects defined by the regularized Boolean operations of
constructive solid geometry. PEX does not have this ability.

3.12.  Motion Blur

The OpenGL accumulation buffer can be used to simulate motion blur. PEX
has no support for it.

3.13.  Depth-of-Field

With appropriate change in viewing parameters, the OpenGL accumulation
buffer can be used to produce an image with finite depth-of-field. This
function is not available in PEX.



4.  Performance

Interactive 3D graphics demands much higher performance than 2D
graphics, and therefore affects more aspects of system design. The
architecture of a successful 3D graphics system must encompass not only
3D transformations, shading, and scan conversion, but also CPU load,
cache behavior, main memory bandwidth requirements, main memory
capacity requirements, bus transaction latency and throughput, address
translation costs, and graphics processor request queueing, among other
things. These issues must be considered for the graphics software,
operating system, window system, and even the applications programs.

There are numerous existence proofs that both PEX and OpenGL can
support high-performance implementations.  However, the two systems do
not place equal emphasis on some of the design issues mentioned above.
We believe that these differences in design would lead to differences
in performance should PEX and OpenGL be implemented on the same
hardware, and we believe that trends in semiconductor technology will
tend to widen the performance gap.

4.1.  Data Reformatting

It is nearly always necessary for applications to maintain private data
structures that are specialized for their particular problem domains.
Graphics data is just one component of these structures.

Current PEX APIs (and the PEX protocol) use their own data structures
for describing graphics primitives. For example, a polygon consists of
a counted list of vertices, each of which contains coordinates in
floating-point format, plus optional data such as colors and surface
normals.

When an application wishes to make an immediate-mode graphics request,
it must extract data from its private data structures, reformat it to
match the PEX interface requirements, write it to a temporary buffer,
and then invoke the PEX API to process it. This has several
consequences:

    The CPU must spend cycles to reformat and copy the data. These
    cycles do no useful work, yet they are lost to the application
    and to other processes in the system that might have made use
    of them. Furthermore, since they are in application code
    outside the scope of the graphics system, no improvement in the
    graphics hardware or software can recover the loss.

    For high-performance interactive 3D graphics applications,
    where a considerable amount of data must be processed for each
    frame, the reformatting process sweeps the system's data cache
    repeatedly. This can drastically reduce the effectiveness of
    the cache, and thus degrade performance for the entire
    workstation.

    During reformatting, the CPU is generating memory transactions
    at a high rate. The bus traffic resulting from these
    transactions interferes with other users of the system bus. One
    critical user of the bus that may be adversely affected by this
    traffic is the graphics processor.

To quantify this problem, consider a hypothetical workstation
(simplified, but similar to many in use today). It has a 32-byte cache
line, a 64-bit-wide bus to memory, and main memory with a 240ns
random-access cycle time and an 80ns page-mode-access cycle time. A
cache line occupies four sequential addresses, so it can be read in
(240ns + 80ns + 80ns + 80ns), or 480ns. We'll ignore the cost of
reformatting. Writing the cache line to memory takes another 480ns.
Therefore we can copy 32 bytes in a total of 960ns, for an average copy
rate of 33MB/s. An independent 3D triangle is composed of three
vertices, each having three coordinates, three color components, and
three surface normal components. If all of these are specified by
single-precision floating-point values, then the total size of the
triangle is 108 bytes. Therefore our 33MB/s copy rate is sufficient to
copy a little more than 300K triangles per second. At this rate, our
workstation is completely saturated by the data reformatting process;
there is no time for drawing the triangles, much less running the
application. This maximum performance level is already below the target
drawing rate for comparable machines that exist today. And any
additional reformatting or copying (for example, to build PEX protocol
packets) reduces the maximum drawing rate still further.

OpenGL uses a different approach. The OpenGL API allows graphics data
to be extracted directly from the application's private data
structures, without reformatting. It is also possible to support this
interface in hardware in such a way that the data need never be
transferred into the CPU, so it will not sweep the CPU's cache.  This
is impractical for PEX on the hardware with which we are familiar.

For years, CPU cycle times have improved more rapidly than main memory
cycle times. We expect this trend to continue, with the result that
PHIGS and PEXlib-style APIs (as well as any new PEX APIs that reformat
data to build PEX protocol packets) will become increasingly limited by
memory access. Since so many cycles will be spent copying data, systems
based on PEX will need to be faster and more expensive to sustain the
same overall level of application performance as systems based on
OpenGL.

