Name EXT_transform_feedback2 Name Strings GL_EXT_transform_feedback2 Contributors Eric Zolnowski Contact Pat Brown, NVIDIA Corporation (pbrown 'at' nvidia.com) Eric Zolnowski, AMD (eric.zolnowski 'at' amd.com) Status In Progress. Version Last Modified Date: 10/12/2009 Author Revision: 3 Number xxx Dependencies The OpenGL Shading Language (GLSL) is required. OpenGL 2.0 or the ARB_shader_objects extension is required. NV_transform_feedback or EXT_transform_feedback is required. EXT_geometry_shader4 trivially interacts with this extension. This extension is written against the OpenGL 2.1 Specification. This extension is written against the EXT_transform_feedback extension specification. Overview The EXT_transform_feedback extension allows applications to capture primitives to one or more buffer objects when transformed by the GL. This extension provides a few additional capabilities to these extensions, making transform feedback mode more useful. First, it provides transform feedback objects which encapsulate transform feedback-related state, allowing applications to replace the entire transform feedback configuration in a single bind call. Second, it provides the ability to pause and resume transform feedback operations. When transform feedback is paused, applications may render without transform feedback or may use transform feedback with different state and a different transform feedback object. When transform feedback is resumed, additional primitives are captured and appended to previously captured primitives for the object. Additionally, this extension provides the ability to draw primitives captured in transform feedback mode without querying the captured primitive count. The command DrawTransformFeedbackEXT() is equivalent to glDrawArrays(, 0, ), where is the number of vertices captured to buffer objects during the last transform feedback capture operation on the transform feedback object used. This draw operation only provides a vertex count -- it does not automatically set up vertex array state or vertex buffer object bindings, which must be done separately by the application. New Procedures and Functions void BindTransformFeedbackEXT(enum target, uint id); void DeleteTransformFeedbacksEXT(sizei n, const uint *ids); void GenTransformFeedbacksEXT(sizei n, uint *ids); boolean IsTransformFeedbackEXT(uint id); void PauseTransformFeedbackEXT(void); void ResumeTransformFeedbackEXT(void); void DrawTransformFeedbackEXT(enum mode, uint id); New Tokens Accepted by the parameter of BindTransformFeedbackEXT: TRANSFORM_FEEDBACK_EXT 0x8E22 Accepted by the parameter of GetBooleanv, GetDoublev, GetIntegerv, and GetFloatv: TRANSFORM_FEEDBACK_BUFFER_PAUSED_EXT 0x8E23 TRANSFORM_FEEDBACK_BUFFER_ACTIVE_EXT 0x8E24 TRANSFORM_FEEDBACK_BINDING_EXT 0x8E25 Additions to Chapter 2 of the OpenGL 2.1 Specification (OpenGL Operation) (Replace the "Section 2.Y" introduced by the EXT_transform_feedback specification.) In transform feedback mode, attributes of the vertices of transformed primitives are written out to one or more buffer objects. The vertices are fed back after vertex color clamping but before clipping. If a geometry shader is active, the vertices recorded are those emitted from the geometry shader. The transformed vertices may be optionally discarded after being stored into one or more buffer objects, or they can be passed on down to the clipping stage for further processing. Section 2.Y.1, Transform Feedback Objects The set of buffer objects used to capture vertex attributes and related state are stored in a transform feedback object. If a vertex or geometry shader is active, the set of attributes captured in transform feedback mode is determined using the state of the active program object; otherwise, it is taken from the state of the currently bound transform feedback object, as described below. The name space for transform feedback objects is the unsigned integers. The name zero designates the default transform feedback object. A transform feedback object is created by calling void BindTransformFeedbackEXT(enum target, uint id) with set to TRANSFORM_FEEDBACK_EXT and set an unused transform feedback object name. The resulting transform feedback object is a new state vector, initialized to the default state values described in Table 6.X. Additionally, the new object is bound to the GL state vector and is used for subsequent transform feedback operations. BindTransformFeedbackEXT can also be used to bind an existing transform feedback object to the GL state for subsequent use. If the bind is successful, no change is made to the state of the newly bound transform feedback object and any previous binding to is broken. While a transform feedback buffer object is bound, GL operations on the target to which it is bound affect the bound transform feedback object, and