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()
  self.update()
  # self.update()
 else:            # non Dorag ing
  self.pselected = -1
  rc = QRectF ( e.pos() - self.a , e.pos() + self.a )
  for i, l in  enumerate( self.plist ) :
   if rc.contains( rc , l , False) :
    self.pselected = i
    self.drawpoint(QRectF( l-self.a , l+self.a ) ,1 )
    break

def drawpoint(self,rc,flag): # 0:black 1:red
 paint=QPainter(self)
 if flag :
  paint.setPen( Qt.red )
 else :
  paint.setPen( Qt.black )
 paint.drawRect( rc )
 # 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( Qt.black , 1, style=Qt.SolidLine ) )
 painter.drawPath(path)

 painter.setPen( QPen( Qt.red , 1, style=Qt.DotLine ) )
 painter.drawLine( plist[0] , plist[1] ) # sub
 painter.drawLine( plist[2] , plist[3] ) # sub

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