Why path.subtracted(path2) is linking my path? How can I delinking? - qt

Question

I am working on a simple Paint application using pyqt5. My goal is:
1. Draw freely according to mouse events. It's done!
2. Erase freely according to mouse events. It isn't work as expected!
I have a QGraphicsView, QGraphicsScene where I'm adding QGraphicsItems, specifically QGraphicsPathItem. My eraser has a rect shape, and what I want is to erase part of my drawing while moving my eraser on it.
On MouseMoveEvent, I check if my eraser shape intersects my drawing path, if so, I just subtract it from my drawing path. Then the awkward behavior occurs, it closes my drawing path.
Figure_1, shows my drawing (QGraphicsPathItem).
Figure_2, shows what happens when my eraser(QGraphicsRectItem) intersects my drawing.
Drawing freely lines working fine. Figure_1
Erasing drawing. Figure_2
As we can see, it draws a line linking my first and end points, closing my path. I dont want it. I just want to erase my drawing path.
from PyQt5.QtCore import QRectF, Qt
from PyQt5.QtGui import QPainterPath, QPen
from PyQt5.QtWidgets import QApplication, QGraphicsScene, \
QGraphicsView, QPushButton, QWidget, \
QVBoxLayout, QGraphicsItem, QGraphicsPathItem, QGraphicsRectItem
class Window(QWidget):
scene = None
def __init__(self):
QWidget.__init__(self)
self.view = View(self)
self.button = QPushButton('Clear View', self)
self.button.clicked.connect(self.handleClearView)
layout = QVBoxLayout(self)
layout.addWidget(self.view)
layout.addWidget(self.button)
def handleClearView(self):
self.view.scene.clear()
class View(QGraphicsView):
def __init__(self, parent):
self.scribing = False
self.erasing = False
QGraphicsView.__init__(self, parent)
self.scene = QGraphicsScene()
self.setScene(self.scene)
self.free_draw_item = None
self.eraser_item = None
def resizeEvent(self, QResizeEvent):
self.setSceneRect(QRectF(self.viewport().rect()))
def mousePressEvent(self, event):
if event.buttons() == Qt.LeftButton:
self.scribing = True
pp = QPainterPath(event.pos())
self.free_draw_item = QGraphicsPathItem(pp)
self.free_draw_item.setPen(QPen(Qt.green))
self.free_draw_item.setFlags(QGraphicsItem.ItemIsSelectable)
self.free_draw_item.setPath(pp)
self.scene.addItem(self.free_draw_item)
if event.buttons() == Qt.RightButton:
self.erasing = True
self.eraser_item = QGraphicsRectItem(event.pos().x() - 5,
event.pos().y() - 5, 10, 10)
self.eraser_item.setPen(QPen(Qt.red))
self.eraser_item.setBrush(Qt.transparent)
self.scene.addItem(self.eraser_item)
def mouseMoveEvent(self, event):
if (event.buttons() & Qt.LeftButton) and self.scribing:
if self.free_draw_item:
path = self.free_draw_item.path()
path.lineTo(event.pos())
self.free_draw_item.setPath(path)
if event.buttons() & Qt.RightButton and self.erasing:
self.eraser_item.setRect(event.pos().x() - 5, event.pos().y() - 5,
10, 10)
for item in self.scene.collidingItems(self.eraser_item):
if isinstance(item, QGraphicsPathItem):
if item.path().intersected(self.eraser_item.shape()):
new = item.path().subtracted(self.eraser_item.shape())
item.setPath(new)
#item.setBrush(Qt.red)
def mouseReleaseEvent(self, event):
self.scribing = False
self.erasing = False
if self.eraser_item != None:
self.scene.removeItem(self.eraser_item)
# if self.free_draw_item != None:
# self.free_draw_item.setSelected(True)
if __name__ == '__main__':
import sys
app = QApplication(sys.argv)
window = Window()
window.resize(640, 480)
window.show()
sys.exit(app.exec_())
This is my code.
Left mouse button draws green path on scene.
Right mouse button erase my drawings.