queries of the target to which a transform feedback object is bound return state from the bound object. When buffer objects are bound for transform feedback, they are attached to the currently bound transform feedback object. Buffer objects are used for transform feedback only if they are attached to the currently bound transform feedback object. In the initial state, a default transform feedback object is bound and treated as a transform feedback object with a name of zero. That object is bound any time BindTransformFeedbackEXT() is called with of zero. The error INVALID_OPERATION is generated by BindTransformFeedbackEXT if the transform feedback operation is active on the currently bound transform feedback object, and that operation is not paused (as described below). Transform feedback objects are deleted by calling void DeleteTransformFeedbacksEXT(sizei n, const uint *ids) contains names of transform feedback objects to be deleted. After a transform feedback object is deleted it has no contents, and its name is again unused. Unused names in are silently ignored, as is the value zero. The default transform feedback object cannot be deleted. The error INVALID_OPERATION is generated by DeleteTransformFeedbacksEXT if the transform feedback operation for any object named by is currently active. The command void GenTransformFeedbacksEXT(sizei n, uint *ids) returns previously unused transform feedback object names in . These names are marked as used, for the purposes of GenTransformFeedbacksEXT only, but they acquire transform feedback state only when they are first bound, just as if they were unused. Section 2.Y.2, Transform Feedback Primitive Capture Transform feedback for the currently bound transform feedback object is started and finished by calling void BeginTransformFeedbackEXT(enum primitiveMode) and void EndTransformFeedbackEXT(void), respectively. Transform feedback is said to be active after a call to BeginTransformFeedbackEXT and inactive after a call to EndTransformFeedbackEXT. is one of TRIANGLES, LINES, or POINTS, and specifies the output type of primitives that will be recorded into the buffer objects bound for transform feedback (see below). Transform feedback commands must be paired; the error INVALID_OPERATION is generated by BeginTransformFeedbackEXT if transform feedback is active, and by EndTransformFeedbackEXT if transform feedback is inactive. Transform feedback is initially inactive. Transform feedback operations for the currently bound transform feedback object may be paused and resumed by calling void PauseTransformFeedbackEXT(void) and void ResumeTransformFeedbackEXT(void), respectively. When transform feedback operations are paused, transform feedback is still considered active and changing most transform feedback state related to the object results in an error. However, a new transform feedback object may be bound while transform feedback is paused. The error INVALID_OPERATION is generated by PauseTransformFeedbackEXT if the currently bound transform feedback is not active or is paused. The error INVALID_OPERATION is generated by ResumeTransformFeedbackEXT if the currently bound transform feedback is not active or is not paused. When transform feedback is active and not paused, all geometric primitives generated must be compatible with the value of passed to BeginTransformFeedbackEXT. The error INVALID_OPERATION is generated by Begin or any operation that implicitly calls Begin (such as DrawElements) if is not one of the allowed modes in Table X.1. If a geometry shader is active, its output primitive type is used instead of the parameter passed to Begin for the purposes of this error check. Any primitive type may be used while transform feedback is paused. Transform Feedback primitiveMode allowed render primitive modes ---------------------- --------------------------------- POINTS POINTS LINES LINES, LINE_LOOP, and LINE_STRIP TRIANGLES TRIANGLES, TRIANGLE_STRIP, TRIANGLE_FAN, QUADS, QUAD_STRIP, and POLYGON Table X.1 Legal combinations between the transform feedback primitive mode, as passed to BeginTransformFeedbackEXT and the current primitive mode. Transform feedback mode captures the values of varying variables written by an active vertex or geometry shader. The error INVALID_OPERATION is generated by BeginTransformFeedbackEXT if no vertex or geometry shader is active. Buffer objects are made to be targets of transform feedback by calling one of the commands void BindBufferRangeEXT(enum target, uint index, uint buffer, intptr offset, sizeiptr size), void BindBufferOffsetEXT(enum target, uint index, uint buffer, or intptr offset), void BindBufferBaseEXT(enum target, uint index, uint buffer), with set to TRANSFORM_FEEDBACK_BUFFER_EXT. There is an array of buffer object binding points that are used while transform feedback