Name ARB_query_buffer_object Name Strings GL_ARB_query_buffer_object Contact Daniel Rakos (daniel.rakos 'at' amd.com) Contributors Daniel Rakos, AMD Graham Sellers, AMD Christophe Riccio, AMD Notice Copyright (c) 2013 The Khronos Group Inc. Copyright terms at http://www.khronos.org/registry/speccopyright.html Specification Update Policy Khronos-approved extension specifications are updated in response to issues and bugs prioritized by the Khronos OpenGL Working Group. For extensions which have been promoted to a core Specification, fixes will first appear in the latest version of that core Specification, and will eventually be backported to the extension document. This policy is described in more detail at https://www.khronos.org/registry/OpenGL/docs/update_policy.php Status Complete. Approved by the ARB on June 3, 2013. Ratified by the Khronos Board of Promoters on July 19, 2013. Version Last Modified Date: 7 March 2016 Author Revision: 3 Number ARB Extension #148 Dependencies OpenGL 1.5 is required. This extension is written against the OpenGL 4.3 (core) specification. Overview Statistics about the operation of the OpenGL pipeline, such as the number of samples that passed the depth test, the elapsed time between two events or the number of vertices written by transform feedback can be retrieved from the GL through query objects. The result of a query object is acquired by the application through the OpenGL API into a client provided memory location. Should the result returned by the API be required for use in a shader, it must be passed back to the GL via a program uniform or some other mechanism. This requires a round-trip from the GPU to the CPU and back. This extension introduces a mechanism whereby the result of a query object may be retrieved into a buffer object instead of client memory. This allows the query rsult to be made available to a shader without a round-trip to the CPU for example by subsequently using the buffer object as a uniform buffer, texture buffer or other data store visible to the shader. This functionality may also be used to place the results of many query objects into a single, large buffer and then map or otherwise read back the entire buffer at a later point in time, avoiding a per-query object CPU-GPU synchronization event. The extension allows acquiring the result of any query object type supported by the GL implementation into a buffer object. The implementation will determine the most efficient method of copying the query result to the buffer. New Procedures and Functions None. New Tokens Accepted by the parameter of GetQueryObjectiv, GetQueryObjectuiv, GetQueryObjecti64v and GetQueryObjectui64v: QUERY_RESULT_NO_WAIT 0x9194 Accepted by the parameter of BindBuffer, BufferData, BufferSubData, MapBuffer, UnmapBuffer, MapBufferRange, GetBufferSubData, GetBufferParameteriv, GetBufferParameteri64v, GetBufferPointerv, ClearBufferSubData, and the and parameters of CopyBufferSubData: QUERY_BUFFER 0x9192 Accepted by the parameter of GetBooleanv, GetIntegerv, GetFloatv, and GetDoublev: QUERY_BUFFER_BINDING 0x9193 Accepted in the bitfield in MemoryBarrier: QUERY_BUFFER_BARRIER_BIT 0x00008000 Additions to Chapter 4 of the OpenGL Core Profile Specification, Version 4.3, "Event Model" Modify Section 4.2.1, "Query Object Queries" Add new paragraph after the fist paragraph on p. 44 ending with "... is the name of a query object." Initially, zero is bound to the QUERY_BUFFER binding point, indicating that is a pointer into client memory. However, if a non-zero buffer object is bound as the current query result buffer (see section 6.1), then is treated as an offset into the designated buffer object. Add new paragraph after the third paragraph on p. 44 ending with "... finite amount of time." If is QUERY_RESULT_NO_WAIT, then the query object's result value is returned as a single integer in if the result is available at the time of the state query. If the result is not available then the destination memory location is not overwritten. Additions to Chapter 6 of the OpenGL Core Profile Specification, Version 4.3, "Buffer Objects" Modify Section 6.1, "Creating and Binding Buffer Objects" Add to Table 6.1: "Buffer object binding targets" +--------------------+--------------------------+-------------------------+ | Target name | Purpose | Described in section(s) | +--------------------+--------------------------+-------------------------+ | QUERY_BUFFER | Query result buffer | 4.2.1 | +--------------------+--------------------------+-------------------------+ Additions to Chapter 7 of the OpenGL Core Profile Specification, Version 4.3, "Programs and Shaders" Modify