| // Copyright (c) 2020 NVIDIA Corporation |
| // |
| // SPDX-License-Identifier: CC-BY-4.0 |
| |
| include::{generated}/meta/{refprefix}VK_NV_fragment_shading_rate_enums.txt[] |
| |
| === Other Extension Metadata |
| |
| *Last Modified Date*:: |
| 2020-09-02 |
| |
| *Contributors*:: |
| - Pat Brown, NVIDIA |
| - Jeff Bolz, NVIDIA |
| |
| === Description |
| |
| This extension builds on the fragment shading rate functionality provided by |
| the VK_KHR_fragment_shading_rate extension, adding support for |
| "`supersample`" fragment shading rates that trigger multiple fragment shader |
| invocations per pixel as well as a "`no invocations`" shading rate that |
| discards any portions of a primitive that would use that shading rate. |
| |
| include::{generated}/interfaces/VK_NV_fragment_shading_rate_enums.txt[] |
| |
| === Issues |
| |
| . Why was this extension created? How should it be named? |
| + |
| -- |
| *RESOLVED*: The primary goal of this extension was to expose support for |
| supersample and "`no invocations`" shading rates, which are supported by the |
| VK_NV_shading_rate_image extension but not by VK_KHR_fragment_shading_rate. |
| Because VK_KHR_fragment_shading_rate specifies the primitive shading rate |
| using a fragment size in pixels, it lacks a good way to specify supersample |
| rates. |
| To deal with this, we defined enums covering shading rates supported by the |
| KHR extension as well as the new shading rates and added structures and APIs |
| accepting shading rate enums instead of fragment sizes. |
| |
| Since this extension adds two different types of shading rates, both |
| expressed using enums, we chose the extension name |
| VK_NV_fragment_shading_rate_enums. |
| -- |
| |
| . Is this a standalone extension? |
| + |
| -- |
| *RESOLVED*: No, this extension requires VK_KHR_fragment_shading_rate. |
| In order to use the features of this extension, applications must enable the |
| relevant features of KHR extension. |
| -- |
| |
| . How are the shading rate enums used, and how were the enum values assigned? |
| + |
| -- |
| *RESOLVED*: The shading rates supported by the enums in this extension are |
| accepted as pipeline, primitive, and attachment shading rates and behave |
| identically. |
| For the shading rates also supported by the KHR extension, the values |
| assigned to the corresponding enums are identical to the values already used |
| for the primitive and attachment shading rates in the KHR extension. |
| For those enums, bits 0 and 1 specify the base two logarithm of the fragment |
| height and bits 2 and 3 specify the base two logarithm of the fragment |
| width. |
| For the new shading rates added by this extension, we chose to use 11 |
| through 14 (10 plus the base two logarithm of the invocation count) for the |
| supersample rates and 15 for the "`no invocations`" rate. |
| None of those values are supported as primitive or attachment shading rates |
| by the KHR extension. |
| -- |
| |
| . Between this extension, VK_KHR_fragment_shading_rate, and |
| VK_NV_shading_rate_image, there are three different ways to specify shading |
| rate state in a pipeline. |
| How should we handle this? |
| + |
| -- |
| *RESOLVED*: We do not allow the concurrent use of VK_NV_shading_rate_image |
| and VK_KHR_fragment_shading_rate; it is an error to enable shading rate |
| features from both extensions. |
| But we do allow applications to enable this extension together with |
| VK_KHR_fragment_shading_rate together. |
| While we expect that applications will never attach pipeline CreateInfo |
| structures for both this extension and the KHR extension concurrently, |
| Vulkan does not have any precedent forbidding such behavior and instead |
| typically treats a pipeline created without an extension-specific CreateInfo |
| structure as equivalent to one containing default values specified by the |
| extension. |
| Rather than adding such a rule considering the presence or absence of our |
| new CreateInfo structure, we instead included a pname:shadingRateType member |
| to slink:VkPipelineFragmentShadingRateEnumStateCreateInfoNV that selects |
| between using state specified by that structure and state specified by |
| slink:VkPipelineFragmentShadingRateStateCreateInfoKHR. |
| -- |
| |
| === Version History |
| |
| * Revision 1, 2020-09-02 (pbrown) |
| - Internal revisions |