4.2.  Data Duplication

There are two ways developers of PEX-based applications can avoid the
dilemma described in the previous section. The first is to rewrite all
data-structure-handling code so that the graphics data is stored in
precisely the form required by PHIGS or PEXlib. In the extreme this
involves dispensing with private data structures entirely, i.e. using
PEX structure store for all graphics data and PEX structure extensions
("GSEs") for private non-graphics data. Not only would this force
applications to be rewritten, but it would also require the use of
interprocess (and sometimes network) communication to access the
application database. In any case, eliminating the application's
specialized data structures is usually not feasible, since those
structures are instrumental in providing the added value that makes the
application successful commercially.

The second approach is to duplicate the graphics data, keeping a copy
in the proper form for a PEX API. This can be done by maintaining
parallel data structures in the application or in PEX structure store.
In practice, we have found that these methods rarely work. In the case
where the graphics data is static, and well-separated from other
application data, maintaining a parallel graphics database can be
effective. Usually, though, the graphics data is being referenced
frequently, is edited, or is intermixed with other data. In such cases
maintaining parallel data structures causes the real memory
requirements of the application to expand dramatically.  Typical
CAD/CAM graphics databases are 20MB-40MB in size today, and are
expected to grow. Customers are reluctant to purchase that much real
memory to add to their systems, especially if they know that it is only
being used for duplicated data! Applications developers are reluctant
to add the code needed to keep the duplicated data in sync with the
main database.

OpenGL avoids all of these problems by making it unnecessary to
duplicate or reformat graphics data.

4.3.  Display Lists

Once a PEX structure is created, it may be edited by inserting or
deleting new structure elements. Once an OpenGL display list is
created, it may only be deleted or replaced; it may not be edited.

OpenGL makes this restriction to allow its display lists to be
"compiled" into a more efficient form at the time they are created.
This is particularly effective for entities like texture maps and
material definitions that may involve a significant amount of
preprocessing. It can also be used to perform data type and format
conversions, or to move drawing requests into special display list
memory for fast access by the graphics hardware.

PEX structures may be compiled, too. However, because they are
editable, a number of complications arise.  For example, since there is
no provision in PEX for "opening" and "closing" a structure, the PEX
extension cannot determine when edits to a structure are complete.
Recompiling a structure after each edit could be a great waste of time,
since another edit might arrive shortly thereafter. To avoid this,
recompilation must be delayed until the structure is actually invoked.
This increases implementation complexity and on systems with graphics
accelerator hardware it can increase overhead in the "fast path" case
for drawing.

Another issue arises when a PEX structure is in main memory, and the
graphics hardware accesses it by DMA.  Since a given structure may be
read by the graphics hardware and written by the structure editor at
the same time, access to it must be interlocked. This interlock must be
fairly fine-grained; one cannot simply lock out edits until structure
traversal is completed, because the PHIGS model for graphics requires
support for continuous traversal and "pending" edits. Usually the
structures being changed are precisely those that need to be accessed
by the graphics hardware to update the display, so conflicts are
frequent. Furthermore the required locks are interprocessor locks, so
they are expensive on many machines.

On a machine with a similar architecture, OpenGL display lists would
also require interlocks. However, the interlocks are needed for just
three operations (display list replacement and deletion must be
interlocked with display list lookup), and continuous traversal is not
an issue. Fewer synchronization operations are needed than for PEX, and
the critical sections complete more quickly (often in a single
instruction).

4.4.  Display List Editing Granularity

PEX structures are edited by inserting, deleting, or replacing
"structure elements," which consist of geometric primitives or
attribute-setting commands. Thus PEX structure editing can be applied
to individual primitives, but not to any entity at a finer
granularity.  This can cause performance problems when editing large
primitives.  For example, to change the location of one vertex in a
large quadrilateral mesh, the entire mesh must be re-transmitted to the
server. In many cases this eliminates the advantage of PEX structure
store.

As noted above in Functionality, OpenGL display lists may invoke other
display lists even within a single primitive. Although display lists
may not be edited, they may be replaced. Together these two features
allow primitives to be edited at a very fine granularity. For example,
a quadrilateral mesh can be defined as a set of calls to display lists
that define its individual vertices. Replacing the display list for a
vertex effects a vertex-level edit. Only the data for the new vertex
need be transmitted to the server.

