Draw[13]> vrenderparams -gi
TKOpenGl.GLSL | Type: Error | ID: 2000 | Severity: High | Message:
  glCompileShader failed to compile a GLSL shader with the following shader info
 log: 'Fragment shader failed to compile with the following errors:
ERROR: 0:1443: error(0000202) No matching overloaded function found: mix
ERROR: 0:1443: error(0000247) Function return does not match type
ERROR: error(#273) 2 compilation errors.  No code generated

'
TKOpenGl | Type: Error | ID: 0 | Severity: High | Message:
  Failed to compile ray-tracing fragment shaders

Draw[15]> vglinfo
OpenGL info:
  GLvendor: ATI Technologies Inc.
  GLdevice: AMD Radeon R9 200 Series
  GLversion: 4.5.13447 Compatibility Profile/Debug Context 16.300.2311.0
  GLSLversion: 4.50
  GLdebug: ON
  Max texture size: 16384
  Max MSAA samples: 8
  Viewport: 409x409
  ResolutionRatio: 1

  return mix (UNIT, ZERO, theWo.z <= 0.f);