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add_mesh_trimesh ignores transparency from trimesh RGBA colors #525
Description
Describe the bug
When a trimesh.Trimesh object is created with RGBA face colors (where the alpha channel defines transparency), and then visualized using server.scene.add_mesh_trimesh, the transparency information is ignored. The resulting mesh in the viser viewer is always rendered as fully opaque.
To Reproduce
This simple script creates three trimesh cylinders. Each is assigned a color with a different alpha value for transparency. When visualized, none of them appear transparent.
#!/usr/bin/env python3
"""
Simple script to test transparency with trimesh objects in viser.
"""
import viser
import trimesh
import numpy as np
import time
def main():
"""Sets up the viser server and adds trimesh objects with alpha values."""
print("\n=== Testing trimesh RGBA color settings ===")
server = viser.ViserServer()
# Create three cylinders
mesh1 = trimesh.creation.cylinder(radius=0.5, height=2.0, sections=16)
mesh2 = trimesh.creation.cylinder(radius=0.5, height=2.0, sections=16)
mesh3 = trimesh.creation.cylinder(radius=0.5, height=2.0, sections=16)
# Set RGBA colors, where the 4th value is alpha (0-255)
# 76/255 is ~30% opaque
mesh1.visual.face_colors = [255, 0, 0, 76]
# 153/255 is 60% opaque
mesh2.visual.face_colors = [0, 255, 0, 153]
# 255/255 is 100% opaque
mesh3.visual.face_colors = [0, 0, 255, 255]
# Add meshes to the scene
server.scene.add_mesh_trimesh(
"red_transparent",
mesh1,
position=(-1.5, 0.0, 0.0)
)
server.scene.add_mesh_trimesh(
"green_transparent",
mesh2,
position=(0.0, 0.0, 0.0)
)
server.scene.add_mesh_trimesh(
"blue_opaque",
mesh3,
position=(1.5, 0.0, 0.0)
)
print("\nViser server started!")
print("View at http://localhost:8080")
print("Press Ctrl+C to exit")
try:
while True:
time.sleep(1)
except KeyboardInterrupt:
print("\nExiting.")
if __name__ == "__main__":
main()
Activity
brentyi commentedon Aug 7, 2025
Hi there!
I spent some time looking into this. Here's a quick workaround:
Some notes on the root cause:
mesh.visualis set to an instance ofColorVisuals, trimesh doesn't populate the"materials"field of the glTF output.TextureVisualsvia.to_texture()fixes the problem.I filed a trimesh issue: mikedh/trimesh#2436, we can figure out next steps based on the response there.
Noietch commentedon Aug 11, 2025
Thanks for your reply, it works for me !
brentyi commentedon Oct 11, 2025
I was revisiting this today. Couldn't reproduce the fix above 🤔
New one:
sea-bass commentedon Oct 14, 2026
I'm not 100% sure this is exactly the same issue, but pointing out I just put up #784 in case this fixes the problem on your end and it's a way simpler fix than #783 ... but again, it could be orthogonal.