PyQt5: Centre line of a QPolygonF

Question

I'm trying to get the centre line of a QPolygonF object, meaning the line that is in the middle of min and max X coordinate, for every single y coordinate from min to max.

The desired output would be a single centre line, yet for some reason the drawn object/ central line has area. Why is that the case & what should I do to get the centre line? As roughly estimated with the black line in the image below

Here's the code:

import PyQt5
from PyQt5 import QtGui
from PyQt5.QtWidgets import QApplication,QWidget, QMainWindow,QVBoxLayout, QGraphicsScene, QGraphicsPixmapItem,QGraphicsView
from PyQt5.QtGui import QPixmap,QPolygonF
from PyQt5.QtCore import Qt,QPointF
import sys
import time
class MainWindow(QMainWindow):

    def __init__(self, *args, **kwargs):
        super(MainWindow, self).__init__(*args, **kwargs)
        self.setFixedSize(1200, 800)

        self.ImageDisplay = QGraphicsView()

        self.qwidget = QWidget()

        qpixmap = QPixmap()
        qpixmap2 = qpixmap.scaledToWidth(self.ImageDisplay.width())
        qgraphicsitem = QGraphicsPixmapItem(qpixmap2)
        qscene = QGraphicsScene(self.ImageDisplay)
        qscene.addItem(qgraphicsitem)
        self.ImageDisplay.setScene(qscene)
        self.ImageDisplay.setMouseTracking(True)
        self.ImageDisplay.setTransformationAnchor(QGraphicsView.AnchorUnderMouse)
        self.ImageDisplay.setResizeAnchor(QGraphicsView.AnchorUnderMouse)
        self.ImageDisplay.setVerticalScrollBarPolicy(Qt.ScrollBarAlwaysOff)
        self.ImageDisplay.setHorizontalScrollBarPolicy(Qt.ScrollBarAlwaysOff)
        start = time.time()

        poly = QPolygonF([QPointF(400, 400), QPointF(100, 10), QPointF(20, 100), QPointF(0, 400)])#, QPointF(105, 5), QPointF(9, 300)])

        min_y, max_y = poly.boundingRect().y(), poly.boundingRect().y() + poly.boundingRect().height()
        min_x,max_x =  poly.boundingRect().x(), poly.boundingRect().x() + poly.boundingRect().width()
        print("min_y="+str(min_y)+", max_y="+str(max_y))
        print("min_x=" + str(min_x) + ", max_x=" + str(max_x))
        end = time.time()
        print(f"runtime: {end - start}")

        middle_points= []
        for y in range(int(min_y),int(max_y)):
           # for x in range(int(min_x),int(max_x)):
            polyline = QPolygonF([QPointF(min_x,y),QPointF(max_x,y),QPointF(min_x,y+0.1),QPointF(max_x,y+0.1)])

            intersection_points = poly.intersected(polyline)


            intersection_points = intersection_points.boundingRect()

            middle_point = QPointF(intersection_points.x()+intersection_points.width()/2,intersection_points.y()+intersection_points.height()/2)
            middle_points.append(middle_point)


        middle_poly = QPolygonF(middle_points)
        self.ImageDisplay.scene().addPolygon(middle_poly, QtGui.QColor(20, 0, 255, 28),
                                             QtGui.QColor(20, 0, 255, 28))

        end = time.time()
        print(f"runtime: {end - start}")






        self.vlayout = QVBoxLayout()
        self.vlayout.addWidget(self.ImageDisplay)

        self.qwidget.setLayout(self.vlayout)

        self.setCentralWidget(self.qwidget)


app = QApplication(sys.argv)
window = MainWindow()
window.show()
app.exec_()

Answer 1

You have to take into account that:

• When adding a QPolygon then a closed figure will be drawn.
• It is not a central line but a central curve that is made up of line segments.
• The code for y in range(int(min_y),int(max_y)): works with (ymax-ymin) are greater than 1 but it fails in other cases.

To draw the central curve then you have to use QPainterPath, in addition to using np.arange().

import sys

from PyQt5.QtCore import QPointF, QRectF
from PyQt5.QtGui import QPainterPath, QPolygonF
from PyQt5.QtWidgets import QApplication, QGraphicsScene, QGraphicsView

import numpy as np


def build_center_line(polygon):
    points = []
    minx, miny, maxx, maxy = polygon.boundingRect().getCoords()
    dy = (maxy - miny) / 200
    for y in np.arange(miny, maxy, dy):
        polyline = QPolygonF(
            QRectF(QPointF(minx, y - dy / 2), QPointF(maxx, y + dy / 2))
        )
        p = polygon.intersected(polyline).boundingRect().center()
        points.append(p)
    return points


def main():
    app = QApplication(sys.argv)
    polygon = QPolygonF(
        [QPointF(400, 400), QPointF(100, 10), QPointF(20, 100), QPointF(0, 400)]
    )

    points = build_center_line(polygon)
    path = QPainterPath()
    if points:
        path.moveTo(points[0])
        for point in points[1:]:
            path.lineTo(point)

    scene = QGraphicsScene()
    view = QGraphicsView(scene)
    scene.addPolygon(polygon)
    scene.addPath(path)
    view.resize(640, 480)
    view.show()

    sys.exit(app.exec_())


if __name__ == "__main__":
    main()