Section 7.12, "Shader Memory Access" Add to the list of MemoryBarrier bullets, p. 138 * QUERY_BUFFER_BARRIER_BIT: Writes of buffer objects via the QUERY_BUFFER binding (see section 4.2.1) after the barrier will reflect data written by shaders prior to the barrier. Additionally, buffer object writes issued after the barrier will wait on the completion of all shader writes initiated prior to the barrier. Additions to the AGL/GLX/WGL Specifications None. GLX Protocol None. Errors Replace second error case for QueryObject* with the following: An INVALID_ENUM error is generated if is not QUERY_RESULT, QUERY_RESULT_NO_WAIT or QUERY_RESULT_AVAILABLE. Add error case for QueryObject* as follows: An INVALID_OPERATION error is generated if the command would cause data to be written beyond the bounds of the buffer currently bound to the QUERY_BUFFER target. New State Append to Table 23.73, "Miscellaneous" +----------------------+------+--------------+---------------+------------------------------+-------+ | Get Value | Type | Get Command | Initial Value | Description | Sec. | +----------------------+------+--------------+---------------+------------------------------+-------+ | QUERY_BUFFER_BINDING | Z+ | GetIntegeriv | 0 | Query result buffer binding. | 4.2.1 | +----------------------+------+--------------+---------------+------------------------------+-------+ New Implementation Dependent State None. Usage Examples Convenient macro definition for specifying buffer offsets: #define BUFFER_OFFSET(i) ((void*)NULL + (i)) Example 1: Using occlusion query result in shader // Create a buffer object for the query result glGenBuffers(1, &queryBuffer); glBindBuffer(GL_QUERY_BUFFER, queryBuffer); glBufferData(GL_QUERY_BUFFER, sizeof(GLuint), NULL, GL_DYNAMIC_COPY); // Perform occlusion query glBeginQuery(GL_SAMPLES_PASSED, queryId) ... glEndQuery(GL_SAMPLES_PASSED); // Get query results to buffer object glBindBuffer(GL_QUERY_BUFFER, queryBuffer); glGetQueryObjectuiv(queryId, GL_QUERY_RESULT, BUFFER_OFFSET(0)); // Bind query result buffer as uniform buffer glBindBufferBase(GL_UNIFORM_BUFFER, 0, queryBuffer); ... --- Shader --- ... uniform queryResult { uint samplesPassed; }; ... void main() { ... if (samplesPassed > threshold) { // complex processing ... } else { // simplified processing ... } ... } Example 2: Using occlusion query result in shader only if result is available // Create a buffer object for the query result glGenBuffers(1, &queryBuffer); glBindBuffer(GL_QUERY_BUFFER, queryBuffer); glBufferData(GL_QUERY_BUFFER, 2 * sizeof(GLuint), NULL, GL_DYNAMIC_COPY); // Perform occlusion query glBeginQuery(GL_SAMPLES_PASSED, queryId) ... glEndQuery(GL_SAMPLES_PASSED); // Get query availability and result (if available) to buffer object glBindBuffer(GL_QUERY_BUFFER, queryBuffer); glGetQueryObjectuiv(queryId, GL_QUERY_RESULT_AVAILABLE, BUFFER_OFFSET(0)); glGetQueryObjectuiv(queryId, GL_QUERY_RESULT_NO_WAIT, BUFFER_OFFSET(4)); // Bind query result buffer as uniform buffer glBindBufferBase(GL_UNIFORM_BUFFER, 0, queryBuffer); ... --- Shader --- ... uniform queryResult { uint resultAvailable; uint samplesPassed; }; ... void main() { ... if (resultAvailable) { if (samplesPassed > threshold) { // complex processing ... } else { // simplified processing ... } } else { // default processing if no query result is available ... } ... } Example 3: Using a default value and QUERY_RESULT_NO_WAIT // Create a buffer object for the query result // Store a default value in the buffer that will be used // if the query results are not available glGenBuffers(1, &queryBuffer); glBindBuffer(GL_QUERY_BUFFER, queryBuffer); GLuint defaultValue = 42; glBufferData(GL_QUERY_BUFFER, sizeof(GLuint), &defaultValue, GL_DYNAMIC_COPY); // Perform occlusion query glBeginQuery(GL_SAMPLES_PASSED, queryId) ... glEndQuery(GL_SAMPLES_PASSED); // Get query results to buffer object with no wait // Default value remains untouched if results are not available glBindBuffer(GL_QUERY_BUFFER, queryBuffer); glGetQueryObjectuiv(queryId, GL_QUERY_RESULT_NO_WAIT, BUFFER_OFFSET(0)); ... Example 4: Using transform feedback query result to fill indirect draw buffer // Create a buffer object for the indirect draw command glGenBuffers(1, &drawIndirectBuffer); // Initialize draw command DrawArraysIndirectCommand cmd = { ... }; glBindBuffer(GL_DRAW_INDIRECT_BUFFER, drawIndirectBuffer); glBufferData(GL_DRAW_INDIRECT_BUFFER, sizeof(DrawArraysIndirectCommand), &cmd, GL_DYNAMIC_COPY); // Perform transform feedback query glBeginQuery(GL_TRANSFORM_FEEDBACK_PRIMITIVES_WRITTEN, queryId) ... glEndQuery(GL_TRANSFORM_FEEDBACK_PRIMITIVES_WRITTEN); // Write query