is active, plus a single general binding point that can be used by other buffer object manipulation functions (e.g., BindBuffer, MapBuffer). All three commands bind the buffer object named by to the general binding point, and additionally bind the buffer object to the binding point in the array given by . The error INVALID_VALUE is generated if is greater than or equal to the value of MAX_TRANSFORM_FEEDBACK_SEPARATE_ATTRIBS_EXT. For BindBufferRangeEXT, specifies a starting offset into the buffer object and specifies the amount of data that can be written to the buffer object while transform feedback mode is active. Both and are in basic machine units. The error INVALID_VALUE is generated if the value of is less than or equal to zero, or if either or are not word-aligned. Calling BindBufferOffsetEXT is equivalent of calling BindBufferRangeEXT with = sizeof(buffer) - , and rounding down so that it is word-aligned. BindBufferBaseEXT is equivalent to calling BindBufferOffsetEXT with an of 0. The set of buffer objects used by a transform feedback object may not change while transform feedback is active. The error INVALID_OPERATION is generated by BindBufferRangeEXT, BindBufferOffsetEXT, or BindBufferBaseEXT if is TRANSFORM_FEEDBACK_BUFER_EXT and transform feedback is currently active. When an individual point, line, or triangle primitive reaches the transform feedback stage while transform feedback is active and not paused, the values of the specified varying variables of each vertex are appended to the buffer objects bound to the transform feedback binding points. The attributes of the first vertex received after BeginTransformFeedbackEXT are written at the starting offsets of the bound buffer objects set by BindBufferRangeEXT, and subsequent vertex attributes are appended to the buffer object. When capturing line and triangle primitives, all attributes of the first vertex are written first, followed by attributes of the subsequent vertices. When writing varying variables that are arrays, individual array elements are written in order. For multi-component varying variables, elements of varying arrays, or transformed vertex attributes, the individual components are written in order. The value for any attribute specified to be streamed to a buffer object but not actually written by a vertex or geometry shader is undefined. When transform feedback is paused, no vertices are recorded. When transform feedback is resumed, subsequent vertices are appended to the buffer objects bound immediately following the last vertex written while transform feedback was paused. When quads and polygons are provided to transform feedback with a primitive mode of TRIANGLES, they will be tessellated and recorded as triangles (the order of tessellation within a primitive is undefined). Individual lines or triangles of a strip or fan primitive will be extracted and recorded separately. Incomplete primitives are not recorded. Transform feedback can operate in either INTERLEAVED_ATTRIBS_EXT or SEPARATE_ATTRIBS_EXT mode. In INTERLEAVED_ATTRIBS_EXT mode, the values of one or more varyings or transformed vertex attributes are written interleaved, into the buffer object bound to the first transform feedback binding point (index = 0). If more than one varying variable is written, they will be recorded in the order specified by TransformFeedbackVaryingsEXT (EXT_transform_feedback specification, section 2.15.3). In SEPARATE_ATTRIBS_EXT mode, the first varying variable or transformed vertex attribute specified is written to the first transform feedback binding point; subsequent varying variables are written to the subsequent transform feedback binding points. The total number of variables or transformed attributes that may be captured in SEPARATE_ATTRIBS_EXT mode is given by MAX_TRANSFORM_FEEDBACK_SEPARATE_ATTRIBS_EXT. If recording the vertices of a primitive to the buffer objects being used for transform feedback purposes would result in either exceeding the limits of any buffer object's size, or in exceeding the end position + - 1, as set by BindBufferRangeEXT, then no vertices of that primitive are recorded in any buffer object, and the counter corresponding to the asynchronous query target TRANSFORM_FEEDBACK_PRIMITIVES_WRITTEN_EXT (see Section 2.Z) is not incremented. In either separate or interleaved modes, all transform feedback binding points that will be written to must have buffer objects bound when BeginTransformFeedbackEXT is called. The error INVALID_OPERATION is generated by BeginTransformFeedbackEXT if any binding point used in transform feedback mode does not have a buffer object bound. In interleaved mode, only the first buffer object binding point is ever written to. (add error paragraph describing conditions where it is illegal to change transform feedback-related state) When