Python-Tutorial-3

P1 Images

In the previous tutorial we didn't succeed to create a P6 image.

So we will try with the P1 version of the portable images format.

Indeed this one is purely textual.

w = 7  # width
h = 10 # height

print(f"P1")
print(f"{w} {h}")

img = []

for y in range(0, h):
  line = []
  for x in range(0, w):
    line.append(0)
  img.append(line)

for y in range(0, h):
  for x in range(0, w):
    c = img[y][x]
    print(f" {c}", end="")
  print('', end="\n")

We will get an image similar to this one:

P1
7 10
0 0 0 0 0 0 0
0 0 1 1 1 0 0
0 1 0 0 0 1 0
0 1 0 0 0 1 0
0 1 0 0 0 1 0
0 1 0 0 0 1 0
0 1 0 0 0 1 0
0 1 0 0 0 1 0
0 0 1 1 1 0 0
0 0 0 0 0 0 0

Here is what we get if we visualise it:

As we saw in the previous tutorial, after we start with a very simple version, we can organise the different parts with small functions:


def create_img(w, h):
  img = []
  for y in range(0, h):
    line = []
    for x in range(0, w):
      line.append(0)
    img.append(line)
  return img

def print_img(img):
  w = len(img[0])
  h = len(img)
  print(f"P1")
  print(f"{w} {h}")
  for y in range(0, h):
    for x in range(0, w):
      c = img[y][x]
      print(f" {c}", end="")
    print('', end="\n")

def draw_rect(img, rect, value):
  _x = rect[0]
  _y = rect[1]
  _w = rect[2]
  _h = rect[3]
  for y in range(_y, _y + _h):
    for x in range(_x, _x + _w):
      img[y][x] = value

def main():
  w = 7  # width
  h = 10 # height
  img = create_img(w, h)
  draw_rect(img, [2, 2, 3, 2], 1)
  print_img(img)

main()

Here there is also the draw_rect() add'd at the same time, but it's better to do it in two steps.

If you add too much code at the same time, the probability to make an error is higher.

Compared to the previous draw_rect() function, here we group all the coords in a single rect value.

Again we can add the bound check after:


def draw_rect(img, rect, value):
  w = len(img[0])
  h = len(img)
  _x = rect[0]
  _y = rect[1]
  _w = rect[2]
  _h = rect[3]
  for y in range(_y, _y + _h):
    for x in range(_x, _x + _w):
      if x >= 0 and x < w:
        if y >= 0 and y < h:
          img[y][x] = value

Here is the result:

Then all the coords of a rectangle can be group'd in a single value as you can see:

  rect = [20, 20, 70, 40]

Then we can try to create a draw_circ() function,

import math

def dist(p1, p2):
  x1 = p1[0]
  y1 = p1[1]
  x2 = p2[0]
  y2 = p2[1]
  dx = x2 - x1
  dy = y2 - y1
  sq_dist = (dx * dx) + (dy * dy)
  return int(math.sqrt(sq_dist))

def draw_circ(img, c, r, value):
  w = len(img[0])
  h = len(img)
  cx = c[0]
  cy = c[1]
  _x1 = cx - r
  _y1 = cy - r
  _x2 = cx + r
  _y2 = cy + r
  for y in range(_y1, _y2 + 1):
    for x in range(_x1, _x2 + 1):
      d = dist([cx, cy], [x, y])
      if d < r:
        img[y][x] = value


def main():
  w = 100 # width
  h = 100 # height
  img = create_img(w, h)
  draw_rect(img, [0, 0, 100, 100], 1)
  draw_rect(img, [20, 20, 40, 40], 0)
  draw_circ(img, [20, 20], 10, 0)
  print_img(img)

main()

For this function, we also need a dist() function. Very often a function will need a sub-function.

Here is the result that we get:

Here the draw_circ() function also need a bound check, just like the draw_rect() function.

Going Further

You can try to draw other kind of shapes.

Usage Notice

License for the tutorial parts:

license: FDL

License for the code parts:

To the extent permitted by law:
 spdx=any

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