1) Introduction

• A picture can be treated as a long list of pixels.
• This lecture shows you how to
  • Create programs to manipulate digital images

2) You Can Skip This Lecture If...

• You know what a pixel is
• You understand what RGB stands for
• You have worked with PIL or a similar image-processing library

3) Pictures and Pixels

Figure 8.1: Pixels in a Picture

• Digital images are made up of pixels, which are tiny dots.
• That's what 1024 x 768 resolution refers to: 1024 pixels wide, 768 pixels high
• Pixel (0, 0) is upper left
• Pixel (1023, 0) is upper right
• Pixel (0, 767) is lower left
• Pixel (1023, 767) is lower right

4) Color Representation

Figure 8.2: Color Combinations

• Colors: combinations of red, green, and blue
• Each part has intensity in range 0 - 255
• Black: (0, 0, 0)
• White: (255, 255, 255)
• Red: (255, 0, 0)
• Green: (0, 255, 0)
• Blue: (0, 0, 255)

5) PyMedia Introduction

media.create_picture(width, height, color)

Create a width by height color picture

media.choose_file()

opens a file dialog and lets the user select a file

media.load_picture(filename)

tells Python to load the picture from file named filename

6) PyMedia: Pictures

media.show(pic)

makes the picture appear on the screen

media.get_width(pic)

Retrieve the width of the picture

media.get_height(pic)

Retrieve the height of the picture

7) PyMedia: Pixels and Colors

pixel = media.get_pixel(pic, x, y)

Retrieve pixel (x, y) from Picture pic.

c = media.get_color(pixel)

Retrieve the color of a pixel

media.set_color(pixel, c)

Set the color of a pixel to c

media.get_green(pixel)

Retrieve the green value (an int) of a pixel

media.set_green(pixel, g)

Set the green value of a pixel to int g

c = media.create_color(r, g, b)

Create color (r, g, b)

8) PyMedia: Iterating over Pixels

import media
import color

pic = media.create_picture(100, 200, media.black)

for i in range(media.get_width(pic)):
  for j in range(media.get_height(pic)):
  pixel = media.get_pixel(pic, i, j)
  media.set_color(pixel,
    media.create_color(i % 255, j % 255, 0))

pic.show()

9) Task: Cheesy Daytime to Sunset

Figure 8.3: Niagara During the Day

Figure 8.4: Niagara at Sunset

10) A Sunset Algorithm

For each pixel in pic:
  get the blue component of the pixel
  reduce that number by 30%
  set the blue component of the pixel to the reduced number

  get the green component of the pixel
  reduce that number by 30%
  set the green component of the pixel to the reduced number

11) Task: Send Greg Around the World

Figure 8.5: Greg Before Travelling

12) Lake Elmore

Figure 8.6: Lake Elmore, Vermont

13) India Tea Plantation

Figure 8.7: India Tea Plantation, Kerala Province

14) Giraffes in Africa

Figure 8.8: Giraffes in Africa

15) Giant Mosquito

Figure 8.9: Giant Mosquito