Here is some ChimeraX Python code that hides lines of a mesh that are not parallel the x or y or z axes. Ilhan Tomris asked about how to show a grid on light microscopy data. The shape command can show a rectangle, and the surface style command can display it as a mesh.

```
shape rectangle width 20 height 20
surface style #3 mesh
```

But the rectangle is made up of triangles so the mesh shows triangles. The Python code hide_diagonals.py below defines a command hidediagonals that hides the lines that are not parallel x or y or z axes.

I also added this to ChimeraX as the surface squaremesh command.

# Define command hidediagonals that hides edges of a mesh surface that are not parallel # the x, or y, or z axes. This was to make a square grid to overlay on microscopy data. def hide_diagonals(session, surface): vertices = surface.vertices from numpy import array, uint8 edge_mask = array([diagonal_mask(t, vertices) for t in surface.triangles], uint8) surface.edge_mask = edge_mask def diagonal_mask(triangle, vertices): ''' The edge mask uses the lowest 3 bits to indicate which of the 3 edges of a triangles should be shown. ''' i0,i1,i2 = triangle v0,v1,v2 = vertices[i0],vertices[i1],vertices[i2] mask = 0 if parallel_x_or_y_or_z(v0,v1): mask |= 0x1 if parallel_x_or_y_or_z(v1,v2): mask |= 0x2 if parallel_x_or_y_or_z(v2,v0): mask |= 0x4 return mask def parallel_x_or_y_or_z(xyz1,xyz2): return (xyz1 == xyz2).sum() >= 2 def register_hide_diagonals_command(logger): from chimerax.core.commands import CmdDesc, register, SurfaceArg desc = CmdDesc( required = [('surface', SurfaceArg)], synopsis = 'Hide diagonal of surface mesh' ) register('hidediagonals', desc, hide_diagonals, logger=logger) register_hide_diagonals_command(session.logger)

Tom Goddard, October 18, 2020