Pixel Calculator Activities - Secret Messages

Look for a message

	Now Load the file mystery.bmp by selecting File then Get Image.
	Draw a rectangle around the trees under the mountain in the image. 	
	Enter: # * 4 then press Compute on the calculator.
	Zoom in on the trees once or twice.

What does the hidden message say?

Secret Messages

Now we're going to learn how to find secret messages hidden inside an image. The technique we're going to use is called image thresholding. Remember that the coordinates are shown on the bottom left of the screen.

	Load the image bus.bmp by selecting File then Get Image.

Now we're going to zoom in on a particular pixel to find out its value.

	Zoom in on the front driver's side tire of the first bus 3 times.
	Zoom in on pixel (261, 80) until you can read the numbers.

What is the gray value of the pixel at (261, 80)?

What are some of the other gray values around the pixel at (261, 80)?

The pixels that equal 13 are actually secret message pixels!

	Select the rectangle tool.
	Draw a rectangle around as much of the tire as you can.
	Enter on the calculator:  # - 12 then press Compute.

All of the message pixels now have value 1, and everything that was greater than 13 is now greater than 2. But its still too hard to read the message!

	Enter on the calculator:  # * 128 then press Compute.

What do you see in the selected rectangle?

Let's try it by hand

Doing the calculations by hand will help you to understand what is going on. Remember that the secret message gray value is 13. There are 3 steps to finding the secret message.

	1.  Zoom in on a pixel and find out its gray value (in this case it was 13).
	2.  Select a region around the pixel and on the calculator enter:
		# - (gray value - 1), then press Compute.
	    (In this case we did # - 12)
	3.  On the calculator enter: # * 128, then press Compute.

Now fill in the boxes.
-12 = * 128 =
-12 = * 128 =
-12 = * 128 =

Let's try it again

	Load the image lion.bmp by selecting File then Get Image.
	Zoom in in somewhere near pixel (233,111), until you can read the numbers.
		(hint: it's on baby Simba)

All the pixels in the secret message have value 213, can you find the message?

	Draw a rectangle around as much of Simba as you can.
	Enter: # - 212 then press Compute on the calculator. 

All of the message pixels now have value 1, and everything that was less than 213 is now 0. But it is still too hard to read the message!

	Enter # * 128 then press Compute on the calculator.

Now all of the message pixels have value of 128 and look gray, and everything else around it is 0 (black) or 255 (white).

What does the secret message say?

But wait, there's more!

	Zoom out until you can see the whole picture.
	Zoom in somewhere near pixel (125, 103), until you can read the numbers.
		(hint: it's in Scar's black mane)

What is the value of the secret message pixel?

	Draw a rectangle around as much of Scar as you can.
	Enter: # - 30 then press Compute on the calculator. 

All of the message pixels now have value 1, and everything that was less than 31 is now 0. But it is still too hard to read the message!

	 Enter # * 128 then press Compute on the calculator. 

What does the secret message say?

Factoid:

Did you know that scientists use image processing to fix up images of the earth, of other planets, and of star galaxies, to find out hidden information?

Secret messages on the mountain

There is a secret message hidden on the mountain in this image.

	Load the image mystery.bmp by selecting File then Get Image.
	Zoom in somewhere near pixel (149, 336), until you can read the numbers. 

(149, 336) is a secret message pixel and has a gray value of 201. Actually all the pixels in the secret message have a gray value 201.

	Zoom out twice. 
	Draw a rectangle around as much of the mountain as you can.
	Enter: # - 200 then press Compute on the calculator.

All of the message pixels now have value 1, and everything that was less than 201 is now 0. But it is still too hard to read the message!

	Enter: # * 128 then press Compute on the calculator.

What does the secret message say?

Secret messages on the map

Here are some secret message hidden on a map of Puget Sound in Washington.

	Load the image puget.bmp by selecting File then Get Image.
	Zoom in somewhere near pixel (229,313), until you can read the numbers.

All the pixels in the secret message have value 20, can you find the message?

	Draw a rectangle around as much of the screen as you can.
	Enter: # - 19 then press Compute on the calculator.

All of the message pixels now have value 1, and everything that was less than 20 is now 0. But it is still too hard to read the message!

 	Use multiplication on the calculator to make all the higher valued pixels 255
		(hint: Enter # *  then press Compute on the calculator).

What does the secret message say?

Now let's find another hidden messages!

	Load a fresh copy of puget.bmp by selecting File then Get Image.

The next message is hidden in Lake Washington, Lake Washington is the dark spot that is just east (to the right) of Seattle.

	Zoom in somewhere near pixel (301, 349), until you can read the numbers. 

The pixel at coordinates (301, 349) is a secret message pixel.

What is the gray value of that pixel?

	Draw a rectangle around as much of the screen as you can.
	Then use subtraction to make all lower valued pixels 0
		(Hint: Enter # - (gray value - 1) then press Compute on the calculator)
 	Use multiplication on the calculator to make all the higher valued pixels 255
		(hint: Enter # *  then press Compute on the calculator).

What is the secret message?

Factoid:

UW is the abbreviation for the University of Washington. That's where this program was written.

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