Name APPLE_vertex_array_range Name Strings GL_APPLE_vertex_array_range Contact Geoff Stahl, Apple Computer (gstahl 'at' apple.com) Status Complete Version $Date: 2009/01/07 22:05:21 $ $Revision: 1.13 $ Number 274 Dependencies APPLE_fence can affect this extension. Overview This extension is designed to allow very high vertex processing rates which are facilitated both by relieving the CPU of as much processing burden as possible and by allowing graphics hardware to directly access vertex data. Because this extension is implemented as an addition to the vertex array specification provided by OpenGL 1.1, applications can continue to use existing vertex submission logic while taking advantage of vertex array ranges to more efficiently process those arrays. The vertex array coherency model provided by OpenGL 1.1 requires that vertex data specified in vertex arrays be transferred from system memory each time Begin, DrawArrays, or DrawElements is called. Further, OpenGL 1.1 requires that the transfer of data be completed by the time End, DrawArrays, or DrawElements returns. Both of these requirements are relaxed by the vertex array range extension. Vertex data may be cached by the GL so there is no guarantee that changes to the vertex data will be reflected in following drawing commands unless it is flushed with FlushVertexArrayRangeAPPLE. The reading of vertex data may be deferred by the GL so there is no guarantee that the GL will be finished reading the data until completion is forced by the use of Finish or the APPLE_fence extension. Vertex array range can be enabled in two ways. EnableClientState can be used with the VERTEX_ARRAY_RANGE_APPLE param to enable vertex array range for the client context. One can also simply set the vertex array storage hint to either STORAGE_CACHED_APPLE or STORAGE_SHARED_APPLE (as discussed below) to enable a particular vertex array range. Once this is done, use of vertex array range requires the definition of a specific memory range for vertex data through VertexArrayRangeAPPLE. It is recommended this data be page aligned (4096 byte boundaries) and a multiple of page size in length for maximum efficiency in data handling and internal flushing, but this is not a requirement and any location and length of data can be defined as a vertex array. This extension provides no memory allocators as any convenient memory allocator can be used. Once a data set is established, using VertexArrayRangeAPPLE, it can be can be drawn using standard OpenGL vertex array commands, as one would do without this extension. Note, if any the data for any enabled array for a given array element index falls outside of the vertex array range, an undefined vertex is generated. One should also understand removing or replacing all calls to vertex array range functions with no-ops or disabling the vertex array range by disabling the VERTEX_ARRAY_RANGE_APPLE client state should not change the results of an application's OpenGL drawing. For static data no additional coherency nor synchronization must be done and the client is free to draw with the specified draw as it sees fit. If data is dynamic, thus to be modified, FlushVertexArrayRangeAPPLE should be used. The command is issued when data has been modified since the last call to VertexArrayRangeAPPLE or FlushVertexArrayRangeAPPLE and prior to drawing with such data. FlushVertexArrayRangeAPPLE only provides memory coherency prior to drawing (such as ensuring CPU caches are flushed or VRAM cached copies are updated) and does not provide any synchronization with previously issued drawing commands. The range flushed can be the specific range modified and does not have to be the entire vertex array range. Additionally, data maybe read immediately after a flush without need for further synchronization, thus overlapping areas of data maybe read, modified and written between two successive flushes and the data will be consistent. To synchronize data modification after drawing two methods can be used. A Finish command can be issued which will not return until all previously issued commands are complete, forcing completely synchronous operation. While this guarantees all drawing is complete it may not be the optimal solution for clients which just need to ensure drawing with the vertex array range or a specific range with the array is compete. The APPLE_fence extension can be used when dynamic data modifications need to be synchronized with drawing commands. Specifically, if data is to be modified, a fence can be set immediately after drawing with the data. Once it comes time to modify the data, the application must test (or finish) this fence to ensure the drawing command has completed. Failure to do this could result in new data being used by the previously issued drawing commands. It should be noted that providing the maximum time between the drawing set fence and the modification test/finish fence allows the most asynchronous behavior and will result in the least stalling waiting for drawing completion. Techniques such as double buffering vertex data can be used to help further prevent stalls based on fence completion but are beyond the scope of this extension. Once an application is finished with a specific vertex array range or at latest prior to exit, and prior to freeing the memory associated with this vertex array, the client should call VertexArrayRangeAPPLE with a data location and length of 0 to allow the internal memory managers to complete any commitments for the array range. In this case once VertexArrayRangeAPPLE returns it is safe to de-allocate the memory. Three types of storage hints are available for vertex array ranges; client, shared, and cached. These hints are set by passing the STORAGE_CLIENT_APPLE, STORAGE_SHARED_APPLE, or