BeginTransformFeedbackEXT is called with an active program object containing a vertex or geometry shader, the set of varying variables captured during transform feedback is taken from the active program object and may not be changed while transform feedback is active. That program object must be active until the EndTransformFeedbackEXT is called, except while the transform feedback object is paused. The error INVALID_OPERATION is generated: * by TransformFeedbackVaryingsEXT if the current transform feedback object is active, even if paused; * by UseProgram if the current transform feedback object is active and not paused; * by LinkProgram if is the name of a program being used by one or more transform feedback objects, even if the objects are not currently bound or are paused; or * by ResumeTransformFeedbackEXT if the program object being used by the current transform feedback object is not active. Section 2.Y.3, Transform Feedback Draw Operations When transform feedback is active, the values of varyings or transformed vertex attributes are captured into the buffer objects attached to the current transform feedback object. After transform feedback is complete, subsequent rendering operations may use the contents of these buffer objects (section 2.9). The number of vertices captured during transform feedback is stored in the corresponding transform feedback object and may be used in conjunction with the command void DrawTransformFeedbackEXT(enum mode, uint id) to replay the captured vertices. This command is equivalent to calling DrawArrays with set to , set to zero, and set to the number of vertices captured the last time transform feedback was active on the transform feedback object named by . The error INVALID_VALUE is generated if is not the name of a transform feedback object. The error INVALID_OPERATION is generated if EndTransformFeedbackEXT has never been called while the object named by was bound. No error is generated if the transform feedback object named by is active; the vertex count used for the rendering operation is set by the previous EndTransformFeedbackEXT command. Additions to Chapter 3 of the OpenGL 2.1 Specification (Rasterization) None. Additions to Chapter 4 of the OpenGL 2.1 Specification (Per-Fragment Operations and the Frame Buffer) None. Additions to Chapter 5 of the OpenGL 2.1 Specification (Special Functions) On p. 244, add a new set of commands to the list of commands not compiled into a display list: Transform feedback objects: GenTransformFeedbacksEXT, DeleteTransformFeedbacksEXT, (note: IsTransformFeedbackEXT is covered by the "Other queries" rule.) Additions to Chapter 6 of the OpenGL 2.1 Specification (State and State Requests) (Add to the "Transform Feedback" query section added by the EXT_transform_feedback extension.) The command boolean IsTransformFeedbackEXT(uint id) returns TRUE if is the name of a transform feedback object. If is a non-zero value that is not the name of a transform feedback object, IsTransformFeedbackEXT() return FALSE. Additions to Appendix A of the OpenGL 2.1 Specification (Invariance) None. Additions to the AGL/GLX/WGL Specifications None. GLX Protocol TBD (Protocol support needs to be added for the new functions.) Dependencies on NV_transform_feedback2 Dependencies on EXT_transform_feedback This language is written against the EXT_transform_feedback specification. Dependencies on EXT_geometry_shader4 If EXT_geometry_shader4 is not supported, remove all references to geometry shaders. Errors The error INVALID_OPERATION is generated by BindTransformFeedbackEXT if a transform feedback operation is active on the currently bound transform feedback object, and that operation is not paused (as described below). The error INVALID_OPERATION is generated by DeleteTransformFeedbacksEXT if the transform feedback operation for any object named by is currently active. The error INVALID_OPERATION is generated by BeginTransformFeedbackEXT if transform feedback is active. The error INVALID_OPERATION is generated by EndTransformFeedbackEXT if transform feedback is inactive. The error INVALID_OPERATION is generated by PauseTransformFeedbackEXT if the currently bound transform feedback is not active or is paused. The error INVALID_OPERATION is generated by ResumeTransformFeedbackEXT if the currently bound transform feedback is not active or is not paused. The error INVALID_OPERATION is generated by Begin or any operation that implicitly calls Begin (such as DrawElements) if transform feedback is active and not paused and if is incompatible with the parameter supplied to BeginTransformFeedbackEXT. The error INVALID_VALUE is generated by BindBufferRangeEXT, BindBufferOffsetEXT, or BindBufferBaseEXT if is TRANSFORM_FEEDBACK_BUFFER_EXT and is greater than or equal to the value of MAX_TRANSFORM_FEEDBACK_SEPARATE_ATTRIBS_EXT. The