VK_EXT_multisampled_render_to_single_sampled
Other Extension Metadata
Last Modified Date
2021-04-16
IP Status
No known IP claims.
Contributors
- Shahbaz Youssefi, Google
- Jan-Harald Fredriksen, Arm
- Jörg Wagner, Arm
- Matthew Netsch, Qualcomm Technologies, Inc.
- Jarred Davies, Imagination Technologies
Description
With careful usage of resolve attachments, multisampled image memory
allocated with VK_MEMORY_PROPERTY_LAZILY_ALLOCATED_BIT, loadOp
not equal to VK_ATTACHMENT_LOAD_OP_LOAD and storeOp not equal to
VK_ATTACHMENT_STORE_OP_STORE, a Vulkan application is able to
efficiently perform multisampled rendering without incurring any additional
memory penalty on some implementations.
Under certain circumstances however, the application may not be able to
complete its multisampled rendering within a single render pass; for example
if it does partial rasterization from frame to frame, blending on an image
from a previous frame, or in emulation of
GL_EXT_multisampled_render_to_texture.
In such cases, the application can use an initial subpass to effectively
load single-sampled data from the next subpass’s resolve attachment and fill
in the multisampled attachment which otherwise uses loadOp equal to
VK_ATTACHMENT_LOAD_OP_DONT_CARE.
However, this is not always possible (for example for stencil in the absence
of VK_EXT_shader_stencil_export) and has multiple drawbacks.
Some implementations are able to perform said operation efficiently in hardware, effectively loading a multisampled attachment from the contents of a single sampled one. Together with the ability to perform a resolve operation at the end of a subpass, these implementations are able to perform multisampled rendering on single-sampled attachments with no extra memory or bandwidth overhead. This extension exposes this capability by allowing a framebuffer and render pass to include single-sampled attachments while rendering is done with a specified number of samples.
New Structures
- Extending VkFormatProperties2:
- Extending VkPhysicalDeviceFeatures2, VkDeviceCreateInfo:
- Extending VkSubpassDescription2, VkRenderingInfo:
New Enum Constants
VK_EXT_MULTISAMPLED_RENDER_TO_SINGLE_SAMPLED_EXTENSION_NAMEVK_EXT_MULTISAMPLED_RENDER_TO_SINGLE_SAMPLED_SPEC_VERSION- Extending VkImageCreateFlagBits:
VK_IMAGE_CREATE_MULTISAMPLED_RENDER_TO_SINGLE_SAMPLED_BIT_EXT
- Extending VkStructureType:
VK_STRUCTURE_TYPE_MULTISAMPLED_RENDER_TO_SINGLE_SAMPLED_INFO_EXTVK_STRUCTURE_TYPE_PHYSICAL_DEVICE_MULTISAMPLED_RENDER_TO_SINGLE_SAMPLED_FEATURES_EXTVK_STRUCTURE_TYPE_SUBPASS_RESOLVE_PERFORMANCE_QUERY_EXT
Issues
1) Could the multisampled attachment be initialized through some form of copy?
RESOLVED: No. Some implementations do not support copying between attachments in general, and find expressing this operation through a copy unnatural.
2) Another way to achieve this is by introducing a new loadOp to load
the contents of the multisampled image from a single-sampled one.
Why is this extension preferred?
RESOLVED: Using this extension simplifies the application, as it does not
need to manage a secondary lazily-allocated image.
Additionally, using this extension leaves less room for error; for example a
single mistake in loadOp or storeOp would result in the
lazily-allocated image to actually take up memory, and remain so until
destruction.
3) There is no guarantee that multisampled data between two subpasses with the same number of samples will be retained as the implementation may be forced to split the render pass implicitly for various reasons. Should this extension require that every subpass that uses multisampled-render-to-single-sampled end in an implicit render pass split (which results in a resolve operation)?
RESOLVED: No. Not requiring this allows render passes with multiple multisampled-render-to-single-sampled subpasses to potentially execute more efficiently (though there is no guarantee).
Version History
- Revision 1, 2021-04-12 (Shahbaz Youssefi)