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Lab 31: Images

As usual, create a directory to hold today's files. All programs that you write today should be stored in this directory. 

$ cd ~/cs120/labs
$ mkdir lab31 
$ cd lab31

Practice Problem 1

Write a function check_color(a_color), which takes as a parameter a list of 3 integers. Each of the integers are guaranteed to be in the range \([0, 256)\). Your function should return \(True\) if the provided parameter is "green". Consider a list green if the value at the first index is less than \(75\), if the second value is greater than \(175\), and the third value is less than \(75\). Your function should return \(False\) in all other cases. 









PPM Module



  Image files can be thought of as just 2-dimensional lists of pixel
  color values.  We can even extend that thought
  to 3-dimensional lists, since every pixel value can be
  represented by a red, green,
  and blue value (typically abbreviated RGB).  The
  one complexity with this representation is how to get an image file
  into a list of this structure.  Luckily, we are giving you a module
  that will do just that.




  The ppm.py module
  holds 4 functions, but you are only concerned with two of them.
  the read_image(file_name) function takes a string file
  name of a PPM image, and returns a 3-dimensional list of pixel color
  values.  For any given pixel color list, the red value is store in
  the 0th index, the green value is stored in the
  1st index, and the blue value is stored in the
  2nd index.




  The write_image(ppm_file_name, image) takes a string
  file name of where you want to write a PPM image, and a
  3-dimensional list of the above form.  This function will actually
  write the image out to the file system, using the file name
  specified in the parameters.







Green Screen



  It is pretty expensive to film movies and TV shows on location
  now-a-days.  However, with a little bit
  of Movie Magic, we can pretend to be
  anywhere in the world.  Ever want to pretend you traveled to the
  expanses of Moscow?  Now is your chance!  Ever want to go to
  Delaware?  Yeah… Delaware.




  Use Google image search to find a green screen image, and a
  background image that is at least the same size, if not
  larger.  You are going to write a program
  that will take these two images, and composite them together to
  create one image.  You will need to convert these pictures to PPM
  files, as demonstrated in class.




Details



  Write a function overlay_greenscreen(greenscreen_list,
  background_list) in a file called greenscreen.py.
  This function takes two parameters: an m ×
  n 2-dimensional list of RGB values (really just a
  3-dimensional list of integers), and a  j ×
  k 2-dimensional list of RGB values (but really just a
  3-dimensional list of integers), where  j ≥
  m and k ≥ n.  This function
  should modify greenscreen_list by replacing pixels that are
  mostly green, with the corresponding pixel
  from background_list.




  As it turns out, it's really hard to get a green screen to be
  exactly green.  Thus, it is not sufficient to check just the
  value of the green portion of a given pixel.  You need to
  make sure the green value is high enough, and that the red
  and blue values are low enough to make sure the color is
  truly "green."




Example


Here is a very simplistic example of what you are trying to do:



  
  + 
  = 




Hint




  
  

    
    
      
  • 
	
    
	  The overlay_greenscreen function will need to
	  analyze every pixel in the greenscreen_list.  Use
	  nested for loops to iterate over all indices for the rows
	  and columns.
	
    
      
    • 
      
  • 
	
    
	  Define three global constants
	  called RED_THRESHOLD, GREEN_THRESHOLD,
	  and BLUE_THRESHOLD.  These should be integer
	  values in the range [0, 255].  With these variables, a given
	  pixel is replaced with a pixel from from the background only
	  if the red and blue colors are below this defined threshold,
	  and the green value is greater than the threshold.
	
    
      
    • 
      
  • 
	
    
	  You will likely have to play around with the threshold
	  values you define for the RGB values to get a better green
	  screen effect.  The thresholds are going to be very
	  different from what you expect.  Can you figure out why this
	  is the case?
	
    
      
    • 
    

  





 



  
Challenge

  

    It's one thing to use a green screen image you found on the
    Internet.  It's another thing entirely to make your own green
    screen images!  Luckily, we have this nice (hopefully pretty
    clean) chalkboard that we could you to mimic a green screen.
  

  

    Take a picture with your cellphone (or a friends) of yourself
    standing in front of the chalkboard.  Email the picture to
    yourself, and try to composite your image with a different image
    you found on the web.  You will likely have to make drastic
    changes to your threshold values, but you should be able to get
    something that looks decent by the end.