error INVALID_VALUE is generated by BindBufferRangeEXT if is less than or equal to zero or either or are not word-aligned. The error INVALID_VALUE is generated by BindBufferOffsetEXT if is not word-aligned. The error INVALID_OPERATION is generated by BindBufferRangeEXT, BindBufferOffsetEXT, or BindBufferBaseEXT if is TRANSFORM_FEEDBACK_BUFFER_EXT and transform feedback is currently active. The error INVALID_OPERATION is generated by BeginTransformFeedbackEXT if any binding point used in transform feedback mode does not have a buffer object bound. The error INVALID_OPERATION is generated by TransformFeedbackVaryingsEXT if the current transform feedback object is active, even if paused. The error INVALID_OPERATION is generated by UseProgram if the current transform feedback object is active and not paused. The error INVALID_OPERATION is generated by LinkProgram if is the name of a program being used by one or more transform feedback objects, even if the objects are not currently bound or are paused. The error INVALID_OPERATION is generated by ResumeTransformFeedbackEXT if the program object being used by the current transform feedback object is not active. The error INVALID_VALUE is generated by DrawTransformFeedbackEXT if is not the name of a transform feedback object. The error INVALID_OPERATION is generated by DrawTransformFeedbackEXT if EndTransformFeedbackEXT has never been called while the object named by was bound. New State (Add a new table: Table 6.X, Transform Feedback Object State) Get Value Type Get Command Init. Value Description Sec Attrib ------------------ ------ -------------- ------------ ------------------------- ----- ------ TRANSFORM_FEEDBACK_ Z+ GetIntegerv 0 Buffer object bound to 6.1.13 - BUFFER_BINDING_EXT generic bind point for transform feedback. TRANSFORM_FEEDBACK_ nxZ+ GetInteger- 0 Buffer object bound to 6.1.13 - BUFFER_BINDING_EXT IndexedvEXT each transform feedback attribute stream. TRANSFORM_FEEDBACK_ nxZ+ GetInteger- 0 Start offset of binding 6.1.13 - BUFFER_START_EXT IndexedvEXT range for each transform feedback attrib. stream TRANSFORM_FEEDBACK_ nxZ+ GetInteger- 0 Size of binding range 6.1.13 - BUFFER_SIZE_EXT IndexedvEXT for each transform TRANSFORM_FEEDBACK_ B GetBooleanv FALSE Is transform feedback 6.1.13 - BUFFER_PAUSED_EXT paused on this object? TRANSFORM_FEEDBACK_ B GetBooleanv FALSE Is transform feedback 6.1.13 - BUFFER_ACTIVE_EXT active on this object? [[ Note: This table includes all transform feedback state provided by EXT_transform_feedback, except for transform feedback-related state belonging to query objects and GLSL program objects. The only other transform feedback-related state not present in these objects is the object binding (below) and implementation-dependent limits. ]] (Modify table 6.10: Transformation State, p. 275) Get Value Type Get Command Init. Value Description Sec Attrib ------------------ ------ -------------- ------------ ------------------------- ----- ------ TRANSFORM_FEEDBACK_ Z+ GetIntegerv 0 Object bound for transform 2.Y - BINDING_EXT feedback operations New Implementation Dependent State None. Issues 1. How should we provide the ability to automatically render primitives captured in transform feedback mode? RESOLVED: Adding a new transform feedback state object provides encapsulation for two useful operations: pause/resume and automatic rendering. When applications pause and possibly switch to different transform feedback state, it is necessary to save the state of the paused transform feedback operation somewhere. The transform feedback object provides a convenient entity to hold this saved state. The transform feedback object is also a convenient place to store final counts for use by actual drawing. Additionally, the transform feedback object is helpful in ensuring that the transform feedback state used when resuming transform feedback is the same as when it was paused without a complicated error check. We simply disallow changing the state in an object while transform feedback is active (even when paused), so the state can't become inconsistent while paused. The same basic consistency rules apply to transform feedback state stored separately in a GLSL program object; you can't change them while transform feedback is active in the original extensions, and this extension treats the paused state as active for the purposes of these restrictions. Alternately, the in-progress transform feedback state (e.g., vertex counts, pointers into buffer objects) could have been stored with the buffer objects used to capture the primitives. 