Lab 4: Modifying Pixels in a Range

Using `range` to modify pixels in a subsection of an image

In the lab, we created a function that used range and a nested for loop to do pixel operations on a subsection of an image.

def clearGreenInRange(picture):
  for x in range(35,127):
    for y in range(10,201):
      p = getPixel(picture,x,y)
      setGreen(p,0)

This function selected a specific region of the image to clear the green component of the region's pixels. We "hard-coded" the range into the function. However, it would be far more useful to design the function to take any range of x and y coordinates.

def clearGreenInRange(picture,x0,y0,x1,y1):
  for x in range(x0,x1+1):
    for y in range(y0,y1+1):
      p = getPixel(picture,x,y)
      setGreen(p,0)

Remember:

Your second value for the range function must be the last index you want plus one.
Formatting using white-space on the left side of your code is critical! Get into the habit of reaching for the space bar (or tab key) as soon as you type a colon, since that usually implies that you're about to start a new, nested block of code (i.e. a new scope).

`if` statements

From the Python Tutorial:

>>> x = int(raw_input("Please enter an integer: "))
>>> if x < 0:
...      x = 0
...      print 'Negative changed to zero'
... elif x == 0:
...      print 'Zero'
... elif x == 1:
...      print 'Single'
... else:
...      print 'More'
...

So, in the case of problem 3.9 from last lab's assignment:

def changeColor(picture,amount,rgb):
  for p in getPixels(picture):
    if (rgb == 1):
      redValue = getRed(p)
      setRed(p,(1+amount)*redValue)
    elif (rgb == 2):
      greenValue = getGreen(p)
      setGreen(p,(1+amount)*greenValue)
    elif (rgb == 3):
      blueValue = getBlue(p)
      setBlue(p,(1+amount)*blueValue)
    else:
      print 'Please enter a 1 (red), 2 (green), or 3 (blue) as the third argument'
      return

Here we used an "if/elif/else" structure to chose the correct color to do our pixel operation. A few things to notice:

The expression between the "if" or "elif" and the colon is commonly referred to as a boolean expression. It is the test condition that, if true, causes Python to run the associated block of code. In Python, it is not required that the expression yield a true boolean value, but that is the common practice and is highly recommended.
It is possible in this particular case that we could have used three separate "if" statements (i.e. not used the "if/elif" construct) and still arrived at our desired behavior, but that is not recommended. Any time you are doing a test on a single variable whose value will determine the behavior of your function or method, you should use the "if/elif" control structure.

MediaTools

setMediaPath and getMediaPath

>>> setMediaPath()                      # this will ask to set a directory where media is located
>>> filePath = getMediaPath('arch.jpg') # this will return path to arch.jpg in media directory
>>> picture = makePicture(filePath)     # this loads the arch image into memory

Picture Tool...

In the MediaTools menu item, there is a "Picture Tool..." option. This is used to garner information about individual pixels for any picture object we have loaded into memory and is directly referenced by an individual variable in the interpreter's scope. So, during the lab, when I created a list of picture objects, these objects were not available to MediaTools until I directly referenced one of them by an individual variable.

I.e.

>>> filePath = getMediaPath('arch.jpg')
>>> pictureList = []
>>> for i in range(5):
...   pictureObject = makePicture(filePath)
...   pictureList.append(pictureObject)
>>> pictureList[0]
Picture, filename Y:\cs106\MediaSources\arch.jpg height 350 width 263
>>>
>>> # MediaTools cannot see pictureList[0]
>>>
>>> archPicture = pictureList[0]
>>>
>>> # MediaTools *can* see archPicture

Code from Lab 4

Here is the code that we worked on during the lab. If you have any questions about it, feel free to email me at cs106ta@cis.uab.edu.

lab4code.py

LAB ASSIGNMENT

Problems 4.1, 4.2, 4.3, 4.4, 4.5 from chapter 4 in the textbook.

These are due by 3:30 next Thursday (9-18-08). After then, they will be considered late, and points will be deducted.

Last modified:	09-12-2008
By:	David O'Gwynn