Drawing Diagonal Lines Across a Picture

Question

I am trying to draw parallel lines diagonally from the top right corner to the bottom left corner of a picture. I want it to look like this (lovely paint pic)

def diagTopLBottomR():
  pic=makePicture(pickAFile())
  w=getWidth(pic)
  h=getHeight(pic)
  x1=0
  y1=0
  x2=0
  y2=0
  i=0
  while i<11:
    x1=10*i
    y2=10*i
    i+=1
    for y in range (y1,y2):
      x = (y-y1)*(x2-x1)/(y2-y1) +x1
      px=getPixel(pic,x,y)
      color=makeColor(0,0,0)
      setColor(px, color)
  x3=0
  y3=h
  x4=w
  y4=0
  j=0
  while j<10:
    x3=10*j
    y4=10*j
    j+=1
    for y in range (y3,y4):
      x = (y-y3)*(x4-x3)/(y4-y3) +x3
      px=getPixel(pic,x,y)
      color=makeColor(0,0,0)
      setColor(px, color)

  return(pic)

You'll note that the x3 will either be max value, causing an outof range exception, or the y range will start with a higher value ie (y3>y4) and doesn't work in reverse, or when I decrement it. It's like a paradox.

The first loop is working, no matter what I try I cannot get the second loop to work. This is what I'm ending up with.

Any ideas? Thanks.

---

Edit

I have played around with the ranges, and either get no result for the second loop, as shown above of an out of range exception.

I have tried:

  x3=0
  y3=h
  x4=w
  y4=0
  j=0
  while j<10:
    x3=10*j
    y4=10*j
    j+=1
    for x in range (x3,x4):
      y = (x-x3)*(y4-y3)/(x4-x3) +y3

---

Stole Unicorns from here.

Answer 1

In the first part, y1 is set to 0 and y2 increases from 0 in the loop, so y1 < y2. This is fine because you use

for y in range (y1,y2)

In the second part, y3 is set to h (128 in your case, I guess) and y4 increases from 0 in the loop, so y3 > y4. This is NOT fine because you use

for y in range (y3,y4)

You could try stepping backwards by giving range() a third parameter indicating a step size of -1. Or you could switch y3 and y4 (be cautious of what this does to the rest of your code).

Answer 2

range() assumes the first parameter is less than the second parameter, and it goes in an ascending order. You have:

for y in range (y3,y4):

where y3=h and y4=0 (on the first pass). Since y3 > y4, this loop does nothing. You can use either:

for y in range(y4,y3):

or

for y in range(y3,y4,-1):

Answer 3

In the second loop, y3 is greater (or equal) than y4. So, try range (y4,y3).

Answer 4

I solved it by making px=getPixel(pic,x,y-1) and using the suggested answers of decrementing y range.

def diagTopLBottomR():
  pic=makePicture(pickAFile())
  w=getWidth(pic)
  h=getHeight(pic)
  x1=0
  y1=0
  x2=0
  y2=0
  i=0
  while i<10:
    x1=10*i
    y2=10*i
    i+=1
    for y in range (y1,y2):
      x = (y-y1)*(x2-x1)/(y2-y1) +x1
      px=getPixel(pic,x,y)
      color=makeColor(0,0,0)
      setColor(px, color)
  x3=0
  y3=h
  x4=w
  y4=0
  j=0
  while j<10:
    x3=10*j
    y4=10*j
    j+=1
    for y in range (y3,y4,-1):#change here
      x = abs((y-y3)*(x4-x3)/(y4-y3) +x3)
      px=getPixel(pic,x,y-1)#change here
      color=makeColor(0,0,0)
      setColor(px, color)

  return(pic)