Name NV_viewport_swizzle Name Strings GL_NV_viewport_swizzle Contact Jeff Bolz, NVIDIA Corporation (jbolz 'at' nvidia.com) Pat Brown, NVIDIA Corporation (pbrown 'at' nvidia.com) Contributors Mathias Heyer, NVIDIA Status Shipping. Version Last Modified Date: April 7, 2015 Revision: 1 Number OpenGL Extension #483 OpenGL ES Extension #258 Dependencies This extension is written against the OpenGL 4.3 specification (Compatibility Profile). This extension interacts with the OpenGL ES 3.1 (March 17, 2014) specification. This extension interacts with NV_viewport_array2. Overview This extension provides a new per-viewport swizzle that can modify the position of primitives sent to each viewport. New viewport swizzle state is added for each viewport, and a new position vector is computed for each vertex by selecting from and optionally negating any of the four components of the original position vector. This new viewport swizzle is useful for a number of algorithms, including single-pass cubemap rendering (broadcasting a primitive to multiple faces and reorienting the vertex position for each face) and voxel rasterization. The per-viewport component remapping and negation provided by the swizzle allows application code to re-orient three-dimensional geometry with a view along any of the X, Y, or Z axes. If a perspective projection and depth buffering is required, 1/W buffering should be used, as described in the single-pass cubemap rendering example in the "Issues" section below. New Procedures and Functions void ViewportSwizzleNV(uint index, enum swizzlex, enum swizzley, enum swizzlez, enum swizzlew); New Tokens Accepted by the , , , and parameters of ViewportSwizzleNV: VIEWPORT_SWIZZLE_POSITIVE_X_NV 0x9350 VIEWPORT_SWIZZLE_NEGATIVE_X_NV 0x9351 VIEWPORT_SWIZZLE_POSITIVE_Y_NV 0x9352 VIEWPORT_SWIZZLE_NEGATIVE_Y_NV 0x9353 VIEWPORT_SWIZZLE_POSITIVE_Z_NV 0x9354 VIEWPORT_SWIZZLE_NEGATIVE_Z_NV 0x9355 VIEWPORT_SWIZZLE_POSITIVE_W_NV 0x9356 VIEWPORT_SWIZZLE_NEGATIVE_W_NV 0x9357 Accepted by the parameter of GetBooleani_v, GetDoublei_v, GetIntegeri_v, GetFloati_v, and GetInteger64i_v: VIEWPORT_SWIZZLE_X_NV 0x9358 VIEWPORT_SWIZZLE_Y_NV 0x9359 VIEWPORT_SWIZZLE_Z_NV 0x935A VIEWPORT_SWIZZLE_W_NV 0x935B Additions to Chapter 13 of the OpenGL 4.3 (Compatibility Profile) Specification (Fixed-Function Vertex Post-Processing) Modify Section 13.2 (Transform Feedback), p. 453 Modify the first paragraph: ...The vertices are fed back after vertex color clamping, but before viewport swizzling and viewport mask expansion, flatshading, and clipping. ... Add a new Section 13.X (Viewport Swizzle) after 13.3 (Primitive Queries) Each primitive sent to a given viewport has a swizzle and optional negation applied to its clip coordinates. The swizzle that is applied depends on the viewport index, and is controlled by the command void ViewportSwizzleNV(uint index, enum swizzlex, enum swizzley, enum swizzlez, enum swizzlew); The viewport specified by has its x,y,z,w swizzle state set to the corresponding , , , value. If the value of VIEWPORT_SWIZZLE_X_NV is denoted by , swizzling computes the new x component of the position as if (swizzlex == VIEWPORT_SWIZZLE_POSITIVE_X_NV) x' = x; if (swizzlex == VIEWPORT_SWIZZLE_NEGATIVE_X_NV) x' = -x; if (swizzlex == VIEWPORT_SWIZZLE_POSITIVE_Y_NV) x' = y; if (swizzlex == VIEWPORT_SWIZZLE_NEGATIVE_Y_NV) x' = -y; if (swizzlex == VIEWPORT_SWIZZLE_POSITIVE_Z_NV) x' = z; if (swizzlex == VIEWPORT_SWIZZLE_NEGATIVE_Z_NV) x' = -z; if (swizzlex == VIEWPORT_SWIZZLE_POSITIVE_W_NV) x' = w; if (swizzlex == VIEWPORT_SWIZZLE_NEGATIVE_W_NV) x' = -w; Similar selections are performed for the y, z, and w coordinates. This swizzling is applied after transform feedback, but before clipping and perspective divide. Errors: - The error INVALID_VALUE is generated if is greater than or equal to the value of MAX_VIEWPORTS. - The error INVALID_ENUM is generated if any of , , , or are not one of