Unnecessarily slow turtle rotations

Bug report

Two similar drawings can widely differ in terms of speed, resulting in a counter-intuitive experience for the end-user. This is independent from the plateform (OS, architecture) and from python version (due to the current implementation of turtle).

Example

Consider the following script (which is a simplified version of a real demo program with animation turned on). it draws two 7-points stars. The first one is drawn normally but the second, while similar, is much slower. This is very confusing (also for the programmer trying to understand what happens).

import turtle
def star7(t, a, b, c):
    t.left(a)
    for i in range(7):
        t.forward(80)
        t.left(b)
        t.forward(80)
        t.left(c)
    t.left(-a)

t = turtle.Turtle()
t.hideturtle()
star7(t, 64.6, -77.7, 129.1)    # draw star 1 (normal speed)
star7(t, 295.4, -282.3, 590.9)  # draw star 2 (really too slow !)

Explanation

The problem comes from turtle rotation: in case of animation (by default speed > 0), the time taken by the rotation is proportional to the amplitude of the angle even if the turtle is not visible resulting in a confusing slow drawing. This can be confirmed with a visible turtle, commenting out:

# t.hideturtle()

Indeed, the second star7() invocation contains "large" angles (these values are actually computed, not written by a human). Obviously, it is possible to give smaller equal angles (here taking the opposite of angles of the first star), resulting in a fast drawing e.g.:

star7(t, -64.6, 77.7, -129.1)

Obviously, the programmer can normalize all rotation angles in case of animation but this is painfull and error-prone (the programmer can misses some). And above all, there is no justification for such delays if the turtle is not shown !

Looking at the code of function _rotate() in class RawTurtle (in turtle.py:3279):

    def _rotate(self, angle):
        """Turns pen clockwise by angle.
        """
        if self.undobuffer:
            self.undobuffer.push(("rot", angle, self._degreesPerAU))
        angle *= self._degreesPerAU                # angle is now in degrees
        neworient = self._orient.rotate(angle)
        tracing = self.screen._tracing
        if tracing == 1 and self._speed > 0:       # test: rotation animation ?
            anglevel = 3.0 * self._speed
            steps = 1 + int(abs(angle)/anglevel)   # animation: split into steps (proportionally to abs(angle))
            delta = 1.0*angle/steps
            for _ in range(steps):
                self._orient = self._orient.rotate(delta)
                self._update()
        self._orient = neworient
        self._update()

In case of animation (speed>0), the rotation is split in several steps, each invoking _update(), thus the delays.

Solutions

A first solution is to only do this if the turtle is visible. This can be achieved replacing the test by:

        if self._shown and tracing == 1 and self._speed > 0:

But maybe this results in a too fast drawing.

A second solution consists in normalizing the angle if the turtle is not visible. This can be achived with:

        if tracing == 1 and self._speed > 0:
            if not self._shown:
                angle -= math.ceil(angle / 360.0 - 0.5) * 360.0     # normalize angle in (-180;180]

This normalization ensures the angle (in degrees since the instruction angle *= self._degreesPerAU above) is now in (-180;180] and prevents too long delays. The result is nice. I'm in favor of this second solution.

What do you think ?

Inizia in turtle.py da RawTurtle._rotate(), quindi esegui l’esempio fornito star7 con l’animazione abilitata e la tartaruga nascosta per riprodurre il ritardo sproporzionato. Confronta i casi con angolo ampio e angolo normalizzato; il lavoro è completato quando rotazioni equivalenti con la tartaruga nascosta non producono più tempi di disegno confusamente diversi.