VK_NV_clip_space_w_scaling(3)

# VK_NV_clip_space_w_scaling(3) Manual Page

## Name

VK_NV_clip_space_w_scaling - device extension

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## Registered Extension Number

88

## Revision

1

## Extension and Version Dependencies

- Requires Vulkan 1.0

### Other Extension Metadata

Last Modified Date

  2017-02-15

Contributors

  - Eric Werness, NVIDIA

  - Kedarnath Thangudu, NVIDIA

### Description

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.

### New Commands

- vkCmdSetViewportWScalingNV

### New Structures

- VkViewportWScalingNV

- Extending VkPipelineViewportStateCreateInfo:

  - VkPipelineViewportWScalingStateCreateInfoNV

### New Enum Constants

- `VK_NV_CLIP_SPACE_W_SCALING_EXTENSION_NAME`

- `VK_NV_CLIP_SPACE_W_SCALING_SPEC_VERSION`

- Extending VkDynamicState:

  - `VK_DYNAMIC_STATE_VIEWPORT_W_SCALING_NV`

- Extending VkStructureType:

  - `VK_STRUCTURE_TYPE_PIPELINE_VIEWPORT_W_SCALING_STATE_CREATE_INFO_NV`

### 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

    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);
    }

### Version History

- Revision 1, 2017-02-15 (Eric Werness)

  - Internal revisions

## See Also

VkPipelineViewportWScalingStateCreateInfoNV, VkViewportWScalingNV, vkCmdSetViewportWScalingNV

## Document Notes

For more information, see the Vulkan Specification

This page is a generated document. Fixes and changes should be made to the generator scripts, not directly.

## Copyright

Copyright (c) 2014-2020 The Khronos Group Inc.

SPDX-License-Identifier: CC-BY-4.0