4.5.  State Save and Restore

PEX semantics require that all rendering pipeline attributes be saved
when a structure is traversed, and restored when the traversal is
completed. This demands an expensive "push and pop" of the complete
state, or a complex copy-on-write scheme that adds cost to
attribute-setting commands.

OpenGL state is saved and restored selectively, and only when the
client application requests it, so the process can be optimized or even
eliminated if it is not needed.

4.6.  Indirect Attributes

In PEX, attributes like the color and style of a primitive may be
stored in "bundles." A flag associated with each primitive determines
whether that primitive's attributes are provided directly (along with
the geometry for the primitive) or indirectly (from a bundle). In
immediate-mode drawing the flag must be tested, and that slows down the
fast drawing path for the primitive. In both immediate and structure
modes, the use of the bundle forces a memory access to a nonsequential
address, which often causes a TLB miss or a random-access penalty for
the memory subsystem. Since the bundle tables may be large, they cannot
always be stored on the graphics subsystem; this makes it difficult to
optimize the graphics pipeline for some important cases (like polygons,
where the style of the primitive has a substantial effect on the
computations needed to display it).

OpenGL pushes attribute lookup back to the level of the application, so
the cost of indirection is incurred only when it is strictly
necessary.  The display list mechanism can be used to implement
attribute indirection straightforwardly, for arbitrary sets of
attributes.



5.  Portability

The single most important reason to standardize a graphics system is to
guarantee portability for the applications that are based on it. PEX
and OpenGL may well succeed or fail depending on how effectively their
APIs and wire protocols support this goal.

5.1.  Subsetting

PEX has three mutually-incompatible subsets: immediate-mode,
structure-mode, and "PHIGS Workstation." Vendors of PHIGS for PEX must
provide three significantly different implementations layered on these
subsets to insure network transparency and application portability. A
single PEXlib is sufficient to support all three subsets, but at the
cost of pushing portability issues all the way up into the
application.  Real software portability does not yet exist in the PEX
world because of the wide variations in commercial versions of PEX.

The existence of these PEX subsets may be indicative of the difficulty
of building a full PEX implementation. If true, this has negative
implications for the quality, availability, and timeliness of PEX.

In order to combat application portability problems caused by
subsetting and optional functionality, a group of PEX vendors has found
it desirable to fund a private organization, the PEX Interoperability
Center, just to provide a PEX porting and testing environment.

OpenGL may not be subsetted. Application developers can be assured that
all OpenGL features will be available on all implementations. Unlike
PEX today, there exists an OpenGL sample implementation with complete
functionality. The need for interoperability testing is greatly reduced
because OpenGL implementations are much more consistent than PEX
implementations.

5.2.  Optional Functionality

Many of the features described in the PEX protocol specification are
optional. For example, the protocol contains provisions for scanline
rendering, Phong shading, and CIE LUV color computations, but none of
these features is guaranteed to be available in any particular PEX
implementation. To use any optional feature, an application must query
the server to determine if the feature is available. If not, the
application must include code for a fallback strategy. This approach is
very general, but it yields portable programs only by imposing a large
burden on every application developer.

In contrast, OpenGL has a much larger set of core features that are
universally supported.

5.3.  Conformance Testing

To our knowledge, there are no comprehensive test suites available to
validate PEX implementations. This makes it difficult to determine
whether a particular implementation conforms to the specification. It
also reduces confidence that an application that runs on one
implementation of PEX will also run correctly on another.

A suite of conformance tests for OpenGL 1.0 ships with every release
kit.

5.4.  Window-System Dependency

PEX as it is defined today is very closely coupled to the X Window
System. The semantics of X resources, drawables,
atomicity/sequentiality requirements, and events are integral to the
design of PEX.

However, there are other commercially-significant window systems.
Microsoft Windows is an example. In order to accommodate these systems,
OpenGL separates the parts of the interface that are window-system
dependent (about ten routines under X11) from those that are
window-system independent. Adding a small new set of window-system
dependent routines allows the remainder of OpenGL to be used without
change.  This capability is being exploited to port OpenGL to Microsoft
Windows/NT, with the expectation that Windows/NT platforms will offer a
large new market for 3D graphics applications.



