Drawing coordinate systems and camera

By Adam Nagy

There are 3 coordinate systems being used inside a drawing: Sheet, DrawingView and Model.

The Sheet and DrawingView coordinate systems are almost identical apart from their origin: they have the same axes and same scaling. The Model coordinate system is the coordinate system of the document that is being shown inside the drawing view.

The API makes it really easy to jump between the coordinate systems. There are functions like ModelToSheetSpace(), to transform a point from one coordinate system to the other. If you want to transform a direction then it's easier to use the transformation matrix, which is also provided, e.g. ModelToSheetTransform matrix, and use that to transform a vector using Vector.TransformBy(Matrix).

We can write a simple function which transforms the origin of the various coordinate systems and the model workpoint into sheet coordinate system, and draws a circle at each position. This might make it easier to see the relationship between them.

Sub DrawCircle(pt As Point2d, s As Sheet)
  Dim sk As DrawingSketch
  
  ' Sheet sketches are in Sheet coordinate system
  ' and DrawingView sketches are in DrawingView
  ' coordinate system
  If s.Sketches.count > 0 Then
    Set sk = s.Sketches(1)
  Else
    Set sk = s.Sketches.Add()
  End If
  
  If Not s.Parent.ActivatedObject Is sk Then
    sk.Edit
  End If
  
  Call sk.SketchCircles.AddByCenterRadius(pt, 1)
End Sub

Sub DrawCircles()
  Dim doc As DrawingDocument
  Set doc = ThisApplication.ActiveDocument
  
  Dim s As Sheet
  Set s = doc.ActiveSheet
  
  Dim tg As TransientGeometry
  Set tg = ThisApplication.TransientGeometry
  
  Dim v As DrawingView
  Set v = s.DrawingViews(1)
  
  ' (1) Show the origin of the sheet
  Call DrawCircle(tg.CreatePoint2d(), s)
  
  ' (2) Show origin of the DrawingView
  Call DrawCircle(v.DrawingViewToSheetSpace(tg.CreatePoint2d()), s)
  ' Same as
  'Call DrawCircle(v.Position, s)
  
  ' (3) Show origin of Model coordinate system
  Call DrawCircle(v.ModelToSheetSpace(tg.CreatePoint()), s)
  
  ' (4) Show WorkPoint of model
  ' transformed to Sheet coordinate system
  Dim prt As PartDocument
  Set prt = v.ReferencedDocumentDescriptor.ReferencedDocument
  
  Dim wp As WorkPoint
  Set wp = prt.ComponentDefinition.WorkPoints("MyPoint")
  
  Call DrawCircle(v.ModelToSheetSpace(wp.Point), s)
End Sub

There is also the Camera of the drawing view which is described with Model coordinates. In our case the Target is the middle of the box and Eye (the position of our eye looking at the model, described with Model coordinates) is in front of it along the (1, 1, 1) vector. Though the UpVector looks as if it's the same as the Model's Y axis, it points behind that axis and equals to (-1, 2, -1). The view direction, which points from the Eye to the Target, and the UpVector are always perpendicular. 

Here is a nice post on Cameras: http://modthemachine.typepad.com/my_weblog/2013/09/working-with-cameras-part-1.html