Arc angle in DXF file

[Massimiliano Zecchetto]

Dear all,

I am trying to calculate the angle of an arc using ezdxf. It should be as simple as:

    abs(arc.dxf.start_angle - arc.dxf.end_angle) 

However I am having troubles in one simple case, when the start angle is equal to 360°.

For instance, in this case I get an angle of about 323° when it should be about 36°.

I am really struggling in find a way to manage this exception case, but to date I was not able to find any valid, effective workaround.

Any hints?

With best regards,

Massimiliano 

[Manfred Moitzi]

A simple solution:

        end = arc.dxf.end_angle

        while end < arc.dxf.start_angle:
            end += 360
        angle_span = end - arc.dxf.start_angle

I am using a while loop, because the modulo operator (angle % 360) for angle normalization has a problem for start/end angle = (0, 360), (360 % 360) - 0 == 0.

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[Manfred Moitzi]

Added a new helper function for this problem, because my previous solution was not correct for all cases:

def arc_angle_span_deg(start: float, end: float) -> float:
    """ Returns the counter clockwise angle from `start` to `end` in degrees.
    """
    # start == end is 0 by definition:
    if math.isclose(start, end):
        return 0.0

    # Special treatment for end angle == 360 deg:
    if not math.isclose(end, 360.0):
        end %= 360.0

    start %= 360.0
    if end < start:
        end += 360.0
    return end - start

This solution covers most (all?) corner cases:

@pytest.mark.parametrize('start, end, expected', [
    (0, 0, 0),  # start == end is always 0 by definition
    (360, 360, 0),  # start == end is always 0 by definition
    (-30, -30, 0),  # start == end is always 0 by definition
    (0, 360, 360),  # end angle counter clockwise (ccw) 360 is 360 deg (special)
    (270, 360, 90),  # end angle ccw 360 is 360 deg (special)
    (-90, 360, 90),  # end angle ccw 360 is 360 deg (special)
    (0, -360, 0),  # end angle clockwise (cw) 360 deg is 0
    (90, -360, 270),  # end angle clockwise (cw) 360 deg is 0
    (-90, -360, 90),  # end angle clockwise (cw) 360 deg is 0
    (360, 0, 0),  # start angle 360 is 0
    (360, 90, 90),  # start angle 360 is 0
    (360, -90, 270),  # start angle 360 is 0
    (-360, 0, 0),  # start angle -360 is 0
    (-360, 90, 90),  # start angle -360 is 0
    (-360, -90, 270),  # start angle -360 is 0
    (30, -30, 300),  # crossing ccw 0 deg
    (-30, 30, 60),  # crossing cw 0 deg
    (180, -180, 0),  # 180 is equal to -180
    (-180, 180, 0),  # 180 is equal to -180
    (90, -90, 180),
    (-90, 90, 180),
    (360, 400, 40),
    (400, 360, 320),
])
def test_arc_angle_span_deg(start, end, expected):
    assert arc_angle_span_deg(start, end) == pytest.approx(expected)

Best regards,

Manfred

Am Mo., 21. Dez. 2020 um 14:57 Uhr schrieb Massimiliano Zecchetto <massimilian...@gmail.com>:

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[Massimiliano Zecchetto]

Thank you so much for your prompt reply Mr. Moitzi.

Your suggestion works fine for me.

Will this be available in ezdxf in the future?

Best regards,

Massimiliano