[Closed] Matrices and coordinate systems

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Mar 18, 2016 8:59 pm

nivao:

I’m working on a script, with both maxscript and C#, to parse an LDraw text file (3D LEGO) to import parts from my own library and assemble a LEGO model. However, LDraw uses a different coordinate system, -Z forward and -Y up. I’ve looked up references to adjust one transformation matrix to another. I have been able to set it up with the following formula:
LDMatrix = (matrix3 [1,0,0] [0,0,-1] [0,1,0] [0,0,0])
         --LDMatrix = (matrix3 [1,0,0] [0,0,1] [0,-1,0] [0,0,0])
         $.transform = LDMatrix * matrixT * inverse LDMatrix
      
I got the LDMatrix values by creating a point helper with axis tripod and rotate it until the Z and Y axes were correct. The top variable has -Z forward and -Y up while the second one has +Y up. For the top matrix, I mirrored the point helper so the Y axis would invert. The “matrixT” variable comes from the C# library which is the unedited transformation matrix extracted from the LDraw file.

Does this work?

tm = $.transform -- Source Matrix
  $.transform = matrix3 tm.row1 tm.row3 -tm.row2 tm.row4

PolyTools3D

Mar 18, 2016 8:59 pm

Looking at the LDraw documentation I found this:

[i][b]x y z[/b] is the x y z coordinate of the part
[b]a b c d e f g h i[/b] is a top left 3x3 matrix of a standard 4x4 homogeneous transformation matrix[/i]

So, the previous code seems to work, but the whole model is build along the wrong axis.
To fix it I think the following line of code should be all you need to correctly import the models:

<node>.transform = rotatex (matrix3 [a,b,c] [g,h,i] -[d,e,f] [x,y,z]) -90

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