hoge一から学ぶベジェ曲線
https://postd.cc/bezier-curves/
このサイト素晴らしい、動画を埋め込むのでは無い、動くページの素晴らしさだな
ペイントの基本
https://qiita.com/kenasman/items/87c12f3a8b63f5948153

・ウイジェットを一つ作って（500-500）そこに直接書く
・まずキューブを一つ書く、端点は四つ、これをマウスでグリグリする
・端点を中心に誤差aでマウスで選択してドラッグできるようにする
　通常は黒い■、マウスオーバーで赤い■にしてマウスプレスで選択、ドラッグ
　ドラッグ中はリアルタイムで描画

```
# =====================
from PyQt5.QtWidgets import QApplication, QWidget
from PyQt5.QtCore import QPointF ,QRectF
from PyQt5.QtGui import QPainterPath ,QPainter 

 class App(QWidget):

    def __init__(self):
        super().__init__()
        self.setWindowTitle('hogetta')
        self.setGeometry( 500, 50 , 500 , 500 )
        self.show()

if __name__ == "__main__":
    app = QApplication(sys.argv)
    ex = App()
    sys.exit(app.exec_())

# =======================
def init

self.plist=[]
self.plist.append( QPointF(10,250) )  # start point
self.plist.append( QPointF(100,100) )  #
self.plist.append( QPointF(300,200) )  #
self.plist.append( QPointF(490,250) )  #
self.a = QPointf(2,2)
self.pselected = -1             # selectedpoint
self.pmove = 0                  # should move
self.draw()

# ========================
def keyPressEvent( self ,e):
 if e.                     # not shift return 

 setPen(black);
 for l in   self.plist :
  DrawRect(l._sub_(a) , l._add_( a) ) # <---- ???

# ========================
def mouseMoveEvent(self ,e):

 setPen(red);
 if self.pmove :  # Dorag ing
  self.plist[self.selected] = e.pos()
  self.draw()
 else:            # non Dorag ing
  for i, l in  enumerate( self.plist ) :
   p1 =  e.pos() -= self.a     # e.pos()._sub_( self.a )
   p2 = e.pos()._add_( self.a )
   if e.pos().x() > p1.x() and e.pos().x() < p2.x() and e.pos().y() > p1.y() and  e.pos().y() < p2.y() :
    self.pselected = i
    paint = QPainter(self)
    paint.drawRect( QRectF( p1 , p2) ) # <---- ???
   else : 
    self.pselected = -1

 self.update()
# ========================
def mousePressEvent(self ,e):
 if self.pselected != -1 :
  self.pmove = True
def mouseReleaseEvent(self ,e):
 self.pmove = False
 self.pselected = -1
# ========================
def draw(self):

 # パスを設定した後でパスをドローする

 # path = QPainterPath()
 # path.moveTo( plist[0] );
 path = QPainterPath( plist[0] )
 path.cubicTo( plist[1] , plist[2] , plist[3] )

 painter= QPainter(self) ;
 painter.setPen(QPen(QColor(79, 106, 25), 1, Qt::SolidLine,
                    Qt::FlatCap, Qt::MiterJoin))
 painter.drawPath(path)


# ===================
```