Tcl Library Source Code

View Ticket
Login
Bounty program for improvements to Tcl and certain Tcl packages.
Ticket UUID: fb4812f82b4894d5b80c0594944cffec516cd3d6
Title: math::geometry::direction and octant counts clockwise instead of counterclockwise
Type: Bug Version: 1.20
Submitter: fredCAD Created on: 2020-08-15 14:01:08
Subsystem: math :: geometry Assigned To:
Priority: 5 Medium Severity: Minor
Status: Closed Last Modified: 2020-09-20 12:25:10
Resolution: None Closed By: arjenmarkus
    Closed on: 2020-09-20 12:25:10
Description:
the math:geometry package determines
 90° -> 0 -1  while sin(90°)  is  1
270* -> 0  1  while sin(270°) is -1

Description for math::geometry::direction:
Given the angle in degrees this command computes and returns the unit vector pointing into this direction. The vector for angle == 0 points to the right (up), and for angle == 90 up (north).

examplee:
foreach angle   {0 90 180 270 360} {
    puts "    $angle -> [::math::geometry::direction $angle]"
}
results in:
      0 ->  1.0                     -0.0
     90 ->  6.123233995736766e-17   -1.0    
    180 -> -1.0                     -1.2246467991473532e-16
    270 -> -1.8369701987210297e-16   1.0
    360 ->  1.0                      2.4492935982947064e-16

question:
is there a reason to go in the opposite of sin() behaviour?
by an angle of 90° the sin() is positiv but [direction 90] gives a negative value for the y-coordinate.
also the octant procedure behaves in this way, only to work together with the behaviour of the dircect procedure


Could it be that this behaviour is motivated by the canvas origin on the top-left instead of bottom-left?
User Comments: arjenmarkus added on 2020-09-20 12:25:10:
I have corrected the two procedures (and solved the problem with the adjacent/touching line segments). So closing this ticket.

More to follow: extending the package with the contributions from fredCAD

arjenmarkus added on 2020-09-20 10:57:19:
I agree that this is wrong: the geometry package should use standard mathematical definitions. I do not know if the sign choice was made because of the canvas, it is quite possible, but it is unexpected from a mathematical point of view.

The [octant] procedure gives the expected octants if you use the [angle] procedure to construct the required vector. So that is indeed consistent.