VK_NV_geometry_shader_passthrough

Other Extension Metadata

Last Modified Date

2017-02-15

Interactions and External Dependencies

• This extension provides API support for GL_NV_geometry_shader_passthrough
• This extension requires the geometryShader feature.

Contributors

• Piers Daniell, NVIDIA
• Jeff Bolz, NVIDIA

Description

This extension adds support for the following SPIR-V extension in Vulkan:

• SPV_NV_geometry_shader_passthrough

Geometry shaders provide the ability for applications to process each primitive sent through the graphics pipeline using a programmable shader. However, one common use case treats them largely as a passthrough. In this use case, the bulk of the geometry shader code simply copies inputs from each vertex of the input primitive to corresponding outputs in the vertices of the output primitive. Such shaders might also compute values for additional built-in or user-defined per-primitive attributes (e.g., Layer) to be assigned to all the vertices of the output primitive.

This extension provides access to the PassthroughNV decoration under the GeometryShaderPassthroughNV capability. Adding this to a geometry shader input variable specifies that the values of this input are copied to the corresponding vertex of the output primitive.

When using GLSL source-based shading languages, the passthrough layout qualifier from GL_NV_geometry_shader_passthrough maps to the PassthroughNV decoration. To use the passthrough layout, in GLSL the GL_NV_geometry_shader_passthrough extension must be enabled. Behavior is described in the GL_NV_geometry_shader_passthrough extension specification.

New Enum Constants

• VK_NV_GEOMETRY_SHADER_PASSTHROUGH_EXTENSION_NAME
• VK_NV_GEOMETRY_SHADER_PASSTHROUGH_SPEC_VERSION

New Variable Decoration

• PassthroughNV in Geometry Shader Passthrough

New SPIR-V Capabilities

• GeometryShaderPassthroughNV

Issues

1) Should we require or allow a passthrough geometry shader to specify the output layout qualifiers for the output primitive type and maximum vertex count in the SPIR-V?

RESOLVED: Yes they should be required in the SPIR-V. Per GL_NV_geometry_shader_passthrough they are not permitted in the GLSL source shader, but SPIR-V is lower-level. It is straightforward for the GLSL compiler to infer them from the input primitive type and to explicitly emit them in the SPIR-V according to the following table.

2) How does interface matching work with passthrough geometry shaders?

RESOLVED: This is described in Passthrough Interface Matching. In GL when using passthrough geometry shaders in separable mode, all inputs must also be explicitly assigned location layout qualifiers. In Vulkan all SPIR-V shader inputs (except built-ins) must also have location decorations specified. Redeclarations of built-in variables that add the passthrough layout qualifier are exempted from the rule requiring location assignment because built-in variables do not have locations and are matched by BuiltIn decoration.

Sample Code

Consider the following simple geometry shader in unextended GLSL:

layout(triangles) in;
layout(triangle_strip) out;
layout(max_vertices=3) out;

in Inputs {
    vec2 texcoord;
    vec4 baseColor;
} v_in[];
out Outputs {
    vec2 texcoord;
    vec4 baseColor;
};

void main()
{
    int layer = compute_layer();
    for (int i = 0; i < 3; i++) {
        gl_Position = gl_in[i].gl_Position;
        texcoord = v_in[i].texcoord;
        baseColor = v_in[i].baseColor;
        gl_Layer = layer;
        EmitVertex();
    }
}

In this shader, the inputs gl_Position, Inputs.texcoord, and Inputs.baseColor are simply copied from the input vertex to the corresponding output vertex. The only interesting work done by the geometry shader is computing and emitting a gl_Layer value for the primitive.

The following geometry shader, using this extension, is equivalent:

#extension GL_NV_geometry_shader_passthrough : require

layout(triangles) in;
// No output primitive layout qualifiers required.

// Redeclare gl_PerVertex to pass through "gl_Position".
layout(passthrough) in gl_PerVertex {
    vec4 gl_Position;
} gl_in[];

// Declare "Inputs" with "passthrough" to automatically copy members.
layout(passthrough) in Inputs {
    vec2 texcoord;
    vec4 baseColor;
} v_in[];

// No output block declaration required.

void main()
{
    // The shader simply computes and writes gl_Layer.  We do not
    // loop over three vertices or call EmitVertex().
    gl_Layer = compute_layer();
}

Version History

• Revision 1, 2017-02-15 (Daniel Koch)
  • Internal revisions