Egert Kanep

Hello!

I am sharing my solution here. I use python script lop to get the matrix4 for projection camera and use that as a value override for mtlx constant node. Add python script anywhere after material network node and point it to the mtlxconstant node. In python script lop set attribute name to be value

Python script (I do not take credit for this, AI wrote this)

import hou
from pxr import Usd, UsdGeom, UsdShade, Gf, Sdf

node = hou.pwd()
stage = node.editableStage()
time = Usd.TimeCode(hou.frame())

CAM_PATH = node.parm("cam_path").eval()
SHADER_PATH = node.parm("shader_path").eval()
ATTR_NAME = node.parm("attr_name").eval()

# ── Camera ──────────────────────────────────────────────────────────────────
cam_prim = stage.GetPrimAtPath(CAM_PATH)
if not cam_prim.IsValid():
raise RuntimeError(f"Camera prim not found: {CAM_PATH}")

usd_cam = UsdGeom.Camera(cam_prim)
gf_cam = usd_cam.GetCamera(time)
frustum = gf_cam.frustum

# ── ViewProjection matrix — NO transpose, MaterialX uses row-major too ──────
view = frustum.ComputeViewMatrix()
proj = frustum.ComputeProjectionMatrix()
vp = view * proj

# ── Aspect ratio ─────────────────────────────────────────────────────────────
aspect = gf_cam.horizontalAperture / gf_cam.verticalAperture

# ── Write to shader ──────────────────────────────────────────────────────────
shader_prim = stage.GetPrimAtPath(SHADER_PATH)
if not shader_prim.IsValid():
raise RuntimeError(f"Shader prim not found: {SHADER_PATH}")

shader = UsdShade.Shader(shader_prim)

inp = shader.GetInput(ATTR_NAME)
if not inp:
inp = shader.CreateInput(ATTR_NAME, Sdf.ValueTypeNames.Matrix4d)

inp.Set(vp, time)

print(f"CAM_PATH: {CAM_PATH}")
print(f"SHADER_PATH: {SHADER_PATH}")
print(f"ATTR_NAME: {ATTR_NAME}")
print(f"Aspect: {aspect:.4f} (should be ~1.777 for 16:9)")
print(f"Matrix OK: {vp}")

For anyone trying to render oriented ribbons in Karma CPU. It works with B-Spline or Catmull-Rom. I did not see this mentioned in the documentation as a requirement.

Hello!

I have stumbled upon a problem where I am unable to render ribbons properly in Karma on H21.0559.
Karma CPU will not generate any uv when normal attribute is present and switching between flat ribbon and oriented ribbon does not make any difference. Ribbon always faces the camera.

Karma XPU generates rounded curves and also uv's. So the present N attribute works for generating uv's. but for some reason CPU does not.

Is there any sample file preset which would demonstrate the proper setup, perhaps I am missing something