2. Are transform feedback objects shared between contexts? RESOLVED: No. The amount of state present in one of these objects is fairly small -- there is not a lot of memory saved by avoiding multiple copies through sharing. Additionally, sharing transform feedback objects between contexts doesn't seem particularly useful -- an object could only really be used by one context at a time and explicit synchronization would be required to use the results of one object. Note that this resolution is consistent with query objects, which is the primary type of object used in the original transform feedback specification. 3. How do the new transform feedback objects interact with GLSL program objects? RESOLVED: The set of varyings captured during transform feedback and the buffer mode (interleaved or separate) were assigned to the program object in the original EXT_transform_feedback specification. That seems sensible given that the varyings themselves belong to the program object. In the original extension, implementations are forbidden to unbind or relink a program object or reassign the set of varyings to capture while transform feedback is active. The same basic restrictions apply in this extension, except that than an application may unbind a program object while transform feedback is paused. In order to resume transform feedback, the same program object must be active. Applications may not relink a program or reassign its captured varyings while it is being actively used for capture in any transform feedback object. The actual buffer objects bound in transform feedback mode were bound to the context (not the program object) in the original transform feedback extension. 4. Should we provide any behavior to "cancel" paused transform feedback operations? If an application fails to assign the correct state (e.g., GLSL program object), a resume operation may fail and the XFB object might get stuck in a paused state indefinitely. RESOLVED: Yes. EndTransformFeedback is defined to cancel an active transform feedback operation, even if it is currently paused. 5. Should buffer object bindings be encapsulated in the new transform feedback object? RESOLVED: Yes. This allows applications the convenience of updating all the transform feedback state in one call. Additionally, it ensures that the set of buffer bindings remains consistent while transform feedback is active -- even if we switch objects while paused. 6. Should we be able to use two different sets of transform feedback state (one for capture, a second for rendering - via DrawTransformFeedbackEXT)? RESOLVED: Yes. We should support the ability to capture primitives in transform feedback that are produced by DrawTransformFeedbackEXT. Requiring that applications use the a single transform feedback object for both operations (if even possible) seems inconvenient. As a result, we provide the ability to use separate objects for capture and rendering. 7. How should the second transform feedback object used for rendering be provided to the GL? RESOLVED: The approach chosen by this extension is to have DrawTransformFeedbackEXT() accept a transform feedback object ID. An alternate approach would have been to provide a second binding point (TRANSFORM_FEEDBACK_RENDER_EXT?) whose bound object would be used by any DrawTransformFeedbackEXT() call. 8. Can a single transform feedback object be used for both capture and drawing (via DrawTransformFeedback) at the same time? RESOLVED: Yes. DrawTransformFeedbackEXT is defined to use the vertex count established when the previous transform feedback operation on that object completes (by an EndTransformFeedbackEXT call). If transform feedback is active for an object (paused or not), the accumulated vertex count for the in-progress operation is never used by DrawTransformFeedbackEXT. 9. Does DrawTransformFeedbackEXT() automatically use the buffer objects from the previous transform feedback operation? If not, does it require that applications set up and use only those buffer manually? RESOLVED: DrawTransformFeedbackEXT() couldn't automatically set up buffer objects, even if we wanted to (which we don't). No mechanism exists to automatically line up per-vertex outputs captured during transform feedback against the inputs of a different vertex shader. Applications are thus required to manually set up their vertex arrays appropriately prior to calling DrawTransformFeedbackEXT(). Applications are not required to use any of the buffer objects written to during the previous transform feedback operation, and are allowed to use other buffer objects (OK) or vertex arrays not stored in a buffer object at all (legal, but not recommended). The only information the draw call uses from the previous transform feedback operation is the total number of vertices captured. 10. Does DrawTransformFeedbackEXT() require that an application use the same primitive type for drawing that was used during the previous transform feedback operation? RESOLVED: No. We do expect that will be the common case, however. 