Pixel Calculator Activities - a Scavenger Hunt

Factoid

Did you know that police use image processing to match up fingerprints of criminals?

Just a refresher

This will help you remember the steps for finding a secret message in a mystery image.

1.  Find the gray value of any pixel of the message in the mystery image.
    Usually you are given the coordinates of one message pixel, and then you zoom
    in to that pixel and write down its gray value. Sometimes you are just given
    hints like "it's the only odd numbered value in the region ((12,45) -(34,89))."

2.  Use the rectangle selection tool to select part or all of the picture. 

3.  Use the calculator to show the message.  First use subtraction to make all lower 
    valued pixels 0: # - (message gray value - 1).  Then use multiplication to make 
	all higher valued pixels 255: # * 128. 

Now let's try it on an example message:

	Load the image winter.bmp by selecting File then Get Image.
	Zoom in to coordinates (214, 235).

What is the gray value of this pixel?

	Use the rectangle selection tool to select the upper right hand corner of the image.
	Use subtraction to make all of the lower valued pixels 0 (hint: # - ).
	Use multiplication to make all of the higher valued pixels 255 (hint: # * ).

What was the hidden message?

A Scavenger hunt

Ok, that was just practice. Now we're going to go on a scavenger hunt. There are three clues to finding the final hidden message. Let's find the first clue:

Clue 1

	Load the image winter.bmp by selecting File then Get Image.
	Zoom in to the coordinates (30,28). 

What is the gray value of this pixel?

Are there any other odd numbers in the area? If so, what?

	Use the rectangle selection tool to select the part of the image that you can see.
	Use the calculator to make all of the lower valued pixels 0 (hint: # - ).
	Use the calculator to make all of the higher valued pixels 255 (hint: # * ).

What was the hidden message?

Clue 2

The last hidden message was the name of the file you should load to continue on the hunt.

	Load the image with the name you found by selecting File then Get Image.
	Zoom in somewhere near the pixel at (160,317), until you can read the numbers. 
	Draw a rectangle around ((160,317)-(167,324)). 

	Use the calculator to make all of the lower valued pixels 0 (hint: # - ).
	Use the calculator to make all of the higher valued pixels 255 (hint: # * ). 

The secret message is the first (X) coordinate of a special pixel. What is it?

Clue 3

	Now, zoom in somewhere near the pixel at (53,350), until you can read 
	   the numbers. 
	Draw a rectangle around ((53,350)-(60,355)). 

What is the odd gray value in the rectangle you drew?

	Use the calculator to make all of the lower valued pixels 0 (hint: # - ).
	Use the calculator to make all of the higher valued pixels 255 (hint: # * ). 

The secret message is the second (Y) coordinate of the special pixel. What is it?

The final message!

Well you've found the file name and both pixel coordinates of a secret message pixel. Now you have all that you need to discover the secret message.

Good luck!

What does the secret message say?

What did you do to find this secret message?

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