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Re: Lines on a tkinter.Canvas

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  Re: Lines on a tkinter.Canvas Peter Otten <__peter__@web.de> - 2014-06-12 09:02 +0200

#73208 — Re: Lines on a tkinter.Canvas

Pedro Izecksohn wrote:

> The code available from:
> http://izecksohn.com/pedro/python/canvas/testing.py
> draws 2 horizontal lines on a Canvas. Why the 2 lines differ on thickness
> and length?
> 
> The Canvas' method create_line turns on at least 2 pixels. But I want to
> turn on many single pixels on a Canvas. How should I do this? Canvas has
> no method create_pixel or create_point.

> #!/usr/bin/python3
> 
> import tkinter as tk
> 
> class Point ():
>   def __init__ (self, x, y):
>     self.x = x
>     self.y = y
> 
> class Board (tk.Frame):
>   def __init__ (self, bg, dimensions):
>     tk.Frame.__init__ (self, tk.Tk())
>     self.pack()
>     self.canvas = tk.Canvas (self, bd = 0, bg = bg, width = dimensions.x, 
height = dimensions.y)
>     self.canvas.pack (side = "top")
>   def drawLine (self, pa, pb, color):
>     self.canvas.create_line (pa.x, pa.y, pb.x, pb.y, fill = color)
>   def drawPoint (self, p, color):
>     self.canvas.create_line (p.x, p.y, p.x, p.y, fill = color)
> 
> dimensions = Point (500, 500)
> board = Board ('black', dimensions)
> color = 'red'
> p = Point (0, 250)
> while (p.x < dimensions.x):
>   board.drawPoint (p, color)
>   p.x += 1
> pa = Point (0, 350)
> pb = Point (499, 350)
> board.drawLine (pa, pb, color)
> board.mainloop()

I just tried your script, and over here the line drawn with

>   def drawLine (self, pa, pb, color):
>     self.canvas.create_line (pa.x, pa.y, pb.x, pb.y, fill = color)

has a width of 1 pixel and does not include the end point. Therefore the 
"line" drawn with

>   def drawPoint (self, p, color):
>     self.canvas.create_line (p.x, p.y, p.x, p.y, fill = color)

does not show up at all. You could try to specify the line width explicitly:

   def drawPoint (self, p, color):
     self.canvas.create_line (p.x, p.y, p.x+1, p.y, fill=color, width=1)

If that doesn't work (or there's too much overhead) use pillow to prepare an 
image and show that.

Another random idea: if you have a high resolution display your OS might 
blow up everything. In that case there would be no fix on the application 
level.

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