CSC111 Adding a Red Disk to an Image

--D. Thiebaut (talk) 21:33, 13 March 2014 (EDT)

---

A Large Image for Processing

and an inspiration for Spring Break...

beach.jpg JPEG 1024x768 1024x768+0+0 8-bit sRGB 154KB 0.000u 0:00.000
786,432 pixels

Version 1: The first-pass program

# addRedDisk.py
# adds a disc in the middle of the image.
# The program measures the execution time of the function addRedDisk()
# and prints it on the screen.  The time is in seconds.

from time import time


# ==================================================================  
# distance()
# computes the distance squared between two points in 
# a cartesian system.
def distance( x1, y1, x2, y2 ):
  return sqrt( (x1-x2)*(x1-x2) + (y1-y2)*(y1-y2) )
  
# ==================================================================  
# addRedDisk: 
# adds a red disk to an image, located at center xCenter, yCenter,
# and with radius "radius"
def addRedDisk( image, xCenter, yCenter, radius ):
  for x in range(0,getWidth(image)):
    for y in range(0,getHeight(image)):
        if distance( x, y, xCenter, yCenter ) < radius:
          pixel = getPixel (image, x, y)
          setColor( pixel, makeColor( 255, 0, 0 ) )
  return image


# ==================================================================  
#                             MAIN PROGRAM
# ==================================================================  
file = pickAFile()
image = makePicture( file ) 
show( image )
  
t1 = time()
image = addRedDisk( image, getWidth(image)/2, getHeight(image)/2, 20 )
t2 = time()

repaint( image )
#writePictureTo( image, file )
printNow( t2 - t1 )

Output

======= Loading Progam =======
2.6670000553131104
>>> 

The program spends 2.66 seconds to run on an aging Mac Pro computer.

Version 2: Restricting the number of iterations

# addRedDisk.py
# adds a disc in the middle of the image.
# The program measures the execution time of the function addRedDisk()
# and prints it on the screen.  The time is in seconds.
from time import time


# distance()
# computes the distance squared between two points in 
# a cartesian system.
def distance( x1, y1, x2, y2 ):
  return sqrt( (x1-x2)*(x1-x2) + (y1-y2)*(y1-y2) )
  
# addRedDisk: 
# adds a red disk to an image, located at center xCenter, yCenter,
# and with radius "radius"
def addRedDisk( image, xCenter, yCenter, radius ):
  for x in range( xCenter-radius, xCenter+radius+1 ):
    for y in range( yCenter-radius, yCenter+radius+1 ):
        if x > 0 and y > 0 \
        and x < getWidth( image ) and y < getHeight( image ) \
        and distance( x, y, xCenter, yCenter ) < radius:
          pixel = getPixel (image, x, y)
          setColor( pixel, makeColor( 255, 0, 0 ) )
  return image


# ==================================================================  
#                             MAIN PROGRAM
# ==================================================================  
file = pickAFile()
image = makePicture( file ) 
show( image )
  
t1 = time()
image = addRedDisk( image, getWidth(image)/2, getHeight(image)/2, 20 )
t2 = time()

repaint( image )
#writePictureTo( image, file )
printNow( t2 - t1 )

Output

>>> 
======= Loading Progam =======
0.13000011444091797
>>> 

The program is now about 20 times faster and takes 0.13 seconds to execute.

Version 3: Squeezing some more Computation out of the Loop

# addRedDisk.py
# adds a disc in the middle of the image.
# The program measures the execution time of the function addRedDisk()
# and prints it on the screen.  The time is in seconds.
from time import time


# distance()
# computes the distance squared between two points in 
# a cartesian system.
def distance2( x1, y1, x2, y2 ):
  return (x1-x2)*(x1-x2) + (y1-y2)*(y1-y2)
  
# addRedDisk: 
# adds a red disk to an image, located at center xCenter, yCenter,
# and with radius "radius"
def addRedDisk( image, xCenter, yCenter, radius ):
  radius2 = radius * radius
  for x in range( xCenter-radius, xCenter+radius+1 ):
    for y in range( yCenter-radius, yCenter+radius+1 ):
        if x > 0 and y > 0 \
        and x < getWidth( image ) and y < getHeight( image ) \
        and distance2( x, y, xCenter, yCenter ) < radius2:
          pixel = getPixel (image, x, y)
          setColor( pixel, makeColor( 255, 0, 0 ) )
  return image


# ==================================================================  
#                             MAIN PROGRAM
# ==================================================================  
file = pickAFile()
image = makePicture( file ) 
show( image )
  
t1 = time()
image = addRedDisk( image, getWidth(image)/2, getHeight(image)/2, 20 )
t2 = time()

repaint( image )
#writePictureTo( image, file )
printNow( t2 - t1 )

Output

>>> 
======= Loading Progam =======
0.0969998836517334
>>> 

The program that doesn't use the square-root function is now about 1/3 faster than the previous one.