11. What happens on if DrawTransformFeedbackEXT() uses a transform feedback object whose last capture operation overflowed, and started dropping primitives. RESOLVED: Any primitives discarded during a transform feedback operation will not affect the vertex count extracted by DrawTransformFeedbackEXT(), as though those primitives never existed. 12. How does the ability to pause/resume transform feedback interact with the TRANSFORM_FEEDBACK_PRIMITIVES_WRITTEN_EXT query object? RESOLVED: There is no explicit interaction in this case. The transform feedback-related query objects and transform feedback objects are completely independent. If multiple transform feedback objects are used between BeginQuery() and EndQuery() calls, the query result reflects the number of primitives written using *any* transform feedback object. Note that the primitives written counter is never incremented when transform feedback is paused, because no primitives will be written to buffers while transform feedback is paused. 13. Should we provide a mechanism to query the number of vertices or primitives recorded in the last transform feedback operation on an object? If so, how? RESOLVED: No, not in the this spec. The existing TRANSFORM_FEEDBACK_ PRIMITIVES_WRITTEN_EXT queries can be used to obtain this information. 14. Can a buffer object be attached to more than one transform feedback object at the same time? RESOLVED: Yes. Applications using transform feedback should avoid cases where transform feedback operations can conflict, including: * using multiple threads that simultaneously write to overlapping regions of a single buffer object; or * using one or multiple threads, where a portion of a buffer object is written using one transform feedback object while another transform feedback operation writing to an overlapping region of the same buffer is paused. 15. When a transform feedback object is active and not paused, binding a different transform feedback object without pausing is specified to result in an INVALID_OPERATION error. Should we instead define the bind to implicitly pause and resume as required? RESOLVED: No. While implicit pauses and resumes would be convenient, they have interaction issues with the current transform feedback API. In particular, transform feedback forbids applications from changing various pieces of relevant state (e.g., transform feedback buffer bindings, active GLSL program object) during an active transform feedback operation. The active GLSL program object may be changed while transform feedback is paused, but it must be restored prior to resuming. Consider two active transform feedback objects (A and B) using two different program objects (C and D, respectively). With the current API, you can switch back and forth as follows: // Perform first half of transform feedback for object A. UseProgram(C); BindTransformFeedbackEXT(TRANSFORM_FEEDBACK_EXT, A); BeginTransformFeedbackEXT(...); ... PauseTransformFeedbackEXT(); // Perform first half of transform feedback for object B. UseProgram(D); BindTransformFeedbackEXT(TRANSFORM_FEEDBACK_EXT, B); BeginTransformFeedbackEXT(...); ... PauseTransformFeedbackEXT(); // Perform second half of transform feedback for object A. UseProgram(C); BindTransformFeedbackEXT(TRANSFORM_FEEDBACK_EXT, A); ResumeTransformFeedbackEXT(); ... EndTransformFeedbackEXT(); // Perform second half of transform feedback for object B. UseProgram(D); BindTransformFeedbackEXT(TRANSFORM_FEEDBACK_EXT, B); ResumeTransformFeedbackEXT(); ... EndTransformFeedbackEXT(); Implicit pauses and resumes would allow applications to omit the PauseTransformFeedbackEXT() and ResumeTransformFeedbackEXT() calls. The problem with this approach is that it's not clear when to change program objects. In the example above, the second call to UseProgram(C) is legal because the bound transform feedback object (B) is paused. If the pause call were removed in favor of implicit pauses, that UseProgram(C) call would be illegal because there is an active transform feedback object. Assume the UseProgram(C) call were moved to be after the BindTransformFeedbackEXT(..., A) call on the next line instead. In that case, the implicit resume on the bind call would be the problem instead -- we'd be resuming a transform feedback operation while the wrong program object (D) is active. Revision History Rev. Date Author Changes ---- -------- -------- ----------------------------------------- 3 10/12/09 ericz Removed some state that was part of NV_transform_feedback and only applied to fixed function. 2 09/10/09 ericz Updated issues that were previously UNRESOLVED, based on the latest NV_transform_feedback2 spec. Allow EndTransformFeedbackEXT to be called while transform feedback is paused. 1 06/13/09 ericz Created an initial EXT_transform_feedback2 spec by forking off the existing NV_transform_feedback 2 spec.