blob: f972560ce859cfbe5487f343f3e9c88a10a1fc2c [file] [log] [blame]
Name Strings
Jeff Bolz, NVIDIA Corporation (jbolz 'at'
Pat Brown, NVIDIA Corporation (pbrown 'at'
Mathias Heyer, NVIDIA
Last Modified Date: April 7, 2015
Revision: 1
OpenGL Extension #483
OpenGL ES Extension #258
This extension is written against the OpenGL 4.3 specification
(Compatibility Profile).
This extension interacts with the OpenGL ES 3.1 (March 17, 2014)
This extension interacts with NV_viewport_array2.
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 <swizzlex>, <swizzley>, <swizzlez>, and <swizzlew>
parameters of ViewportSwizzleNV:
Accepted by the <pname> parameter of GetBooleani_v, GetDoublei_v,
GetIntegeri_v, GetFloati_v, and GetInteger64i_v:
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 <index> has its x,y,z,w swizzle state set to the
corresponding <swizzlex>, <swizzley>, <swizzlez>, <swizzlew> value. If the
value of VIEWPORT_SWIZZLE_X_NV is denoted by <swizzlex>, 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.
- The error INVALID_VALUE is generated if <index> is greater than or equal
to the value of MAX_VIEWPORTS.
- The error INVALID_ENUM is generated if any of <swizzlex>, <swizzley>,
<swizzlez>, or <swizzlew> are not one of
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 <i> is set in the mask, the primitive is emitted to viewport
<i> and transformed and swizzled using the state of viewport <i>. ...
New Implementation Dependent State
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
GLX Protocol
The error INVALID_VALUE is generated by ViewportSwizzleNV if <index> is
greater than or equal to the value of MAX_VIEWPORTS.
The error INVALID_ENUM is generated by ViewportSwizzleNV if any of
<swizzlex>, <swizzley>, <swizzlez>, or <swizzlew> are not one of
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.
(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)) {
// 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;
With passthrough geometry shaders, this can be done using a much simpler
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
Note that the use of the passed through input <normal> 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 <normal> 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 <normal> 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
Revision History
Revision 1
- Internal revisions.