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 w coordinate of the vertices in the clip space before perspective divide. The clip space w coordinate of the vertices can 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 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 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.

Examples

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:

// 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);
}

VK_NV_clip_space_w_scaling

Name String

VK_NV_clip_space_w_scaling

Extension Type

Device extension

Registered Extension Number

88

Revision

1

Contact

• Eric Werness ewerness-nv

Other Extension Metadata

Last Modified Date

2017-02-15

Contributors

• Eric Werness, NVIDIA
• Kedarnath Thangudu, NVIDIA

void vkCmdSetViewportWScalingNV(
    VkCommandBuffer                             commandBuffer,
    uint32_t                                    firstViewport,
    uint32_t                                    viewportCount,
    const VkViewportWScalingNV*                 pViewportWScalings);