include::meta/VK_NV_clip_space_w_scaling.txt[]

*Last Modified Date*::
    2017-02-15
*Contributors*::
  - Eric Werness, NVIDIA
  - Kedarnath Thangudu, NVIDIA

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 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 [eq]#w# coordinate of the vertices in the
clip space before perspective divide.
The clip space [eq]#w# coordinate of the vertices can: be offset as of a
function of [eq]#x# and [eq]#y# coordinates as follows:

[eq]#w' = w + Ax + By#

In the intended use case for viewport position scaling, an application
should use a set of four viewports, one for each of the four 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 [eq]#A# and [eq]#B# coefficients of the
[eq]#w#-scaling equation above, that have the same value, but different
signs, for each of the viewports.
The signs of [eq]#A# and [eq]#B# should match the signs of [eq]#x# and
[eq]#y# for the quadrant that they represent such that the value of [eq]#w'#
will always be greater than or equal to the original [eq]#w# value for the
entire image.
Since the offset to [eq]#w#, ([eq]#Ax + By#), is always positive, and
increases with the absolute values of [eq]#x# and [eq]#y#, the effective
resolution will fall off linearly from the center of the image to its edges.

=== New Object Types

None.

=== New Enum Constants

  * Extending elink:VkStructureType:
  ** ename:VK_STRUCTURE_TYPE_PIPELINE_VIEWPORT_W_SCALING_STATE_CREATE_INFO_NV
  * Extending elink:VkDynamicState:
  ** ename:VK_DYNAMIC_STATE_VIEWPORT_W_SCALING_NV

=== New Enums

None.

=== New Structures

  * slink:VkViewportWScalingNV
  * slink:VkPipelineViewportWScalingStateCreateInfoNV

=== New Functions

  * flink:vkCmdSetViewportWScalingNV

=== Issues

1) Is the pipeline struct name too long?

*RESOLVED*: It fits with the naming convention.

2) Separate W scaling section or fold into coordinate transformations?

*RESOLVED*: Leaving it as its own section for now.

=== Examples

[source,c++]
--------------------------------------

VkViewport viewports[4];
VkRect2D scissors[4];
VkViewportWScalingNV scalings[4];

for (int i = 0; i < 4; i++) {
    int x = (i & 2) ? 0 : currentWindowWidth / 2;
    int y = (i & 1) ? 0 : currentWindowHeight / 2;

    viewports[i].x = 0;
    viewports[i].y = 0;
    viewports[i].width = currentWindowWidth;
    viewports[i].height = currentWindowHeight;
    viewports[i].minDepth = 0.0f;
    viewports[i].maxDepth = 1.0f;

    scissors[i].offset.x = x;
    scissors[i].offset.y = y;
    scissors[i].extent.width = currentWindowWidth/2;
    scissors[i].extent.height = currentWindowHeight/2;

    const float factor = 0.15;
    scalings[i].xcoeff = ((i & 2) ? -1.0 : 1.0) * factor;
    scalings[i].ycoeff = ((i & 1) ? -1.0 : 1.0) * factor;
}

VkPipelineViewportWScalingStateCreateInfoNV vpWScalingStateInfo = { VK_STRUCTURE_TYPE_PIPELINE_VIEWPORT_W_SCALING_STATE_CREATE_INFO_NV };

vpWScalingStateInfo.viewportWScalingEnable = VK_TRUE;
vpWScalingStateInfo.viewportCount = 4;
vpWScalingStateInfo.pViewportWScalings = &scalings[0];

VkPipelineViewportStateCreateInfo vpStateInfo = { VK_STRUCTURE_TYPE_PIPELINE_VIEWPORT_STATE_CREATE_INFO };
vpStateInfo.viewportCount = 4;
vpStateInfo.pViewports = &viewports[0];
vpStateInfo.scissorCount = 4;
vpStateInfo.pScissors = &scissors[0];
vpStateInfo.pNext = &vpWScalingStateInfo;

--------------------------------------

Example shader to read from a w-scaled texture:

[source,c++]
--------------------------------------

// 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);
}
--------------------------------------

=== Version History

 * Revision 1, 2017-02-15 (Eric Werness)
   - Internal revisions