STORAGE_CACHED_APPLE param to VertexArrayParameteriAPPLE with VERTEX_ARRAY_STORAGE_HINT_APPLE pname. Client storage, the default OpenGL behavior, occurs when VERTEX_ARRAY_RANGE_APPLE is disabled AND the STORAGE_CLIENT_APPLE hint is set. Note, STORAGE_CLIENT_APPLE is also the default hint setting. Shared memory usage is normally used for dynamic data that is expected to be modified and is likely mapped to AGP memory space for access by both the graphics hardware and client. It is set when either VERTEX_ARRAY_RANGE_APPLE is enabled, without the STORAGE_CACHED_APPLE hint being set, or in all cases when the STORAGE_SHARED_APPLE hint is set. Finally, the cached storage is designed to support static data and data which could be cached in VRAM. This provides maximum access bandwidth for the vertex array and occurs when the STORAGE_CACHED_APPLE hint is set. The following pseudo-code represents the treatment of a vertex array range memory depending on the hint setting and whether vertex array range is enabled for the client context: if (VERTEX_ARRAY_STORAGE_HINT_APPLE == STORAGE_CACHED_APPLE) vertex array is treated as cached else if (VERTEX_ARRAY_STORAGE_HINT_APPLE == STORAGE_SHARED_APPLE) vertex array is treated as shared else if (VERTEX_ARRAY_RANGE_APPLE enabled) vertex array is treated as shared else vertex array is treated as client Note, these hints can affect how array flushes are handled and the overhead associated with flushing the array, it is recommended that data be handled as shared unless it really is static and there are no plans to modify it. To summarize the vertex array range extension provides relaxed synchronization rules for handling vertex array data allowing high bandwidth asynchronous data transfer from client memory to graphics hardware. Different flushing and synchronization rules are required to ensure data coherency when modifying data. Lastly, memory handling hints are provided to allow the tunning of memory storage and access for maximum efficiency. Issues How does one get the current VERTEX_ARRAY_STORAGE_HINT_APPLE (storage hint) for a vertex array range? RESOLUTION: The current VERTEX_ARRAY_STORAGE_HINT_APPLE can be retrieved via GetIntegerv with VERTEX_ARRAY_STORAGE_HINT_APPLE as the pname. How does this extension interact with the compiled_vertex_array extension? RESOLUTION: They are independent and not interfere with each other. In practice, if you use APPLE_vertex_array_range, you can surpass the performance of compiled_vertex_array Should we give a programmer a sense of how big a vertex array range they can specify? RESOLUTION: Not completely resolved. There should be a query for determining the maximum safe size of a vertex array range which, we do not have as of yet. Currently, the vertex array range size plus the command buffers must not exceed available GART space, which in real world terms, means the smallest maximum vertex array range size is about 24 MB on any currently supported hardware on Mac OS X and get's larger normally with more RAM and/or more advanced graphics hardware. Failure modes are likely either failure to speed up the vertex processing or failure to draw all the vertex data. So clients should plan on single vertex array ranges of less than 24 MB if they intend to run on all Mac OS X hardware. Should Flush be the same as FlushVertexArrayRangeAPPLE? RESOLUTION: No. A Flush is a different concept than FlushVertexArrayRangeAPPLE. a Flush submits pending OpenGL commands to the OpenGL engine for processing while, a FlushVertexArrayRangeAPPLE just ensures memory coherency for the vertex array range and does not perform a Flush in OpenGL terms. New Procedures and Functions void VertexArrayRangeAPPLE(sizei length, void *pointer); void FlushVertexArrayRangeAPPLE(sizei length, void *pointer); void VertexArrayParameteriAPPLE(enum pname, int param); New Tokens Accepted by the parameter of EnableClientState, DisableClientState, and IsEnabled: VERTEX_ARRAY_RANGE_APPLE 0x851D Accepted by the parameter of GetBooleanv, GetIntegerv, GetFloatv, and GetDoublev: VERTEX_ARRAY_RANGE_LENGTH_APPLE 0x851E MAX_VERTEX_ARRAY_RANGE_ELEMENT_APPLE 0x8520 Accepted by the parameter of GetPointerv: VERTEX_ARRAY_RANGE_POINTER_APPLE 0x8521 Accepted by the parameter of VertexArrayParameteriAPPLE, GetBooleanv, GetIntegerv, GetFloatv, and GetDoublev: VERTEX_ARRAY_STORAGE_HINT_APPLE 0x851F Accepted by the parameter of VertexArrayParameteriAPPLE: STORAGE_CLIENT_APPLE 0x85B4 STORAGE_CACHED_APPLE 0x85BE STORAGE_SHARED_APPLE 0x85BF Additions to Chapter 2 of the OpenGL 1.1 Specification (OpenGL Operation) After the discussion of vertex arrays (Section 2.8) add a description of the vertex array range: "The command void VertexArrayRangeAPPLE(sizei length, void *pointer) specifies the current vertex array range. When the vertex array range is specified and valid, vertex transfers from within the vertex array range are potentially faster. The vertex array range is a contiguous region of address space for placing vertex arrays. The "pointer" parameter is a pointer to the base of the vertex array range. The "length" pointer is the length of the vertex array range in basic machine units (typically unsigned bytes). Memory associated with a vertex array range should be allocated by the client and the responsibility for maintaining it rests with the client as long as it is being used as a vertex array range. The vertex array range address space region extends from "pointer" to "pointer + length - 1" inclusive. When specified, vertex array vertex transfers from within the vertex array range are potentially faster. There is some system burden associated with establishing a vertex array range (typically, the memory range must be locked down). If either the vertex array range pointer