Answer 1

I've had same problem several and several times.
After some headache and a thousand unsuccessful researches about this specific problem I've come to on answer.
Forget having only one path
You cant subtract without closing the path turning it into a closed polygon
Think as having many paths and working around
Now you have all sub items and can "delete" them when colliding with your eraser
Use the subtract on your favor
Have many paths with 3 not align points each path(least points on a plan to have a polygon)
Group it as QGraphicsItemGroup() if you want to manipulate the whole path made of several (3 points path) as a single item
See my example
-
from PyQt5.QtCore import QPoint
from PyQt5.QtCore import QRectF, Qt
from PyQt5.QtGui import QBrush
from PyQt5.QtGui import QPainter
from PyQt5.QtGui import QPainterPath, QPen
from PyQt5.QtWidgets import QApplication, QGraphicsScene, \
QGraphicsView, QPushButton, QWidget, \
QVBoxLayout, QGraphicsItem, QGraphicsPathItem, QGraphicsRectItem
from PyQt5.QtWidgets import QGraphicsItemGroup
from core.IQGraphicsPathItem import InteractQGraphicsPathItem
class Window(QWidget):
scene = None
def __init__(self):
QWidget.__init__(self)
self.view = View(self)
self.button = QPushButton('Clear View', self)
self.button.clicked.connect(self.handleClearView)
layout = QVBoxLayout(self)
layout.addWidget(self.view)
layout.addWidget(self.button)
def handleClearView(self):
self.view.scene.clear()
class View(QGraphicsView):
dic = {}
num_item = 0
_from_x_ = None
_from_y_ = None
pp = None
pen = None
group = None
def __init__(self, parent):
self.scribing = False
self.erasing = False
QGraphicsView.__init__(self, parent)
self.scene = QGraphicsScene()
self.setScene(self.scene)
self.setRenderHint(QPainter.Antialiasing)
self.free_draw_item = QGraphicsPathItem()
# self.free_draw_item.setPen(QPen(Qt.green))
# self.free_draw_item.setFlags(QGraphicsItem.ItemIsSelectable)
self.eraser_item = None
self.pen = QPen(Qt.green,8)
self.pen.setBrush(QBrush(Qt.green))
self.pen.setWidth(1)
self.group = QGraphicsItemGroup()
def resizeEvent(self, QResizeEvent):
self.setSceneRect(QRectF(self.viewport().rect()))
def mousePressEvent(self, event):
if event.buttons() == Qt.LeftButton:
self._from_x_ = event.pos().x()
self._from_y_ = event.pos().y()
self.scribing = True
self.group = QGraphicsItemGroup()
self.group.setFlags(QGraphicsItem.ItemIsSelectable)
self.scene.addItem(self.group)
self.pp = QPainterPath(event.pos())
self.free_draw_item = QGraphicsPathItem(self.pp)
self.free_draw_item.setPen(self.pen)
# self.free_draw_item.setFlags(QGraphicsItem.ItemIsSelectable)
self.free_draw_item.setPath(self.pp)
self.scene.addItem(self.free_draw_item)
self.group.addToGroup(self.free_draw_item)
if event.buttons() == Qt.RightButton:
self.erasing = True
self.eraser_item = QGraphicsRectItem(event.pos().x() - 5,
event.pos().y() - 5, 10, 10)
self.eraser_item.setPen(QPen(Qt.red))
self.eraser_item.setBrush(Qt.white)
self.scene.addItem(self.eraser_item)
def mouseMoveEvent(self, event):
x_adjust = 1
y_adjust = 1
# if event.pos().x() >= self._from_x_:
# x_adjust = 1
# else:
# x_adjust = -1
# if event.pos().y() >= self._from_y_:
# y_adjust = 1
# else:
# y_adjust = -1
if (event.buttons() & Qt.LeftButton) and self.scribing:
if self.free_draw_item:
# path = self.free_draw_item.path()
# path.lineTo(event.pos())
# self.free_draw_item.setPath(path)
self.pp2 = QPainterPath(QPoint(self._from_x_, self._from_y_))
self.pp2.lineTo(event.pos().x(), event.pos().y())
self.pp2.lineTo(event.pos().x()+x_adjust, event.pos().y()+y_adjust)
self.pp2.lineTo(self._from_x_ + x_adjust, self._from_y_ + y_adjust)
self.free_draw_item.setPen(QPen(Qt.green, 8))
self.free_draw_item.path = QGraphicsPathItem()
self.free_draw_item.path.setPath(self.pp2)
self.scene.addItem(self.free_draw_item.path)
self.group.addToGroup(self.free_draw_item)
self._from_x_ = event.pos().x()
self._from_y_ = event.pos().y()
if event.buttons() & Qt.RightButton and self.erasing:
self.eraser_item.setRect(event.pos().x() - 5, event.pos().y() - 5,
10, 10)
for item in self.scene.collidingItems(self.eraser_item):
new = item.path() - self.eraser_item.shape()
item.setPath(new)
print('collided')
def mouseReleaseEvent(self, event):
self.scribing = False
self.erasing = False
self.group.setSelected(True)
print(self.scene.items())
if self.eraser_item != None:
self.scene.removeItem(self.eraser_item)
# if self.free_draw_item != None:
# self.free_draw_item.setSelected(True)
if __name__ == '__main__':
import sys
app = QApplication(sys.argv)
window = Window()
window.resize(640, 480)
window.show()
sys.exit(app.exec_())
If you want to personalize your own Pen I suggest you to overwrite your own QGraphicsPathItem().
I think it'll work pretty great.