VIEWPORT_SWIZZLE_{POSITIVE,NEGATIVE}_{X,Y,Z,W}. Modify Section 13.6.1 (Controlling the Viewport) (modify the first paragraph, p. 470, as edited by NV_viewport_array2, using "transformed and swizzled" instead of "transformed") Multiple viewports are available ... The primitive is transformed and swizzled using the state of the selected viewport. ... ... If bit is set in the mask, the primitive is emitted to viewport and transformed and swizzled using the state of viewport . ... New Implementation Dependent State None. New State Get Value Get Command Type Initial Value Description Sec. Attribute --------- ----------- ---- ------------- ----------- ---- --------- VIEWPORT_SWIZZLE_X_NV GetIntegeri_v nxZ8 VIEWPORT_SWIZZLE- coordinate and sign for 13.X viewport POSITIVE_X viewport swizzling VIEWPORT_SWIZZLE_Y_NV GetIntegeri_v nxZ8 VIEWPORT_SWIZZLE- coordinate and sign for 13.X viewport POSITIVE_Y viewport swizzling VIEWPORT_SWIZZLE_Z_NV GetIntegeri_v nxZ8 VIEWPORT_SWIZZLE- coordinate and sign for 13.X viewport POSITIVE_Z viewport swizzling VIEWPORT_SWIZZLE_W_NV GetIntegeri_v nxZ8 VIEWPORT_SWIZZLE- coordinate and sign for 13.X viewport POSITIVE_W viewport swizzling Additions to the AGL/GLX/WGL Specifications None. GLX Protocol None. Errors The error INVALID_VALUE is generated by ViewportSwizzleNV if is greater than or equal to the value of MAX_VIEWPORTS. The error INVALID_ENUM is generated by ViewportSwizzleNV if any of , , , or are not one of VIEWPORT_SWIZZLE_{POSITIVE,NEGATIVE}_{X,Y,Z,W}. Interactions with OpenGL ES 3.1 Remove references to GetDoublei_v and GetBooleani_v. Also remove the reference to 'vertex color clamping'. Interactions with NV_viewport_array2 This specification modifies language added/changed by NV_viewport_array2. There are no functional interactions between the two extensions, though we expect that all implementations of this extension will support NV_viewport_array2 or similar functionality. Issues (1) Where does viewport swizzling occur in the pipeline? RESOLVED: Despite being associated with the viewport, viewport swizzling must happen prior to the viewport transform. In particular, it needs to be performed before clipping and perspective division. The viewport mask expansion (NV_viewport_array2) and the viewport swizzle could potentially be performed before or after transform feedback, but feeding back several viewports worth of primitives with different swizzles doesn't seem particularly useful. This specification applies the viewport mask and swizzle after transform feedback, and makes primitive queries only count each primitive once. (2) Any interesting examples of how this extension, NV_viewport_array2, and NV_geometry_shader_passthrough can be used together in practice? RESOLVED: One interesting use case for this extension is for single-pass rendering to a cubemap. In this example, the application would attach a cubemap texture to a layered FBO where the six cube faces are treated as layers. Vertices are sent through the vertex shader without applying a projection matrix, where the gl_Position output is (x,y,z,1) and the center of the cubemap is at (0,0,0). With unextended OpenGL, one could have a conventional instanced geometry shader that looks something like the following: layout(invocations = 6) in; // separate invocation per face layout(triangles) in; layout(triangle_strip) out; layout(max_vertices = 3) out; in Inputs { vec2 texcoord; vec3 normal; vec4 baseColor; } v[]; out Outputs { vec2 texcoord; vec3 normal; vec4 baseColor; }; void main() { int face = gl_InvocationID; // which face am I? // Project gl_Position for each vertex onto the cube map face. vec4 positions[3]; for (int i = 0; i < 3; i++) { positions[i] = rotate(gl_in[i].gl_Position, face); } // If the primitive doesn't project onto this face, we're done. if (shouldCull(positions)) { return; } // Otherwise, emit a copy of the input primitive to the // appropriate face (using