result to the primCount field of the indirect draw command glBindBuffer(GL_QUERY_BUFFER, drawIndirectBuffer); glGetQueryObjectuiv(queryId, GL_QUERY_RESULT, BUFFER_OFFSET(offsetof(DrawArraysIndirectCommand, primCount))); // Execute the indirect draw command glDrawArraysIndirect(GL_TRIANGLES, BUFFER_OFFSET(0)); ... Issues 1) What is QUERY_RESULT_NO_WAIT useful for? RESOLVED: The application may decide that it does not want to wait for the result of the query object if it's not available at the time. This is not a problem without this extension as the application can always query the availability using QUERY_RESULT_AVAILABLE and decide to actually get the results using QUERY_RESULT only if the result is available. However, when using query buffers, QUERY_RESULT_AVAILABLE and QUERY_RESULT alone cannot provide the same flexibility for shader based decision making as the results are either always available (if QUERY_RESULT was used) or never. QUERY_RESULT_NO_WAIT provides a way to query the result only if it's available. Combined with QUERY_RESULT_AVAILABLE, the shader can decide to use the result or not based on the availability (see usage example 2). 2) Should QUERY_RESULT_NO_WAIT be accepted by GetQueryObject* in case there is no buffer bound to QUERY_BUFFER? RESOLVED: YES, for completeness. 3) Is there any guarantee that GetQueryObject* will not wait for the query results to become available if is QUERY_RESULT_NO_WAIT? RESOLVED: There is no need to have such guarantee. An implementation may choose to wait for the results even in case of QUERY_RESULT_NO_WAIT, however, that may incur a potential performance hit in case the application expects it to not wait. 4) Should the INVALID_OPERATION error be generated if a GetQueryObject* command would access data outside the range of the bound query buffer? RESOLVED: YES. This requires considering the value of and the size of the type of the written value to determine the maximum addressed byte for the state query command. Note: This follows the precedence of the language introduced by ARB_pixel_buffer_object. 5) Should we support 64 bit versions of the state query commands (GetQueryObjecti64v and GetQueryObjectui64v)? RESOLVED: YES, both for completeness and to support timer queries. In this case a 64 bit integer value is written to the buffer. 6) Should the extension support all query types supported by the current GL implementation? RESOLVED: YES. There is not much value in providing additional capability queries that would allow the application to find out which query object types are supported. Also, the GL implementation can always choose to implement the functionality through a GPU-CPU rount-trip for query types that cannot be resolved to a buffer by the hardware. 7) Is there any precedence that the pointer parameter of a glGet* command is treated as an offset into a bound buffer object? RESOLVED: YES, glGetTexImage accepts an offset into the pixel pack buffer in case a pixel pack buffer is bound. As pixel buffer objects are part of the core specification since version 2.1, no precedence is introduced by this extension. 8) What should be written to the query buffer when QUERY_RESULT_NO_WAIT is used but the results are not available? DISCUSSION: Leaving the written value undefined is an option, however in many cases the application can benefit from having a defined value in the query buffer if the results are not available. The following options were considered to support such use cases: a. Write a predefined fixed value to the buffer. b. Write a user specified value to the buffer. c. Don't write anything to the buffer. The problem with option a. is that it may be difficult to select such a predefined value that would not potentially conflict with a valid value. Option b. could be fine, however it requires new API to specify this default value. Thus, option c. is considered to most preferable. RESOLVED: Nothing, the current value in the specified offset of the query buffer is left untouched. 9) How does ARB_query_buffer_object differ from AMD_query_buffer_object? RESOLVED: This extension introduced a new barrier bit accepted by MemoryBarrier that controls the relative ordering of shader buffer writes and query buffer writes. Revision History Rev. Date Author Changes ---- -------- -------- ---------------------------------------------- 3 03/07/2016 drakos Fixed typo in example #4 2 04/02/2013 drakos Added missing function names to the list of functions accepting QUERY_BUFFER as parameter 1 03/08/2013 drakos Initial draft based on AMD_query_buffer_object Removed suffixes Updated spec language to GL 4.3 Added QUERY_BUFFER_BARRIER_BIT