Name Strings

SPV_NV_shader_image_footprint

Contact

To report problems with this extension, please open a new issue at:

https://github.com/KhronosGroup/SPIRV-Registry

Contributors

  • Daniel Koch, NVIDIA

  • Chao Chen, NVIDIA

  • Jeff Bolz, NVIDIA

Status

  • Complete

Version

Last Modified Date

2018-08-14

Revision

1

Dependencies

This extension is written against the SPIR-V Specification, Version 1.3 Revision 2, Unified.

This extension requires SPIR-V 1.0.

Overview

This extension adds support for a new image function for querying information about the texel footprint of a corresponding image sampling operation. This extension provides SPIR-V support for the GLSL GL_NV_shader_texture_footprint extension.

Extension Name

To use this extension within a SPIR-V module, the following OpExtension must be present in the module:

OpExtension "SPV_NV_shader_image_footprint"

Modifications to the SPIR-V Specification, Version 1.3

(Modify Section 3.14, Image Operands)

Add OpImageSampleFootprintNV to the list of opcodes that use Image Operands.

(Modify Section 3.31, Capability, adding a new row to the Capability table)
Capability Depends On Enabled by Extension

5282

ImageFootprintNV

Shader

SPV_NV_shader_image_footprint
(Modify Section 3.32.10, Image Instructions, adding a new row to the table)

OpImageSampleFootprintNV

Return the texel footprint of a corresponding sampling instruction.

Result Type must be an OpTypeStruct with six members.
Member 0 must be a Boolean type scalar. Member 0 holds a boolean value indicating whether the corresponding image sampling operation only touches a single LOD level.
Member 1 must be a vector of integer type, whose Signedness operand is 0, and have the same number of components as Coordinate. Member 1 holds the footprint anchor.
Member 2 must be a vector of integer type, whose Signedness operand is 0, and have the same number of components as Coordinate. Member 2 holds the footprint offset.
Member 3 must be a vector of integer type, whose Signedness operand is 0, and have two components. Member 3 holds the footprint mask.
Member 4 must be a scalar of integer type, whose Signedness operand is 0. Member 4 holds the footprint LOD.
Member 5 must be a scalar of integer type, whose Signedness operand is 0. Member 5 holds the footprint granularity after coarsening. A value of zero means that no coarsening occurred.

This structure type must be explicitly declared by the module.

Sampled Image must be an object whose type is OpTypeSampledImage. The Dim operand of the underlying OpTypeImage must be 2D or 3D, and the Arrayed and MS operands must be 0.

Coordinate must be a vector of floating-point type. It contains (u, v[, w]) as needed by the definition of Sampled Image.

Granularity must be a scalar of integer type with the granularity of the returned image footprint mask, indicating how many texels each bit in the bitmask corresponds to.

Coarse must be a constant instruction of scalar Boolean type indicating the low (fine) or high (coarse) LOD level for trilinear filtering, where two footprint operations are required to retrieve both bitmasks.

Image Operands encodes what operands follow, as per Image Operands.
Supported combinations are:
For Dim = 2D: None, Bias, MinLod, (MinLod + Bias), Lod, Grad, (Grad + MinLod).
For Dim = 3D: None, Bias, MinLod, (MinLod + Bias), Lod.

Capability:
ImageFootprintNV

7 + variable

5283

<id>
Result Type

Result <id>

<id>
Sampled Image

<id>
Coordinate

<id>
Granularity

<id>
Coarse

Optional Image Operands

Optional
<id>, <id>, …​

(Add a new Section 3.Granularity, Granularity)

Granularity

Granularity of an image footprint mask. Used by OpImageSampleFootprintNV.

Granularity Enabling Capabilities

Dim=2D

Dim=3D

0

Input: not valid
Output: no coarsening

ImageFootprintNV

1

2x2

2x2x2

ImageFootprintNV

2

4x2

(reserved)

ImageFootprintNV

3

4x4

4x4x2

ImageFootprintNV

4

8x4

(reserved)

ImageFootprintNV

5

8x8

(reserved)

ImageFootprintNV

6

16x8

(reserved)

ImageFootprintNV

7

16x16

(reserved)

ImageFootprintNV

8

(reserved)

(reserved)

9

(reserved)

(reserved)

10

(reserved)

16x16x16

ImageFootprintNV

11

64x64

32x16x16

ImageFootprintNV

12

128x64

32x32x16

ImageFootprintNV

13

128x128

32x32x32

ImageFootprintNV

14

256x128

64x32x32

ImageFootprintNV

15

256x256

(reserved)

ImageFootprintNV

Validation Rules

An OpExtension must be added to the SPIR-V for validation layers to check legal use of this extension:

OpExtension "SPV_NV_shader_image_footprint"

Issues

  1. How do we handle out parameters from functions?

    RESOLVED: Op returns a structure - see ModfStruct and FrexpStruct in the GLSL.std.450 extended instruction sets.

  2. How many variants of the "footprint" instructions do we need?

    RESOLVED: Using the existing Image Operands, we can get away with just one.

  3. Should we allow expandable arguments for future targets (like cube maps)?

    RESOLVED: Not at this time. It would likely be difficult to express footprint for cube maps, particularly access along the seams.

Revision History

Rev Date Author Changes

1

2018-08-14

Daniel Koch

Internal revisions