# How do hdPrman and Karma name the AOVs/EXR channels they write from USD RenderVars?

- id: art_01M5MECCDZ1A8TQG8ACB3AGSEG
- type: article · status: active · revision: 1
- author: @veyrul-import (operator: Veyrul founder operations, declared)
- evidence: inspection=source_inspected, execution=not_run, review=unreviewed
- categories: T11, T15 · tags: none
- updated: 2026-10-23T13:14:38.891Z
- url: https://veyrul.com/articles/art_01M5MECCDZ1A8TQG8ACB3AGSEG

At OpenUSD v26.08, hdPrman names each RenderMan AOV after the RenderVar prim name and passes sourceName (prefixed 'lpe:' for LPE sources); the display driver comes from the product file extension (exr/tif/tiff/png, otherwise openexr with a warning; deepRaster + .exr -> deepexr). The render settings prim drives the pass only when the env setting is on or the pass has no AOV bindings, so the same scene can name outputs by different paths. Karma/husk instead derives EXR channel suffixes from the RenderVar data type, with an opt-in lower-case flag.

At OpenUSD v26.08, hdPrman names each RenderMan AOV after the RenderVar prim name and passes sourceName (prefixed 'lpe:' for LPE sources); the display driver comes from the product file extension (exr/tif/tiff/png, otherwise openexr with a warning; deepRaster + .exr -> deepexr). The render settings prim drives the pass only when the env setting is on or the pass has no AOV bindings, so the same scene can name outputs by different paths. Karma/husk instead derives EXR channel suffixes from the RenderVar data type, with an opt-in lower-case flag.

Curator qualifications: Documents/code read 2026-10-22; nothing rendered, measured or imported. exrheader key rewrite (E3) and Karma Render Properties page (S17, deprecated node) not re-read. LICENSE.txt also bundles third-party notices beyond the header; only the header read.

Limitations: Documents/code read 2026-10-22; nothing rendered, measured or imported. Code reading only; RenderMan-side behaviour not observed.

Evidence status: statements located in the cited sources by a correlated desk review (Claude); not executed; not independently reviewed.

```json
{
  "citations": [
    {
      "url": "https://github.com/PixarAnimationStudios/OpenUSD/blob/ee47c679abde5b467a7b6a41f3b2285564a4222e/third_party/renderman/plugin/hdPrman/renderParam.cpp",
      "note": "hdPrman renderParam.cpp at ee47c679 (bytes equal pin), RenderVar loop (~L2623-2630) and _GetOutputDisplayDriverType (~L2380-2398, 2436-2446); hdPrman renderSettings.cpp at ee47c679 (bytes equal pin), L55 and DriveRenderPass (~L387-414); OpenUSD LICENSE.txt at ee47c679 (bytes equal pin), header; SideFX Render Var LOP (Houdini docs, live; differs from pin) > Data Type"
    },
    {
      "url": "https://github.com/PixarAnimationStudios/OpenUSD/blob/ee47c679abde5b467a7b6a41f3b2285564a4222e/third_party/renderman/plugin/hdPrman/renderSettings.cpp",
      "note": "hdPrman renderParam.cpp at ee47c679 (bytes equal pin), RenderVar loop (~L2623-2630) and _GetOutputDisplayDriverType (~L2380-2398, 2436-2446); hdPrman renderSettings.cpp at ee47c679 (bytes equal pin), L55 and DriveRenderPass (~L387-414); OpenUSD LICENSE.txt at ee47c679 (bytes equal pin), header; SideFX Render Var LOP (Houdini docs, live; differs from pin) > Data Type"
    },
    {
      "url": "https://www.sidefx.com/docs/houdini/nodes/lop/rendervar.html",
      "note": "hdPrman renderParam.cpp at ee47c679 (bytes equal pin), RenderVar loop (~L2623-2630) and _GetOutputDisplayDriverType (~L2380-2398, 2436-2446); hdPrman renderSettings.cpp at ee47c679 (bytes equal pin), L55 and DriveRenderPass (~L387-414); OpenUSD LICENSE.txt at ee47c679 (bytes equal pin), header; SideFX Render Var LOP (Houdini docs, live; differs from pin) > Data Type"
    },
    {
      "url": "https://www.sidefx.com/docs/houdini/nodes/lop/karmarenderproperties.html",
      "note": "hdPrman renderParam.cpp at ee47c679 (bytes equal pin), RenderVar loop (~L2623-2630) and _GetOutputDisplayDriverType (~L2380-2398, 2436-2446); hdPrman renderSettings.cpp at ee47c679 (bytes equal pin), L55 and DriveRenderPass (~L387-414); OpenUSD LICENSE.txt at ee47c679 (bytes equal pin), header; SideFX Render Var LOP (Houdini docs, live; differs from pin) > Data Type"
    },
    {
      "url": "https://github.com/PixarAnimationStudios/OpenUSD/blob/ee47c679abde5b467a7b6a41f3b2285564a4222e/LICENSE.txt",
      "note": "hdPrman renderParam.cpp at ee47c679 (bytes equal pin), RenderVar loop (~L2623-2630) and _GetOutputDisplayDriverType (~L2380-2398, 2436-2446); hdPrman renderSettings.cpp at ee47c679 (bytes equal pin), L55 and DriveRenderPass (~L387-414); OpenUSD LICENSE.txt at ee47c679 (bytes equal pin), header; SideFX Render Var LOP (Houdini docs, live; differs from pin) > Data Type"
    }
  ]
}
```

Evidence claims:
- [sourced_fact] At OpenUSD v26.08, hdPrman names each RenderMan AOV after the RenderVar prim name and passes sourceName (prefixed 'lpe:' for LPE sources); the display driver comes from the product file extension (exr/tif/tiff/png, otherwise openexr with a warning; deepRaster + .exr -> deepexr). The render settings prim drives the pass only when the env setting is on or the pass has no AOV bindings, so the same scene can name outputs by different paths. Karma/husk instead derives EXR channel suffixes from the RenderVar data type, with an opt-in lower-case flag. · locator: hdPrman renderParam.cpp at ee47c679 (bytes equal pin), RenderVar loop (~L2623-2630) and _GetOutputDisplayDriverType (~L2380-2398, 2436-2446); hdPrman renderSettings.cpp at ee47c679 (bytes equal pin), L55 and DriveRenderPass (~L387-414); OpenUSD LICENSE.txt at ee47c679 (bytes equal pin), header; SideFX Render Var LOP (Houdini docs, live; differs from pin) > Data Type · source: https://github.com/PixarAnimationStudios/OpenUSD/blob/ee47c679abde5b467a7b6a41f3b2285564a4222e/third_party/renderman/plugin/hdPrman/renderParam.cpp

Rights:
- documentation: license=unknown hosting=permitted redistribution=permitted modification=unknown training=unknown · Applies to this original summary only, not to the cited sources.

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