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Lines on a tkinter.Canvas

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  Lines on a tkinter.Canvas Pedro Izecksohn <izecksohn@yahoo.com> - 2014-06-11 18:57 -0700
    Re: Lines on a tkinter.Canvas Gregory Ewing <greg.ewing@canterbury.ac.nz> - 2014-06-12 23:38 +1200
      Re: Lines on a tkinter.Canvas Terry Reedy <tjreedy@udel.edu> - 2014-06-12 13:44 -0400
      Re: Lines on a tkinter.Canvas Pedro Izecksohn <izecksohn@yahoo.com> - 2014-06-12 13:33 -0700
      Re: Lines on a tkinter.Canvas Pedro Izecksohn <izecksohn@yahoo.com> - 2014-06-12 16:11 -0700
        Re: Lines on a tkinter.Canvas Gregory Ewing <greg.ewing@canterbury.ac.nz> - 2014-06-13 12:31 +1200

#73200 — Lines on a tkinter.Canvas

  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.

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#73216

Pedro Izecksohn wrote:
> The Canvas' method create_line turns on at least 2 pixels. But I want to turn
> on many single pixels on a Canvas.

You could try using a 1x1 rectangle instead.

However, be aware that either of these will use quite a
lot of memory per pixel. If you are drawing a very large
number of pixels, this could cause performance problems.
In that case, you might want to use a different approach,
such as creating an image and telling the canvas to display
the image.

-- 
Greg

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#73231

On 6/12/2014 7:38 AM, Gregory Ewing wrote:
> Pedro Izecksohn wrote:
>> The Canvas' method create_line turns on at least 2 pixels. But I want
>> to turn
>> on many single pixels on a Canvas.
>
> You could try using a 1x1 rectangle instead.
>
> However, be aware that either of these will use quite a
> lot of memory per pixel. If you are drawing a very large
> number of pixels, this could cause performance problems.

Pedro, the tkinter canvas is a vector graphics canvas, not a bitmap 
image canvas as in paint programs.

> In that case, you might want to use a different approach,
> such as creating an image and telling the canvas to display
> the image.

-- 
Terry Jan Reedy

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#73240

----- Original Message -----

> From: Gregory Ewing
> To: python-list@python.org
> Sent: Thursday, June 12, 2014 8:38 AM
> Subject: Re: Lines on a tkinter.Canvas
> 
> Pedro Izecksohn wrote:
>>  The Canvas' method create_line turns on at least 2 pixels. But I want 
> to turn
>>  on many single pixels on a Canvas.
> 
> You could try using a 1x1 rectangle instead.

pedro@microboard:~/programming/python/tk/canvas$ diff old/testing.001.py testing.py
19c19
<     self.canvas.create_line (p.x, p.y, p.x, p.y, fill = color)
---
>     self.canvas.create_rectangle (p.x, p.y, p.x, p.y, fill = color)

  The result that I got is: The line drawn by it is not shown.

> However, be aware that either of these will use quite a
> lot of memory per pixel. If you are drawing a very large
> number of pixels, this could cause performance problems.
> In that case, you might want to use a different approach,
> such as creating an image and telling the canvas to display
> the image.

  I did not try this yet.

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#73241

----- Original Message -----

> From: Gregory Ewing
> To: python-list@python.org
> Cc: 
> Sent: Thursday, June 12, 2014 8:38 AM
> Subject: Re: Lines on a tkinter.Canvas
> 
> Pedro Izecksohn wrote:
>>  The Canvas' method create_line turns on at least 2 pixels. But I want 
> to turn
>>  on many single pixels on a Canvas.
> 
> You could try using a 1x1 rectangle instead.
> 
> However, be aware that either of these will use quite a
> lot of memory per pixel. If you are drawing a very large
> number of pixels, this could cause performance problems.
> In that case, you might want to use a different approach,
> such as creating an image and telling the canvas to display
> the image.

  Thank you Greg. Your second approach works and the script became:

#!/usr/bin/python3

import tkinter as tk

BITMAP = '''
#define im_width 1
#define im_height 1
static char im_bits[] = {
0xff
};
'''

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")
    self.objects_drawn = []
  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):
    bitmap = tk.BitmapImage (data=BITMAP, foreground = color)
    self.objects_drawn.append (bitmap)
    self.canvas.create_image (p.x, p.y, image = bitmap)

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()

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#73244

Pedro Izecksohn wrote:
>   Thank you Greg. Your second approach works and the script became:

That's not really what I meant; doing it that way,
you're still incurring the overhead of a tk canvas
object for each point that you draw. However, if
there are only 250 points or so, it might not matter.

-- 
Greg

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