如何以任意角度绘制文本并平行于一条线？

Question

I want to place a text centrally above the line (controlled with variable distance ). There is a method drawLineDescription() for this. This method takes the start and endpoints of the lines and then calculates the center point x, y . I have already worked with angle to ensure that the text is placed correctly. Unfortunately, I can't figure out how to place the text vertically over the line at every angle, ie depending on the rotation the variables x, y have to be able to move. How can I supplement this?

def drawLineDescription(canvas, startX, startY, endX, endY, distance):
        lengthX = endX - startX
        lengthY = endY - startY
        x = int(startX+((lengthX)/2))
        y = int(startY+((lengthY)/2))

        angle = math.degrees(math.atan2(lengthY, lengthX)*-1)
        angle = round(angle)
        if angle < -90 or angle > 90:
           angle += 180

        canvas.create_text(x, y, angle=angle, font=("Arial", 12), text="exampleText")

In the end it should look like this (example of multiple lines with text - the lines never cross their text): Example result

Answer 1

lengthX = endX - startX
lengthY = endY - startY
fullLength = math.sqrt(lengthX**2 + lengthY**2)
#unit direction vector
ux = lengthX / fullLength
uy = lengthY / fullLength
#unit normal
if ux < 0:
    nx, ny = -uy, ux
else:
    nx, ny = uy, -ux
#text center point (D at the picture)
cx = x + nx * distance
cy = y + ny * distance

#if you need start of text (S at the picture)
sx = x + nx * distance - ux * halfwidth
sy = y + ny * distance - uy * halfwidth

Answer 2

You can draw rotated text on tkinter if you are using tcl > 8.6 with the following instructions: canvas_item = tk.create_text , and canvas.itemconfig(canvas_item, angle=rotation_angle)

In order to achieve what you want, you need a little bit of geometry, notably the coordinates of the line segment, its mid point, an offset vector perpendicular to the segment, and the angle of the segment.

I encapsulated the arithmetic necessary to calculate the proper geometric elements in a class point , and in c class Vector . These classes are not bullet proof, but they give you a starting point for basic geometry.

I added an example of a line defined by two points, with the text placed as you need and rotated to match the direction of the line segment.

import math
import tkinter as tk


class Point:
    def __init__(self, x, y):
        self.x = x
        self.y = y

    def __add__(self, other: 'Vector'):
        return Vector(other.x + self.x, other.y + self.y)

    def __sub__(self, other):
        return Vector(self.x - other.x, self.y - other.y)

    def __iter__(self):
        yield self.x
        yield self.y

    def __str__(self):
        return f'{self.__class__.__name__}({self.x}, {self.y})'


class Vector:
    def __init__(self, x, y):
        self.x = x
        self.y = y

    def __add__(self, other):
        return Point(other.x + self.x, other.y + self.y)

    def __sub__(self, other):
        return Vector(other.x - self.x, other.y - self.y)

    def scale(self, scalar):
        return Vector(self.x * scalar, self.y * scalar)

    def normal(self):
        norm = self.norm()
        return Vector(self.x / norm, self.y / norm)

    def norm(self):
        return math.hypot(self.x, self.y)

    def perp(self):
        x, y = self.normal()
        return Vector(y, -x)

    def angle(self):
        return math.atan2(-self.y, self.x) * (180 / math.pi)

    def __iter__(self):
        yield self.x
        yield self.y

    def __str__(self):
        return f'{self.__class__.__name__}({self.x}, {self.y})'


if __name__ == '__main__':

    root = tk.Tk()
    canvas = tk.Canvas(root, width=500, height=500)

    p0, p1 = Point(100, 40), Point(200, 300)
    segment = p1 - p0
    mid_point = segment.scale(0.5) + p0
    # canvas.create_oval(*(mid_point - Vector(2, 2)), *(Vector(2, 2) + mid_point))

    line = canvas.create_line(*p0, *p1)
    offset = segment.perp().scale(20)
    # canvas.create_line(*mid_point, *(mid_point+offset))

    txt = canvas.create_text(*(offset + mid_point), text='example')
    canvas.itemconfig(txt, angle=segment.angle())

    canvas.pack()
    root.mainloop()