Indicate scale with label close to an edge

Question

I used the following code to plot three 3D boxes:

x = Graphics3D[{Blue, Opacity[0.2
    ], Parallelepiped[{0, 0, 
     0}, {{.3, 0, 0}, {0, .3, 0}, {0, 0, 1}}]}, Axes -> True, 
  AxesEdge -> {{-1, -1}, {-1, -1}, {-1, -1}}]
y = Graphics3D[{Blue, Opacity[0.2
    ], Parallelepiped[{2, 0, 
     0}, {{.3, 0, 0}, {0, .3, 0}, {0, 0, 1}}]}, Axes -> True, 
  AxesEdge -> {{-1, -1}, {-1, -1}, {-1, -1}}]
z = Graphics3D[{Blue, Opacity[0.2
    ], Parallelepiped[{1, 2, 
     0}, {{.3, 0, 0}, {0, .3, 0}, {0, 0, 1}}]}, Axes -> True, 
  AxesEdge -> {{-1, -1}, {-1, -1}, {-1, -1}}]
Show[x, y, z, Boxed -> False, Ticks -> False, 
 PlotStyle -> Opacity[.5]]

I'm trying to give a generic indication of scale for the edges. Is it possible to add labels near each edge? Maybe with a curly bracket, or just a letter close to it? For example, the small side is $\delta$ and the big one is $\lambda$.

Answer 1

One quick and dirty way is to add text halfway onto the edges $x+v_i$ of the Parallelepiped, here using a new Method defined for this purpose LabeledParallelepiped:

LabeledParallelepiped[x_,vi_,lables_,styles_]:=Module[{l,p},
  l=MapThread[Text[#1,x+0.5*#2]&,{lables,vi}];
  p=Flatten[{styles,Parallelepiped[x,vi]}];
  Graphics3D[{p,Black,l}]
]

x=LabeledParallelepiped[{0,0,0},{{.3,0,0},{0,.3,0},{0,0,1}},{"\[Delta]","\[Beta]","\[Lambda]"},{Blue,Opacity[0.2]}];
y=LabeledParallelepiped[{2,0,0},{{.3,0,0},{0,.3,0},{0,0,1}},{"\[Delta]","\[Beta]","\[Lambda]"},{Blue,Opacity[0.2]}];
z=LabeledParallelepiped[{1,2,0},{{.3,0,0},{0,.3,0},{0,0,1}},{"\[Delta]","\[Beta]","\[Lambda]"},{Blue,Opacity[0.2]}];

Show[x,y,z,Boxed->False,Ticks->False,Axes->True,AxesEdge->{{-1,-1},{-1,-1},{-1,-1}}]

which generates

This is arguably not the nicest looking plot/diagram for the intended visualization but improvements would probably require additional parameters (like specifying the edge onto which a label should be placed, an offset (see e.g. the Offset option for Text) or even distance markers (like curly brackets or lines). But the present solution might be a good starting point.