Drawing diagonal lines on an image

Question

Hi im trying to draw diagonal lines across an image top right to bottom left here is my code so far.

  width = getWidth(picture)
  height = getHeight(picture)
           


        

Answer 1

I would like to add some math considerations to the discussion...

(Just because it is sad that JES's addLine function draws black lines only and is quite limited...)

Note : The following code uses the Bresenham's Line Algorithm pointed out by MartinStettner (so thanks to him).

The Bresenham's line algorithm is an algorithm which determines which order to form a close approximation to a straight line between two given points. Since a pixel is an atomic entity, a line can only be drawn on a computer screen by using some kind of approximation.

Note : To understand the following code, you will need to remember a little bit of your basic school math courses (line equation & trigonometry).

Code :

# The following is fast implementation and contains side effects...

import random

# Draw point, with check if the point is in the image area
def drawPoint(pic, col, x, y):
   if (x >= 0) and (x < getWidth(pic)) and (y >= 0) and (y < getHeight(pic)):
     px = getPixel(pic, x, y)
     setColor(px, col)


# Draw line segment, given two points
# From Bresenham's line algorithm
# http://en.wikipedia.org/wiki/Bresenham%27s_line_algorithm
def drawLine(pic, col, x0, y0, x1, y1):

   dx = abs(x1-x0)
   dy = abs(y1-y0) 
   sx = sy = 0

   #sx = 1 if x0 < x1 else -1
   #sy = 1 if y0 < y1 else -1

   if (x0 < x1): 
     sx = 1 
   else: 
     sx = -1
   if (y0 < y1):
     sy = 1 
   else: 
     sy = -1

   err = dx - dy

   while (True):

     drawPoint(pic, col, x0, y0)

     if (x0 == x1) and (y0 == y1): 
       break

     e2 = 2 * err
     if (e2 > -dy):
       err = err - dy
       x0 = x0 + sx

     if (x0 == x1) and (y0 == y1):
       drawPoint(pic, col, x0, y0)
       break

     if (e2 <  dx):
       err = err + dx
       y0 = y0 + sy 


# Draw infinite line from segment
def drawInfiniteLine(pic, col, x0, y0, x1, y1):
   # y = m * x + b
   m = (y0-y1) / (x0-x1)
   # y0 = m * x0 + b   =>   b = y0 - m * x0
   b = y0 - m * x0

   x0 = 0
   y0 = int(m*x0 + b)
   # get a 2nd point far away from the 1st one
   x1 = getWidth(pic) 
   y1 = int(m*x1 + b)

   drawLine(pic, col, x0, y0, x1, y1)


# Draw infinite line from origin point and angle
# Angle 'theta' expressed in degres
def drawInfiniteLineA(pic, col, x, y, theta):

   # y = m * x + b
   dx = y * tan(theta * pi / 180.0)  # (need radians)
   dy = y

   if (dx == 0):
     dx += 0.000000001 # Avoid to divide by zero 

   m = dy / dx

   # y = m * x + b   =>   b = y - m * x
   b = y - m * x

   # get a 2nd point far away from the 1st one
   x1 = 2 * getWidth(pic)
   y1 = m*x1 + b

   drawInfiniteLine(pic, col, x, y, x1, y1)


# Draw multiple parallele lines, given offset and angle
def multiLines(pic, col, offset, theta, randOffset = 0):
   # Range is [-2*width, 2*width] to cover the whole surface
   for i in xrange(-2*getWidth(pic), 2*getWidth(pic), offset):
      drawInfiniteLineA(pic, col, i + random.randint(0, randOffset), 1, theta)

# Draw multiple lines, given offset, angle and angle offset
def multiLinesA(pic, col, offsetX, offsetY, theta, offsetA):
   j = 0
   # Range is [-2*width, 2*width] to cover the whole surface
   for i in xrange(-2*getWidth(pic), 2*getWidth(pic), offsetX):
      drawInfiniteLineA(pic, col, i, j, theta)
      j += offsetY
      theta += offsetA



file = pickAFile()
picture = makePicture(file)
color = makeColor(0, 65, 65) #pickAColor()
#drawline(picture, color, 10, 10, 100, 100)
#drawInfiniteLine(picture, color, 10, 10, 100, 100)
#drawInfiniteLineA(picture, color, 50, 50, 135.0)
#multiLines(picture, color, 20, 56.0)
#multiLines(picture, color, 10, 56.0, 15)
multiLinesA(picture, color, 10, 2, 1.0, 1.7) 

show(picture)

Output (Painting by Pierre Soulages) :

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Hope this gave some fun and ideas to JES students... And to others as well...