HowTo:Fix unexpected invisibility

One thing to check is whether your object is declared AND instanciated in the scene. So in the case of pre declared objects you could often find something like this :

// -- The bare necessities are here : Camera & light --
camera {
  location  <0.0, 0.5, -4.0>
  direction 1.5*z
  right     x*image_width/image_height
  look_at   <0.0, 0.0,  0.0>
}

light_source {
  <0, 0, 0>            // light's position (translated below)
  color rgb <1, 1, 1>  // light's color
  translate <-30, 30, -30>
}

// ----------------------------------------

// The object to be shown is declared here:
#declare MySphere = 
sphere {
  0, 0.5 
  pigment {color rgb <0.9,0.5,0.3>}
}
  
// ----------------------------------------
// But if one ommited the line below or commented it out :
object { MySphere } //Name could typcally followed by some transforms
//Then the object would only be declared but never instanciated