pyqtgraph - 将 ArrowItem 的原点移动到本地中心

Question

我正在使用 pyqtgraph 绘制机器人的轨迹（机器人行驶的路径）。现在我想在绘图中添加一个标记来指示机器人的当前位置和航向。我认为 ArrowItem 将是正确的选择，因为它是比例不变的并且可以轻松旋转。然而，箭头的本地原点在它的尖端，就像这样

但我希望它像这样在中心

我该怎么做？我也很欣赏这个问题的不同解决方案。

更新

应用 eyllansec 的代码后，我遇到了一些渲染问题。一个最小的例子（必须缩放或移动视图以禁用自动缩放）：

import pyqtgraph as pg
import numpy as np
import time

class CenteredArrowItem(pg.ArrowItem):
    def paint(self, p, *args):
        p.translate(-self.boundingRect().center())
        pg.ArrowItem.paint(self, p, *args)


if __name__ == '__main__':
    app = pg.QtGui.QApplication([])  
    window = pg.GraphicsWindow(size=(1280, 720))
    window.setAntialiasing(True)
    tracker = window.addPlot(title='Tracker')

    while True:
        for i in range(300):         
            arrow = CenteredArrowItem(angle=i, headLen=40, tipAngle=45, baseAngle=30)   
            arrow.setPos(i / 300, i / 300)
            tracker.addItem(arrow)
            app.processEvents()
            time.sleep(0.02)
            tracker.removeItem(arrow)

您可能已经注意到，我在每次迭代中都添加和删除了箭头。这是因为arrow.setStyle(angle=i) 不工作，因为它不更新箭头的旋转（可能是一个错误）。

Answer 1

一个可能的解决方案是覆盖ArrowItem的paint方法并移动QPainter：

import numpy as np
from pyqtgraph.Qt import QtGui, QtCore
import pyqtgraph as pg

class MyArrowItem(pg.ArrowItem):
    def paint(self, p, *args):
        p.translate(-self.boundingRect().center())
        pg.ArrowItem.paint(self, p, *args)

app = QtGui.QApplication([])

w = QtGui.QMainWindow()
p = pg.PlotWidget()
p.showGrid(x = True, y = True, alpha = 0.3)
w.show()
w.resize(640, 480)
w.setCentralWidget(p)
w.setWindowTitle('pyqtgraph example: Arrow')


a = pg.ArrowItem(angle=-160, tipAngle=60, headLen=40, tailLen=40, tailWidth=20, pen={'color': 'w', 'width': 3},  brush='r')
b = MyArrowItem(angle=-160, tipAngle=60, headLen=40, tailLen=40, tailWidth=20, pen={'color': 'w', 'width': 3})

a.setPos(10,0)
b.setPos(10,0)

p.addItem(a)
p.addItem(b)

## Start Qt event loop unless running in interactive mode or using pyside.
if __name__ == '__main__':
    import sys
    if (sys.flags.interactive != 1) or not hasattr(QtCore, 'PYQT_VERSION'):
        QtGui.QApplication.instance().exec_()

如下图，红色箭头为默认ArrowItem，蓝色为偏移量，两者相对于绘图位于同一位置。

更新：

问题是由默认使用变换中心旋转作为坐标中心的item的方法引起的，即(0, 0)，我们必须移动它：

import pyqtgraph as pg
import numpy as np
import time

from pyqtgraph.Qt import QtGui, QtCore
from pyqtgraph import functions as fn

class CenteredArrowItem(pg.ArrowItem):
    def setStyle(self, **opts):
        # http://www.pyqtgraph.org/documentation/_modules/pyqtgraph/graphicsItems/ArrowItem.html#ArrowItem.setStyle
        self.opts.update(opts)

        opt = dict([(k,self.opts[k]) for k in ['headLen', 'tipAngle', 'baseAngle', 'tailLen', 'tailWidth']])
        tr = QtGui.QTransform()
        path = fn.makeArrowPath(**opt)
        tr.rotate(self.opts['angle'])
        p = -path.boundingRect().center()
        tr.translate(p.x(), p.y())
        self.path = tr.map(path)
        self.setPath(self.path)

        self.setPen(fn.mkPen(self.opts['pen']))
        self.setBrush(fn.mkBrush(self.opts['brush']))

        if self.opts['pxMode']:
            self.setFlags(self.flags() | self.ItemIgnoresTransformations)
        else:
            self.setFlags(self.flags() & ~self.ItemIgnoresTransformations)

if __name__ == '__main__':
    app = pg.QtGui.QApplication([])  
    window = pg.GraphicsWindow(size=(1280, 720))
    window.setAntialiasing(True)
    tracker = window.addPlot(title='Tracker')

    while True:
        for i in range(300):         
            arrow = CenteredArrowItem(angle=i, headLen=40, tipAngle=45, baseAngle=30)   
            arrow.setPos(i / 300, i / 300)
            tracker.addItem(arrow)
            app.processEvents()
            time.sleep(0.02)
            tracker.removeItem(arrow)

Answer 2

在挖掘 pyqtgraph 的源代码后，我最终得到了一个适合我需要的特殊功能。我在创建箭头路径时应用翻译，而不是在渲染它时。幸运的是，这也解决了旋转错误（无论出于何种原因）。

例子：

import pyqtgraph as pg
import numpy as np
import time
import pyqtgraph.functions

class CenteredArrowItem(pg.ArrowItem):
    def setData(self, x, y, angle):
        self.opts['angle'] = angle
        opt = dict([(k, self.opts[k]) for k in ['headLen', 'tipAngle', 'baseAngle', 'tailLen', 'tailWidth']])
        path = pg.functions.makeArrowPath(**opt)
        b = path.boundingRect()
        tr = pg.QtGui.QTransform()              
        tr.rotate(angle)
        tr.translate(-b.x() - b.width() / 2, -b.y() - b.height() / 2)  
        self.path = tr.map(path)
        self.setPath(self.path)
        self.setPos(x, y)


if __name__ == '__main__':
    app = pg.QtGui.QApplication([])  
    window = pg.GraphicsWindow(size=(1280, 720))
    window.setAntialiasing(True)
    tracker = window.addPlot(title='Tracker')

    arrow = CenteredArrowItem(headLen=40, tipAngle=45, baseAngle=30)   
    tracker.addItem(arrow)
    tracker.addItem(pg.InfiniteLine(pos=(0,0), angle=45))
    center = pg.ScatterPlotItem([], [], brush='r')
    tracker.addItem(center)

    while True:
        for i in range(300):
            arrow.setData(i, i, i)     
            center.setData([i], [i])    
            app.processEvents()
            time.sleep(0.02)