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Common attributes standardisation proposal

Draft 1

Table of contents

  1. Motivation
  2. Standard Attribute Names
    1. Renderer Infos
    2. Camera
    3. Geometry
      1. Geometric quantities
      2. Identifiers
      3. classifications
    4. Rendering

Motivation

OSL has been tremendously successful at standardising shading but there are still non standardised areas, especially around each renderer’s internal nomenclature. Although the same compiled shaders can run in any renderer, it is difficult in 2024 to write a shader that performs correctly in multiple renderers because each renderer names primitive attributes differently and may return some quantities in different spaces.

For example, if you wish to use the surface tangent:

Renderer OSL call
RenderMan getattribute("builtin", "Tn", tangent)
Arnold Vector tangent = normalize(dPdu)
Vray Vector tangent = normalize(dPdu)
Cycles getattribute("geom:tangent", tangent)
RedShift Vector tangent = normalize(dPdu)
3Delight getattribute("myUVs", uvSet);
Vector tangent = normalize(Dx(uvSet));1

In this case, the shader writer hopes that u, v is not the parametric uv on meshes, otherwise dPdu may be non-smooth and pretty useless. But there is no guarantee.

Note

In the spec, u and v are "The 2D parametric coordinates of P (on the particular geometric primitive you are shading)". When dPdu is usable, it means the renderer is computing dPds/dPdt instead of dPdu/dPdv, and straying from the spec.

Standard attribute names

Here is a list of proposed names for common attributes, inspired by various sources. Of course, some of these attributes should be computed on-demand, if possible.

Renderer Infos

Shader writers have no way to tell which renderer is executing our code. I think this would be useful as there are other types of discrepancies that need to be taken care of by shader writers, like the space in which some attributes are returned or the meaning of uv (face or mesh-wide coordinates ?).

This could be fixed by adding 2 standard attributes.

Attribute Type Description
"osl:version" int Major*10000 + Minor*100 + patch. std
"shader:shadername" string Name of the shader master. std
"shader:layername" string Name of the layer instance. std
"shader:groupname" string Name of the shader group. std
"renderer:name" string lower-case name i.e. "renderman" new
"renderer:version" int Major*10000 + Minor*100 + patch. new

Camera

These attributes are already defined in the OSL standard, so this is just for completeness.

Name Type Description
"camera:resolution" int[2] Image resolution. std
"camera:pixelaspect" float Pixel aspect ratio. std
"camera:projection" string Projection type (e.g., "perspective", "orthographic", etc.) std
"camera:fov" float Field of fiew. std
"camera:clip_near" float Near clip distance. std
"camera:clip_far" float Far clip distance. std
"camera:clip" float[2] Near and far clip distances. std
"camera:shutter_open" float Shutter open time. std
"camera:shutter_close" float Shutter close time. std
"camera:shutter" float[2] Shutter open and close times. std
"camera:screen_window" float[4] Screen window (xmin, ymin, xmax, ymax). std

Geometry

Note

We should also standardize the space in which geometric quantities are returned. It should ALWAYS be "common".

Geometric Quantities

Attribute Type Description
"geom:tangent" vector The normalised surface tangent 12. new
"geom:undisplaced_P" point The surface position before displacement. new
"geom:undisplaced_N" normal The surface normal before displacement. new
"geom:reference_P" normal The surface position in reference pose in object space. new
"geom:reference_N" normal The surface normal in reference pose in object space. new
"geom:reference_WP" normal The surface position in reference pose in world space. new
"geom:reference_WN" normal The surface normal in reference pose in world space. new

Identifiers

Attribute Type Description
"geom:id" int A unique object id, or first part of 64 bits id. new
"geom:id2" int The second part of a 64 bits unique object id. new
"geom:instance_id" int A unique object instance id. new
"geom:curve_id" int A unique curve id. new
"geom:point_id" int A unique point id. new

classifications

Attribute Type Description
"geom:is_mesh" int 1 if object is a mesh, 0 otherwise new
"geom:is_subdiv" int 1 if object is a subdivision surface, 0 otherwise new
"geom:is_curve" int 1 if object is a curve, 0 otherwise new
"geom:is_point" int 1 if object is a point, 0 otherwise new
"geom:is_volume" int 1 if object is a point, 0 otherwise new

Rendering

Attribute Type Description
"stage:displace" int 1 if running in displacement context, 0 otherwise. new
"stage:shade" int 1 if running in shading context, 0 otherwise. new
"hit:direct" int 1 if running on a direct ray hit, 0 otherwise. new
"hit:indirect" int 1 if running on an indirect ray hit, 0 otherwise. new
"hit:roughness" int The incoming ray's spread, with [0:1] range. new

Footnotes

  1. u,v are parametric, so dPdu and dPdv won't be smooth on meshes: need a uv set and compute its derivative. ↩ ↩2

  2. I didn't define a bitangent attribute, as it is easily computed with N and T. ↩