Name
NV_clip_space_w_scaling
Name Strings
GL_NV_clip_space_w_scaling
Contact
Kedarnath Thangudu, NVIDIA Corporation (kthangudu 'at' nvidia.com)
Contributors
Eric Werness, NVIDIA Corporation
Ingo Esser, NVIDIA Corporation
Pat Brown, NVIDIA Corporation
Mark Kilgard, NVIDIA Corporation
Status
Shipping in NVIDIA release 367.XX drivers and up.
Version
Last Modified Date: June 1, 2016
NVIDIA Revision: 2
Number
OpenGL Extension #486
Dependencies
This extension is written against OpenGL 4.5 Specification
(Compatibility Profile).
Overview
Virtual Reality (VR) applications often involve a post-processing step to
apply a "barrel" distortion to the rendered image to correct the
"pincushion" distortion introduced by the optics in a VR device. The
barrel distorted image has lower resolution along the edges compared to
the center. Since the original image is rendered at high resolution,
which is uniform across the complete image, a lot of pixels towards the
edges do not make it to the final post-processed image.
This extension also provides a mechanism to render VR scenes at a
non-uniform resolution, in particular a resolution that falls linearly
from the center towards the edges. This is achieved by scaling the "w"
coordinate of the vertices in the clip space before perspective divide.
The clip space "w" coordinate of the vertices may be offset as of a
function of "x" and "y" coordinates as follows:
w' = w + Ax + By
In the intended use case for viewport position scaling, an application
should use a set of 4 viewports, one for each of the 4 quadrants of a
Cartesian coordinate system. Each viewport is set to the dimension of the
image, but is scissored to the quadrant it represents. The application
should specify A and B coefficients of the w-scaling equation above,
that have the same value, but different signs, for each of the viewports.
The signs of A and B should match the signs of X and Y for the quadrant
that they represent such that the value of "w'" will always be greater
than or equal to the original "w" value for the entire image. Since the
offset to "w", (Ax + By), is always positive and increases with the
absolute values of "x" and "y", the effective resolution will fall off
linearly from the center of the image to its edges.
New Procedures and Functions
void ViewportPositionWScaleNV(uint index, float xcoeff, float ycoeff)
New Tokens
Accepted by the parameter of Enable, Disable, IsEnabled:
VIEWPORT_POSITION_W_SCALE_NV 0x937C
Accepted by the parameter of GetBooleani_v, GetDoublei_v,
GetIntegeri_v, GetFloati_v, and GetInteger64i_v:
VIEWPORT_POSITION_W_SCALE_X_COEFF_NV 0x937D
VIEWPORT_POSITION_W_SCALE_Y_COEFF_NV 0x937E
Additions to Chapter 13 of the OpenGL 4.5 (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
stereo view expansion, viewport mask expansion, viewport swizzle,
w coordinate warping, flat-shading, and clipping. ...
Add a new Section 13.X (Viewport W Coordinate Scaling)
If VIEWPORT_POSITION_W_SCALE_NV is enabled, the w coordinates for each
primitive sent to a given viewport will be scaled as a function of
its x and y coordinates using the following equation:
w' = xcoeff * x + ycoeff * y + w;
The coefficients for "x" and "y" used in the above equation depend on the
viewport index, and are controlled by the command
void ViewportPositionWScaleNV(uint index, float xcoeff, float ycoeff);
The viewport specified by has its coefficients for "x" and "y"
set to the and values. Specifying these coefficients
enables rendering images at a non-uniform resolution, in particular a
resolution that falls off linearly from the center towards the edges,
which is useful for VR applications. VR applications often involve a
post-processing step to apply a "barrel" distortion to the rendered image
to correct the "pincushion" distortion introduced by the optics in a VR
device. The barrel distorted image, has lower resolution along the edges
compared to the center. Since the original image is rendered at high
resolution, which is uniform across the complete image, a lot of pixels
towards the edges do not make it to the final post-processed image.
VR applications may use the w-scaling to minimize the processing of unused
fragments. To achieve the intended effect, applications should use a set of
4 viewports one for each of the 4 quadrants of a Cartesian coordinate
system. Each viewport is set to the dimension of the image, but is
scissored to the quadrant it represents. The application should specify
the x and y coefficients of the w-scaling equation above, that have the
same value, but different signs, for each of the viewports. The signs of
and should match the signs of X and Y for the quadrant
that they represent such that the value of "w'" will always be greater
than or equal to the original "w" value for the entire image. Since the
offset to "w", (Ax + By), is always positive and increases with the
absolute values of "x" and "y", the effective resolution will fall off
linearly from the center of the image to its edges.
Errors:
- The error INVALID_VALUE is generated if is greater than or equal
to the value of MAX_VIEWPORTS.
New Implementation Dependent State
None.
New State
Initial
Get Value Get Command Type Value Description Sec. Attribute
------------------------------------ ----------- ---- ------- ----------- ---- ---------
VIEWPORT_POSITION_W_SCALE_NV IsEnabled B FALSE Enable W coordinate Scaling 13.X enable
VIEWPORT_POSITION_W_SCALE_X_COEFF_NV GetFloati_v R 0 x coefficient for the w 13.X viewport
coordinate scaling equation
VIEWPORT_POSITION_W_SCALE_Y_COEFF_NV GetFloati_v R 0 y coefficient for the w 13.X viewport
coordinate scaling equation
Additions to the AGL/GLX/WGL/EGL Specifications
None.
GLX Protocol
None.
Errors
None.
Issues
(1) Does this extension provide any functionality to convert the w-scaled
image to the barrel distorted image used in VR?
RESOLVED: No. VR applications would still require a post-processing step to
generate a barrel distorted image to compensate for the lens distortion.
The following vertex and fragment shader pair un-warps a w-scaled image.
It can be incorporated into an existing post-processing shader to directly
convert a w-scaled image to the barrel distorted image.
// Vertex Shader
// Draw a triangle that covers the whole screen
const vec4 positions[3] = vec4[3](vec4(-1, -1, 0, 1),
vec4( 3, -1, 0, 1),
vec4(-1, 3, 0, 1));
out vec2 uv;
void main()
{
vec4 pos = positions[ gl_VertexID ];
gl_Position = pos;
uv = pos.xy;
}
// Fragment Shader
uniform sampler2D tex;
uniform float xcoeff;
uniform float ycoeff;
out vec4 Color;
in vec2 uv;
void main()
{
// Handle uv as if upper right quadrant
vec2 uvabs = abs(uv);
// unscale: transform w-scaled image into an unscaled image
// scale: transform unscaled image int a w-scaled image
float unscale = 1.0 / (1 + xcoeff * uvabs.x + xcoeff * uvabs.y);
//float scale = 1.0 / (1 - xcoeff * uvabs.x - xcoeff * uvabs.y);
vec2 P = vec2(unscale * uvabs.x, unscale * uvabs.y);
// Go back to the right quadrant
P *= sign(uv);
Color = texture(tex, P * 0.5 + 0.5);
}
(2) In the standard use case a application sets up 4 viewports, one for
each quadrant. Does each primitive have to be broadcast to all the 4
viewports?
RESOLVED: No. Applications may see a better performance if the viewport
mask for each primitive is limited to the viewports corresponding
to the quadrants it falls in.
Revision History
Revision 1
- Internal revisions.
Revision 2
- Add _NV suffixes to _COEFF tokens