or size is set to zero, the previously established vertex array range is released (typically, unlocking the memory). This should always be done prior to freeing of the memory by the client. The vertex array range may not be established for operating system dependent reasons, and therefore, not valid. Reasons that a vertex array range cannot be established include exceeding the maximum vertex array range size, the memory could not be locked down, etc. The vertex array range is considered enabled after VERTEX_ARRAY_RANGE_APPLE client state is enabled and as soon as a valid vertex array range is specified and disabled once the size length and/or pointer is set to zero or VERTEX_ARRAY_RANGE_APPLE client state is disabled. When the vertex array range is enabled, ArrayElement commands may generate undefined vertices if and only if any indexed elements of the enabled arrays are not within the vertex array range or if the index is negative or greater or equal to the implementation-dependent value of MAX_VERTEX_ARRAY_RANGE_ELEMENT_APPLE. If an undefined vertex is generated, an INVALID_OPERATION error may or may not be generated. The vertex array coherency model specifies when vertex data must be extracted from the vertex array memory. When the vertex array range is not valid, (quoting the specification) `Changes made to array data between the execution of Begin and the corresponding execution of End may effect calls to ArrayElement that are made within the same Begin/End period in non-sequential ways. That is, a call to ArrayElement that precedes a change to array data may access the changed data, and a call that follows a change to array data may access the original data.' When the vertex array range is valid, the vertex array coherency model is relaxed so that changes made to array data may affect calls to ArrayElement in non-sequential ways. That is a call to ArrayElement that precedes a change to array data may access the changed data, and a call that follows a change to array data may access original data. This requires in two points of synchronization to maintain coherency. The first point where synchronization must occur to maintain coherency is post data modification, prior to drawing. FlushVertexArrayRangeAPPLE should be used by the client on all ranges of memory which have been modified since the last call to VertexArrayRangeAPPLE or FlushVertexArrayRangeAPPLE. The second point of synchronization is after drawing with a vertex array range and prior to modifying it's data. In this case either Finish or a fence must be used. Finish will create a synchronization point for all drawing an may not be the optimal method to ensure drawing completion prior to data modification. A fence, defined in the APPLE_fence extension, on the other hand allows more selective synchronization. The client can set a fence immediately after drawing with the data in question and test or finish that fence prior to modifying the data. See the APPLE_fence extension for more details. To maintain full coherency, once a vertex array range is enabled, requires the client to both flush the vertex array after data modification, prior to drawing, and synchronize with Finish or a fence after drawing, prior to modifying the data. The command void VertexArrayParameteriAPPLE(enum pname, int param) allows the client to hint at the expected use of the vertex array range. pname must be VERTEX_ARRAY_STORAGE_HINT_APPLE. param can either be STORAGE_CACHED_APPLE or STORAGE_SHARED_APPLE and can be used by the system to tune the handling of the vertex array range data. These parameters are just hints and require no specific handling by the system. The default state is STORAGE_SHARED_APPLE which, indicates that the vertex data is expected to be dynamic and should be handled in a way to optimize modification and flushing of the vertex array range, if possible. STORAGE_CACHED_APPLE indicates the data is expected to static and techniques such as VRAM caching could be employed to optimize memory bandwidth to the vertex array range. Proper use of FlushVertexArrayRangeAPPLE guarantees memory coherency in all cases and will result in deterministic defined behavior in all cases, whether hints are employed or not. The client state required to implement the vertex array range consists of an enable bit, a memory pointer, an integer size, and a valid bit." Additions to Chapter 5 of the OpenGL 1.1 Specification (Special Functions) Add to the end of Section 5.4 "Display Lists" "VertexArrayRangeAPPLE and FlushVertexArrayRangeAPPLE are not complied into display lists but are executed immediately. If a display list is compiled while VERTEX_ARRAY_RANGE_APPLE is enabled, the commands ArrayElement, DrawArrays, DrawElements, and DrawRangeElements are accumulated into a display list as if VERTEX_ARRAY_RANGE_APPLE is disabled." Additions to the CGL interface: None Additions to the WGL interface: None Additions to the GLX Specification None GLX Protocol None Errors INVALID_OPERATION is generated if VertexArrayRangeAPPLE or FlushVertexArrayRangeAPPLE is called between the execution of Begin and the corresponding execution of End. INVALID_OPERATION may be generated if an undefined vertex is generated. New State Initial Get Value Get Command Type Value Attrib --------- ----------- ---- ------- ------------ VERTEX_ARRAY_RANGE_APPLE IsEnabled B False vertex-array VERTEX_ARRAY_RANGE_POINTER_APPLE GetPointerv Z+ 0 vertex-array VERTEX_ARRAY_RANGE_LENGTH_APPLE GetIntegerv Z+ 0 vertex-array VERTEX_ARRAY_STORAGE_HINT_APPLE GetIntegerv Z3 STORAGE_CLIENT_APPLE vertex-array New Implementation Dependent State Get Value Get Command Type Minimum Value --------- ----------- ----- ------------- MAX_VERTEX_ARRAY_RANGE_ELEMENT_APPLE GetIntegerv Z+ 65535 Revision History None