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
New SPIR-V Capabilities
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.
Input Layout | Implied Output Layout |
---|---|
points | |
lines | |
triangles |
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