1// Copyright (c) 2017-2020 NVIDIA Corporation 2// 3// SPDX-License-Identifier: CC-BY-4.0 4 5include::{generated}/meta/{refprefix}VK_NV_geometry_shader_passthrough.txt[] 6 7=== Other Extension Metadata 8 9*Last Modified Date*:: 10 2017-02-15 11*Interactions and External Dependencies*:: 12 - This extension requires 13 {spirv}/NV/SPV_NV_geometry_shader_passthrough.html[`SPV_NV_geometry_shader_passthrough`] 14 - This extension provides API support for 15 https://www.khronos.org/registry/OpenGL/extensions/NV/NV_geometry_shader_passthrough.txt[`GL_NV_geometry_shader_passthrough`] 16 - This extension requires the pname:geometryShader feature. 17*Contributors*:: 18 - Piers Daniell, NVIDIA 19 - Jeff Bolz, NVIDIA 20 21=== Description 22 23This extension adds support for the following SPIR-V extension in Vulkan: 24 25 * `SPV_NV_geometry_shader_passthrough` 26 27Geometry shaders provide the ability for applications to process each 28primitive sent through the graphics pipeline using a programmable shader. 29However, one common use case treats them largely as a "`passthrough`". 30In this use case, the bulk of the geometry shader code simply copies inputs 31from each vertex of the input primitive to corresponding outputs in the 32vertices of the output primitive. 33Such shaders might also compute values for additional built-in or 34user-defined per-primitive attributes (e.g., code:Layer) to be assigned to 35all the vertices of the output primitive. 36 37This extension provides access to the code:PassthroughNV decoration under 38the code:GeometryShaderPassthroughNV capability. 39Adding this to a geometry shader input variable specifies that the values of 40this input are copied to the corresponding vertex of the output primitive. 41 42When using GLSL source-based shading languages, the code:passthrough layout 43qualifier from `GL_NV_geometry_shader_passthrough` maps to the 44code:PassthroughNV decoration. 45To use the code:passthrough layout, in GLSL the 46`GL_NV_geometry_shader_passthrough` extension must be enabled. 47Behaviour is described in the `GL_NV_geometry_shader_passthrough` extension 48specification. 49 50include::{generated}/interfaces/VK_NV_geometry_shader_passthrough.txt[] 51 52=== New Variable Decoration 53 54 * <<geometry-passthrough-passthrough,code:PassthroughNV>> in 55 <<geometry-passthrough,Geometry Shader Passthrough>> 56 57=== New SPIR-V Capabilities 58 59 * <<spirvenv-capabilities-table-GeometryShaderPassthroughNV,GeometryShaderPassthroughNV>> 60 61=== Issues 62 631) Should we require or allow a passthrough geometry shader to specify the 64output layout qualifiers for the output primitive type and maximum vertex 65count in the SPIR-V? 66 67*RESOLVED*: Yes they should be required in the SPIR-V. 68Per GL_NV_geometry_shader_passthrough they are not permitted in the GLSL 69source shader, but SPIR-V is lower-level. 70It is straightforward for the GLSL compiler to infer them from the input 71primitive type and to explicitly emit them in the SPIR-V according to the 72following table. 73 74[options="header"] 75|==== 76| Input Layout | Implied Output Layout 77| points | `layout(points, max_vertices=1)` 78| lines | `layout(line_strip, max_vertices=2)` 79| triangles | `layout(triangle_strip, max_vertices=3)` 80|==== 81 822) How does interface matching work with passthrough geometry shaders? 83 84*RESOLVED*: This is described in <<geometry-passthrough-interface, 85Passthrough Interface Matching>>. 86In GL when using passthough geometry shaders in separable mode, all inputs 87must also be explicitly assigned location layout qualifiers. 88In Vulkan all SPIR-V shader inputs (except built-ins) must also have 89location decorations specified. 90Redeclarations of built-in varables that add the passthrough layout 91qualifier are exempted from the rule requiring location assignment because 92built-in variables do not have locations and are matched by code:BuiltIn 93decoration. 94 95 96=== Sample Code 97 98Consider the following simple geometry shader in unextended GLSL: 99 100[source,c] 101--------------------------------------------------- 102layout(triangles) in; 103layout(triangle_strip) out; 104layout(max_vertices=3) out; 105 106in Inputs { 107 vec2 texcoord; 108 vec4 baseColor; 109} v_in[]; 110out Outputs { 111 vec2 texcoord; 112 vec4 baseColor; 113}; 114 115void main() 116{ 117 int layer = compute_layer(); 118 for (int i = 0; i < 3; i++) { 119 gl_Position = gl_in[i].gl_Position; 120 texcoord = v_in[i].texcoord; 121 baseColor = v_in[i].baseColor; 122 gl_Layer = layer; 123 EmitVertex(); 124 } 125} 126--------------------------------------------------- 127 128In this shader, the inputs code:gl_Position, code:Inputs.texcoord, and 129code:Inputs.baseColor are simply copied from the input vertex to the 130corresponding output vertex. 131The only "`interesting`" work done by the geometry shader is computing and 132emitting a code:gl_Layer value for the primitive. 133 134The following geometry shader, using this extension, is equivalent: 135 136[source,c] 137--------------------------------------------------- 138#extension GL_NV_geometry_shader_passthrough : require 139 140layout(triangles) in; 141// No output primitive layout qualifiers required. 142 143// Redeclare gl_PerVertex to pass through "gl_Position". 144layout(passthrough) in gl_PerVertex { 145 vec4 gl_Position; 146} gl_in[]; 147 148// Declare "Inputs" with "passthrough" to automatically copy members. 149layout(passthrough) in Inputs { 150 vec2 texcoord; 151 vec4 baseColor; 152} v_in[]; 153 154// No output block declaration required. 155 156void main() 157{ 158 // The shader simply computes and writes gl_Layer. We do not 159 // loop over three vertices or call EmitVertex(). 160 gl_Layer = compute_layer(); 161} 162--------------------------------------------------- 163 164 165=== Version History 166 167 * Revision 1, 2017-02-15 (Daniel Koch) 168 - Internal revisions 169