VK_NV_corner_sampled_image

Other Extension Metadata

Last Modified Date

2018-08-13

Contributors

• Jeff Bolz, NVIDIA
• Pat Brown, NVIDIA
• Chris Lentini, NVIDIA

Description

This extension adds support for a new image organization, which this extension refers to as corner-sampled images. A corner-sampled image differs from a conventional image in the following ways:

• Texels are centered on integer coordinates. See Unnormalized Texel Coordinate Operations
• Normalized coordinates are scaled using coord × (dim - 1) rather than coord × dim, where dim is the size of one dimension of the image. See normalized texel coordinate transform.
• Partial derivatives are scaled using coord × (dim - 1) rather than coord × dim. See Scale Factor Operation.
• Calculation of the next higher LOD size goes according to ⌈dim / 2⌉ rather than ⌊dim / 2⌋. See Image Mip Level Sizing.
• The minimum level size is 2x2 for 2D images and 2x2x2 for 3D images. See Image Mip Level Sizing.

This image organization is designed to facilitate a system like Ptex with separate textures for each face of a subdivision or polygon mesh. Placing sample locations at pixel corners allows applications to maintain continuity between adjacent patches by duplicating values along shared edges. Additionally, using the modified mipmapping logic along with texture dimensions of the form 2ⁿ+1 allows continuity across shared edges even if the adjacent patches use different LOD values.

New Structures

• Extending VkPhysicalDeviceFeatures2, VkDeviceCreateInfo:
  • VkPhysicalDeviceCornerSampledImageFeaturesNV

New Enum Constants

• VK_NV_CORNER_SAMPLED_IMAGE_EXTENSION_NAME
• VK_NV_CORNER_SAMPLED_IMAGE_SPEC_VERSION
• Extending VkImageCreateFlagBits:
  • VK_IMAGE_CREATE_CORNER_SAMPLED_BIT_NV
• Extending VkStructureType:
  • VK_STRUCTURE_TYPE_PHYSICAL_DEVICE_CORNER_SAMPLED_IMAGE_FEATURES_NV

Issues

1. What should this extension be named?
   DISCUSSION: While naming this extension, we chose the most distinctive aspect of the image organization and referred to such images as corner-sampled images. As a result, we decided to name the extension NV_corner_sampled_image.
2. Do we need a format feature flag so formats can advertise if they support corner-sampling?
   DISCUSSION: Currently NVIDIA supports this for all 2D and 3D formats, but not for cube maps or depth-stencil formats. A format feature might be useful if other vendors would only support this on some formats.
3. Do integer texel coordinates have a different range for corner-sampled images?
   RESOLVED: No, these are unchanged.
4. Do unnormalized sampler coordinates work with corner-sampled images? Are there any functional differences?
   RESOLVED: Yes. Unnormalized coordinates are treated as already scaled for corner-sample usage.
5. Should we have a diagram in the Image Operations chapter demonstrating different texel sampling locations?
   UNRESOLVED: Probably, but later.

Version History

• Revision 1, 2018-08-14 (Daniel Koch)
  • Internal revisions
• Revision 2, 2018-08-14 (Daniel Koch)
  • ???