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Make drawing only possible on top of old path

Hello!
In my app, a shape is drawn consisting of many points, which are drawn with a ui.Path, somewhat like this:
for i in range(1,len(self.points)): x1 = self.points[i].x y1 = self.points[i].y x2 = self.points[i + 1].x y2 = self.points[i + 1].y self.path.move_to(x1, y1) self.path.line_to(x2, y2) self.set_needs_display()
After this is done I would like to be able to draw on top of that shape with a different color and only on that shape. I basically want to be able to recolor parts of the path.
I was thinking that using add_clip() could be used on the path that drew the shape, constricting new drawing operations to the shape. Unfortunately I have not been able to make add_clip work.
Does my approach make sense? If so, where would I use add_clip?

I would recommend to use Image.getpixel(xy) and change the color.
Path.hit_test(x,y) checks only the object bounds, not if a line is hit.
So if you are working only with the coordinates, you have to do a lot of calculations to check for a real hit. 
I tried something similar with changing the image to a numpy pixel array and the checking the touched pixel and its surroundings. Unfortunately this turned out to not work that great, since it didn't paint all the surrounding pixels, especially when it searched a large area. When I try Image.getpixel I get the error "'module' object has no attribute 'getpixel'.
Any ideas how else I might accomplish this?
Here is the code of my attempt with the numpy array:
def touch_began(self,touch): self.x1, self.y1 = touch.location x = int(self.x1) y = int(self.y1) x = x * 2 y = y * 2 if pix[y,x] < 255: self.drawDot(touch.location,self.dotSize) self.check_area(touch.location, 15) return def check_area(self, t, size): # Checks area around touch for dark pixels and paints them red self.x1, self.y1 = t x = int(self.x1) y = int(self.y1) x = x * 2 y = y * 2 ptsx = [] ptsy = [] thresh = 120 for xx in range(size): print('x: '+str(xx)) for yy in range(size): print('y: '+str(yy)) xn = 0 yn = 0 if pix[y + yy, x + xx] < thresh: xn = (x + xx) * 0.5 yn = (y + yy) * 0.5 # print(xn) xn = math.ceil(xn) # print(xn) yn = math.ceil(yn) ptsx.append(xn) ptsy.append(yn) if pix[y  yy, x  xx] < thresh: xn = (x  xx) * 0.5 yn = (y  yy) * 0.5 xn = math.ceil(xn) yn = math.ceil(yn) ptsx.append(xn) ptsy.append(yn) if pix[y + yy, x  xx] < thresh: xn = (x  xx) * 0.5 yn = (y + yy) * 0.5 xn = math.ceil(xn) yn = math.ceil(yn) ptsx.append(xn) ptsy.append(yn) if pix[y  yy, x + xx] < thresh: xn = (x + xx) * 0.5 yn = (y  yy) * 0.5 xn = math.ceil(xn) yn = math.ceil(yn) # self.drawDotCoords(xn,yn,self.dotSize) ptsx.append(xn) ptsy.append(yn) # if xn != 0: # ptsx.append(xn) # ptsy.append(yn) if len(ptsx) > 0: self.drawDotCoords(ptsx,ptsy,self.dotSize) # Draws a onepixel dot on each surrounding dark pixel

Try
ui.set_blend_mode(mode)
with ui.BLEND_COLOR 

@JonB Can you give me some pointers how i would use this function? I'm not sure where it would work. This would also mean that I could only color the contour in one color right?

you would use that within
draw
, at the point you change the color and start drawing the new segments.maybe post a gist with your code?
I am assuming you are starting frim the sketch example?

@JonB This is the link to my github code, sorry it's very messy:
https://github.com/HenningTm/drawing_touchpad
The .xlsx file in the repository contains the point coordinates for the shape and is redrawn in lines 474489.