DXF reading and writing: align the calculation of the position of the dimension text in the middle and above

AutoCAD The law of text position

Observe the following figure , Come to the conclusion
First, calculate the midpoint of the text , Then offset the specified distance upward along the vertical direction of the dimension line . The key here is how to determine the direction of the dimension line , Through research , Dimension line direction is not equal to dimension second point minus dimension first point .

Determination of dimension line direction

• Find the initial dimension direction at the starting and ending points = Mark the second point - Mark the first point

• according to +X Determine the final dimension direction , There are two options , as follows ：
  Use dot product to judge
  If the dimension direction is the same as x The axis is parallel in the positive direction , The marking direction is +x;
  otherwise , If the dimension direction is the same as x The axis is vertical , The marking direction is +y;
  otherwise , If the dimension direction is the same as x Axial dot product < 0, Then the marking direction is reversed
  Otherwise, the dimension direction will not change

  Use angleTo Find two vectors [0-pi] The angle between
  if 0 or pi, And x Axis parallel , The marking direction is +x
  Otherwise, if pi/2, And x The axis is vertical , The marking direction is +y;
  Otherwise, if >pi/2, Then the marking direction is reversed
  Otherwise, the dimension direction will not change

Upward offset direction

The direction of the dimension line is around Z Counterclockwise rotation of shaft 90 Degree is enough

Find the final text position

Text location = Mark midpoint + (drwDimStyle.dimgap + drwDimStyle.dimtxt / 2.0) * drwDimStyle.dimscale * Upward offset direction );