gl_Layer). for (int i = 0; i < 3; i++) { gl_Layer = face; gl_Position = positions[i]; texcoord = v[i].texcoord; normal = v[i].normal; baseColor = v[i].baseColor; EmitVertex(); } } With passthrough geometry shaders, this can be done using a much simpler shader: layout(triangles) in; layout(passthrough) in Inputs { vec2 texcoord; vec3 normal; vec4 baseColor; } layout(passthrough) in gl_PerVertex { vec4 gl_Position; } gl_in[]; layout(viewport_relative) out int gl_Layer; void main() { // Figure out which faces the primitive projects onto and // generate a corresponding viewport mask. uint mask = 0; for (int i = 0; i < 6; i++) { if (!shouldCull(face)) { mask |= 1U << i; } } gl_ViewportMask = mask; gl_Layer = 0; } The application code is set up so that each of the six cube faces has a separate viewport (numbered 0..5). Each face also has a separate swizzle, programmed via the ViewportSwizzleNV() command. The viewport swizzle feature performs the coordinate transformation handled by the rotate() function in the original shader. The "viewport_relative" layout qualifier says that the viewport number (0..5) is added to the base gl_Layer value of zero to determine which layer (cube face) the primitive should be sent to. Note that the use of the passed through input in this example suggests that the fragment shader in this example would perform an operation like per-fragment lighting. The viewport swizzle would transform the position to be face-relative, but would remain in the original coordinate system. It seems likely that the fragment shader in either version of the example would want to perform lighting in the original coordinate system. It would likely do this by reconstructing the position of the fragment in the original coordinate system using gl_FragCoord, a constant or uniform holding the size of the cube face, and the input gl_ViewportIndex (or gl_Layer), which identifies the cube face. Since the value of is in the original coordinate system, it would not need to be modified as part of this coordinate transformation. Note that while the rotate() operation in the regular geometry shader above could include an arbitrary post-rotation projection matrix, the viewport swizzle does not support arbitrary math. To get proper projection, 1/W buffering should be used. To do this: (1) Program the viewport swizzles to move the pre-projection W eye coordinate (typically 1.0) into the Z coordinate of the swizzle output and the eye coordinate component used for depth into the W coordinate. For example, the viewport corresponding to the +Z face might use a swizzle of (+X, -Y, +W, +Z). The Z normalized device coordinate computed after swizzling would then be z'/w' = 1/Z_eye. (2a) On NVIDIA implementations supporting floating-point depth buffers with values outside [0,1], prevent unwanted near plane clipping by enabling DEPTH_CLAMP. Ensure that the depth clamp doesn't mess up depth testing by programming the depth range to very large values, such as glDepthRangedNV(-z, +z), where z == 2^127. It should be possible to use IEEE infinity encodings also (0xFF800000 for -INF, 0x7F800000 for +INF). Even when near/far clipping is disabled, primitives extending behind the eye will still be clipped because one or more vertices will have a negative W coordinate and fail X/Y clipping tests. (2b) On other implementations, scale X, Y, and Z eye coordinates so that vertices on the near plane have a post-swizzle W coordinate of 1.0. For example, if the near plane is at Z_eye = 1/256, scale X, Y, and Z by 256. Also, ideally, program the depth range transformation to be a NOP by using a clip control depth mode (OpenGL 4.5) of ZERO_TO_ONE. (3) Adjust depth testing to reflect the fact that 1/W values are large near the eye and small away from the eye. Clear the depth buffer to zero (infinitely far away) and use a depth test of GREATER instead of LESS. Revision History Revision 1 - Internal revisions.