Compute surface normals for binary STL files if the file normals are zero

[goddard@…]

The following bug report has been submitted:
Platform:        macOS-13.1-x86_64-i386-64bit
ChimeraX Version: 1.6.dev202301091802 (2023-01-09 18:02:01 UTC)
Description
Would be nice to compute surface normals for binary STL files where normals in the file are zero.

Log:
Could not find tool "Tabbed Toolbar"  
UCSF ChimeraX version: 1.6.dev202301091802 (2023-01-09)  
© 2016-2023 Regents of the University of California. All rights reserved.  
How to cite UCSF ChimeraX  

> open /Users/goddard/Downloads/motor.stl format stl

Opened STL file motor.stl containing 25144 triangles  




OpenGL version: 4.1 ATI-4.9.51
OpenGL renderer: AMD Radeon Pro 580 OpenGL Engine
OpenGL vendor: ATI Technologies Inc.

Python: 3.9.11
Locale: en_US.UTF-8
Qt version: PyQt6 6.4.0, Qt 6.4.0
Qt runtime version: 6.4.1
Qt platform: cocoa
Hardware:

    Hardware Overview:

      Model Name: iMac
      Model Identifier: iMac18,3
      Processor Name: Quad-Core Intel Core i7
      Processor Speed: 4.2 GHz
      Number of Processors: 1
      Total Number of Cores: 4
      L2 Cache (per Core): 256 KB
      L3 Cache: 8 MB
      Hyper-Threading Technology: Enabled
      Memory: 32 GB
      System Firmware Version: 499.40.2.0.0
      OS Loader Version: 564.40.4~55
      SMC Version (system): 2.41f2

Software:

    System Software Overview:

      System Version: macOS 13.1 (22C65)
      Kernel Version: Darwin 22.2.0
      Time since boot: 21 hours, 43 minutes

Graphics/Displays:

    Radeon Pro 580:

      Chipset Model: Radeon Pro 580
      Type: GPU
      Bus: PCIe
      PCIe Lane Width: x16
      VRAM (Total): 8 GB
      Vendor: AMD (0x1002)
      Device ID: 0x67df
      Revision ID: 0x00c0
      ROM Revision: 113-D000AA-931
      VBIOS Version: 113-D0001A1X-025
      EFI Driver Version: 01.00.931
      Metal Support: Metal 2
      Displays:
        iMac:
          Display Type: Built-In Retina LCD
          Resolution: Retina 5K (5120 x 2880)
          Framebuffer Depth: 30-Bit Color (ARGB2101010)
          Main Display: Yes
          Mirror: Off
          Online: Yes
          Automatically Adjust Brightness: Yes
          Connection Type: Internal


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[Tom Goddard]

Done.

To avoid slowing down the reading of STL files that have normals, the code only checks the first vertex normal (average of STL triangle normals sharing that vertex) to decide if they should be computed.