6.  Business Issues

Although the primary focus of this paper is on the technical
differences between PEX and OpenGL, it is also necessary to consider
the most critical business issues: licensing, processes for changing
the systems, market acceptance, and availability.

6.1.  Licensing

The PEX specifications and sample implementation are available as part
of the X11 distribution, at no additional fee. Vendors may release
their commercial-quality implementations of PEX as bundled or layered
products without royalty. To cover costs, the members of the X
Consortium make direct financial contributions for support of X and
PEX; for example, the PEX Sample Implementation cost contributors a
total of roughly $750K.

Anyone can license OpenGL from Silicon Graphics at nominal prices.
Manual pages are available by anonymous ftp at no cost. A Level 1
license (including the OpenGL specifications, manual pages, C bindings,
GLU utilities, demo applications, compliance suite, and the right to
redistribute an implementation) is $25K. A Level 2 license (all of
Level 1 plus reference implementation, optimized portable sample
implementation, X11 server extension, and porting documentation) is
$100K. (This was chosen to be less than the fully-burdened cost of one
engineer for a year.) A university license is available for $500. There
is a royalty of $5 per revenue copy. These fees allow SGI to recover
expenses for distribution and maintenance of OpenGL, in much the same
way consortium membership fees allow the X Consortium to provide
maintenance for PEX.

6.2.  Mechanisms for Change

In the case of PEX, multiple independent entities (ANSI, ISO, the
PHIGS-PLUS committee, and the X Consortium) are responsible for
defining various parts of the APIs, rendering semantics, and wire
protocol. This has led to some problems of timeliness and
compatibility, particularly with respect to PEX support for PHIGS-
PLUS. It has also contributed to the proliferation of PEX APIs.
However, the process is open to essentially anyone.

A single organization (Silicon Graphics) defined the original
specification for OpenGL, and a single entity (the OpenGL Architecture
Review Board) is now responsible for all aspects of the API and wire
protocol. The bylaws ensure that each of the six companies on the ARB
has an equal vote on all API and governance issues.  All decisions
require a two-thirds majority vote. Compared to the organization used
by PEX, we believe this structure will be more responsive to the needs
of OpenGL users and more likely to maintain the technical integrity of
the design as it evolves.

SGI has full-time engineering resources assigned to implement the
decisions of the OpenGL ARB. This allows new code to be written and
tested quickly, and thus brought to market earlier or with more
user-requested features. The X Consortium funds two full-time positions
for PEX; other development work is provided by sponsoring companies on
a volunteer basis.

6.3.  Vendor Support

Presently we know of five commercial-quality PEX products. Twenty
companies are participating in the PEX Interoperability Center in
Dallas, Texas, and therefore many should be expected to release
commercial PEX products sometime in the next year.

Today SGI, IBM, Nth Graphics, and DuPont Pixel Systems are shipping
products compatible with IRIS GL, the predecessor to OpenGL. Twenty
personal computer and workstation companies have licensed OpenGL, and
commercial implementations will be available sometime in the next
year.

A considerable number of vendors have stated their intent to support
both PEX and OpenGL.

The most compelling difference in vendor support is the endorsement of
OpenGL by Microsoft and Intel. The OpenGL API will be bundled with
future versions of Microsoft's Windows/NT environment, providing appli-
cations based on OpenGL with a personal computer market at least an
order of magnitude larger than the workstation arena.

6.4.  Software Applications Base

PEX per se has a small base of applications, because it has not been
widely available until recently. The existing base of PHIGS code, much
of which is found in CAD/CAM, government, and European markets, has a
logical porting path to PEX. PEXlib is an alternate API for developers
who find PHIGS inappropriate, but it requires substantial recoding
because it is essentially a new API. There is some concern that ISVs
will delay ports to PEXlib, because the PEX designers have chosen to
make incompatible changes to the protocol for the next PEX release, and
this could force incompatible changes to the PEXlib interface.

