VK_EXT_depth_clip_control

Other Extension Metadata

Last Modified Date

2021-11-09

Contributors

• Spencer Fricke, Samsung Electronics
• Shahbaz Youssefi, Google
• Ralph Potter, Samsung Electronics

Description

This extension allows the application to use the OpenGL depth range in NDC, i.e. with depth in range [-1, 1], as opposed to Vulkan’s default of [0, 1]. The purpose of this extension is to allow efficient layering of OpenGL over Vulkan, by avoiding emulation in the pre-rasterization shader stages. This emulation, which effectively duplicates gl_Position but with a different depth value, costs ALU and consumes shader output components that the implementation may not have to spare to meet OpenGL minimum requirements.

New Structures

• Extending VkPhysicalDeviceFeatures2, VkDeviceCreateInfo:
  • VkPhysicalDeviceDepthClipControlFeaturesEXT
• Extending VkPipelineViewportStateCreateInfo:
  • VkPipelineViewportDepthClipControlCreateInfoEXT

New Enum Constants

• VK_EXT_DEPTH_CLIP_CONTROL_EXTENSION_NAME
• VK_EXT_DEPTH_CLIP_CONTROL_SPEC_VERSION
• Extending VkStructureType:
  • VK_STRUCTURE_TYPE_PHYSICAL_DEVICE_DEPTH_CLIP_CONTROL_FEATURES_EXT
  • VK_STRUCTURE_TYPE_PIPELINE_VIEWPORT_DEPTH_CLIP_CONTROL_CREATE_INFO_EXT

Issues

1) Should this extension include an origin control option to match GL_LOWER_LEFT found in ARB_clip_control?

RESOLVED: No. The fix for porting over the origin is a simple y-axis flip. The depth clip control is a much harder problem to solve than what this extension is aimed to solve. Adding an equivalent to GL_LOWER_LEFT would require more testing.

2) Should this pipeline state be dynamic?

RESOLVED: Yes. The purpose of this extension is to emulate the OpenGL depth range, which is expected to be globally fixed (in case of OpenGL ES) or very infrequently changed (with glClipControl in OpenGL).

3) Should the control provided in this extension be an enum that could be extended in the future?

RESOLVED: No. It is highly unlikely that the depth range is changed to anything other than [0, 1] in the future. Should that happen a new extension will be required to extend such an enum, and that extension might as well add a new struct to chain to VkPipelineViewportStateCreateInfo::pNext instead.

Version History

• Revision 0, 2020-10-01 (Spencer Fricke)
  • Internal revisions
• Revision 1, 2020-11-26 (Shahbaz Youssefi)
  • Language fixes