IRIS GL has the majority of applications in markets requiring
high-performance 3D graphics - CAD/CAM (particularly finite element and
structural analyses), animation, creative graphics, visual simulation,
and scientific visualization. Since OpenGL is not backward-compatible
with IRIS GL, existing IRIS GL applications must be ported. The
majority of the current base of 1500 IRIS GL applications are expected
to port to OpenGL, given its similarity to IRIS GL, the need to stay
current with SGI and its product release schedule, and the wider mar-
ket that is provided by OpenGL. According to a poll taken at the last
SGI Developers Conference, 85% of the current IRIS GL ISVs have already
decided to port to OpenGL. Because OpenGL evolved from IRIS GL, much of
the conversion process can be automated; SGI demonstration programs
were ported at the rate of 5000 lines per engineer-day. SGI has a
vested interest in simplifying the move from IRIS GL to OpenGL, because
OpenGL is the native interface for future SGI hardware.

6.5.  Availability

The PEX 5.1 specification review process will be complete very shortly,
and we can expect PEX 5.1 implementations to be available in the Fall
of 1992.

The OpenGL 1.0 specification and sample implementation shipped on June
30, 1992. The first commercial implementations are expected in the
first quarter of 1993.

The PEX design committee has chosen to make the next update of PEX
incompatible with previous versions.  The specification for PEX 6.0
will include some of the advanced rendering features of OpenGL, and
should be complete near the middle of 1993. First implementations could
appear as early as 1994.



7.  Conclusions

We believe diversity in computer graphics interfaces can be desirable
if it encourages real innovation in hardware, system software, and
applications. However, if the computer graphics community feels it must
standardize on one graphics system, OpenGL is a better choice than
PEX:

    OpenGL has a single well-defined API. There are two major
    competing APIs for PEX, and one of them (PHIGS) is not under
    the control of the organization defining PEX.

    OpenGL has more rendering functionality than PEX. In addition,
    more OpenGL functionality is guaranteed to be present in all
    implementations than is the case for PEX.

    When the entire computing system is analyzed, OpenGL has
    several important performance advantages over PEX. These
    advantages apply to both immediate-mode and structure-mode
    rendering.

    OpenGL applications can be made portable with less effort than
    is needed for applications based on PEX, because OpenGL
    implementations are more consistent functionally and are
    subject to validation by a conformance test. OpenGL is also
    less dependent on the window system under which it is run.

    Both OpenGL and PEX are available on generous terms, and both
    have a good deal of support in the industry.  The X Consortium
    process for defining PEX is more democratic and participatory,
    but OpenGL's governance process will likely result in better
    technology coming to market more quickly.

    Both systems offer porting paths from existing code bases: from
    stand-alone PHIGS to PHIGS on PEX, and from IRIS GL to OpenGL.
    However, the initial OpenGL application base will be
    substantially larger.

    OpenGL will be a standard part of Microsoft Windows/NT. This
    provides OpenGL applications a vastly larger potential market
    than is available to PEX applications, and will make OpenGL
    more attractive to independent software developers.

We welcome your comments and suggestions.



References

Goldfeather, Jack, et. al., "Fast Constructive Solid Geometry Display
in the Pixel-Powers Graphics System", Computer Graphics 20:4 (SIGGRAPH
1986 Conference Proceedings), August, 1986, pp. 107-116.

Haeberli, Paul, and Akeley, Kurt, "The Accumulation Buffer: Hardware
Support for High Quality Rendering," Computer Graphics 24:4 (SIGGRAPH
1990 Conference Proceedings), August, 1990, pp. 309-318.

Mammen, Abraham, "Transparency and Antialiasing Algorithms Implemented
with the Virtual Pixel Maps Technique," IEEE Computer Graphics and
Applications, July, 1989.

Segal, Mark, and Akeley, Kurt, The OpenGL Graphics System: A
Specification (Version 1.0), June 30, 1992, Silicon Graphics, Inc.

Womack, Paula (ed.), PEX Protocol Specification Version 5.1P - X Public
Review Draft, May 11, 1992, Massachusetts Institute of Technology.



The Author

Allen Akin is an Engineering Manager in the OpenGL group at Silicon
Graphics. He is currently integrating OpenGL with Microsoft
Windows/NT.  His graphics experience includes a year at MacroMedia,
where he completed the MacroMind Three-D animation system for the
Apple Macintosh, and five years at Digital Equipment Corporation, where
he served as Engineering Manager for the first commercial
implementation of PEX and as Graphics Software Architect for RISC
workstations. He can be reached at the Internet address [e-mail].



Copyright 1992, Silicon Graphics, Inc. All rights reserved.