Steganography Coding Example

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Aus dem Kurs von Duke University

Programming Foundations with JavaScript, HTML and CSS

5042 Bewertungen

Duke University

Programming Foundations with JavaScript, HTML and CSS

5042 Bewertungen

Kurs 1 von 5 in Specialization Java Programming and Software Engineering Fundamentals

Learn foundational programming concepts (e.g., functions, for loops, conditional statements) and how to solve problems like a programmer. In addition, learn basic web development as you build web pages using HTML, CSS, JavaScript. By the end of the course, will create a web page where others can upload their images and apply image filters that you create. After completing this course, you will be able to: 1. Think critically about how to solve a problem using programming; 2. Write JavaScript programs using functions, for loops, and conditional statements; 3. Use HTML to construct a web page with paragraphs, divs, images, links, and lists; 4. Add styles to a web page with CSS IDs and classes; and 5. Make a web page interactive with JavaScript commands like alert, onClick, onChange, adding input features like an image canvas, button, and slider.

Aus der Unterrichtseinheit

MiniProject: Image Filters on the Web

Steganography Part I6:46

Steganography Part II8:28

Steganography Part III4:52

Steganography Coding Example7:34

Treffen Sie die Kursleiter

Susan H. Rodger
Professor of the Practice
Computer Science
Robert Duvall
Lecturer
Computer Science
Owen Astrachan
Professor of the Practice
Computer Science
Andrew D. Hilton
Associate Professor of the Practice
Electrical and Computer Engineering

Okay, we've walked through some of the concepts and

we told you we would walk you through the code to hide one image in another.

We've started here with the clear bits function which we'd seen before in

a previous video which just takes the Math.floor of the color

value divided by 16 and multiplies it by 16 to clear out the lower bit.

We've previously talked about the chop2hide function, and

we've written down here the steps for the algorithm for

that and we're just going to turn those into code.

So we need for(var px of Image.values),

that's how you've seen to iterate over the pixels in an image before.

And we want to clear the low bits of the red, so px.setRed to (clearbits),

this is the function we wrote to do the math for us of (px.getRed).

So we want to take the current red value

call clear bits to zero out the lower bits.

And then set the red to that new value.

We can do the same thing for the green.

And the blue.

And then when we're finished, our image that we've been modifying is the answer to

our whole function, that's what we're we want to give back to whoever called us.

Now, I've got some placeholders for

the other functions we're going to write here in a second.

And, at the bottom here, I've got this algorithm that we saw as our

high level algorithm where we combine chop to hide, shift and combined.

So at this point, these two functions don't do anything so

printing our overall answer is not going to do us much good.

I'm going to comment these lines out.

And instead, I'm just going to print start so

that we can make sure that works pretty well before we do anything else.

So if I run this code, I'm working with our picture of Ussain Bolt,

I get something that looks pretty similar as with the previous videos.

You can notice that the background colors have changed just a little

because I've been modifying these color values.

Great, now let's go work on shift.

So remember, this is where we're going to take those most significant bits, and move

them over to the least significant bit position for the image we want to hide.

So our algorithm is going to call for us to iterate over each Pixel in the image.

And what we want to do here is to shift over the red bits.

So we want to px.setred.

And remember we can do this by dividing by 16.

(Px.get Red / 16) because 16 is 2 to

the 4th power that would move everything over four binary digits or bits.

Now, we want the px.setGreen to (px.getGreen () /16).

And similarly, px.setBlue(px.getBlue() /16).

And again we've written this little bit of code,

we'd like to make sure that it works, or

at least look like it works before we move on.

We're not testing very thoroughly in this video,

we're going to encourage you to do more extensive testing of your own code.

So what if we now hide and we print out hide?

Oops, we left off, if we go back up here, we got sort of a weird error message,

cannot read property 'getString' of undefined.

What is that mean?

Well something was undefined that's generally a bad thing.

If I go back and look at this, I forgot to return my answer, ops.

And now we get an image that is mostly black.

If you remember from before that's what we want.

We had hidden all of this information with significant bits.

We can't really see it.

Even though we just fixed one little compiler error,

it was really good that we got that fixed before we had a big mess of code

where it might have been harder to find that problem.

Finally, we need this combine function.

We want to make a new image, we're going to call it answer.

It's going to be a new simple image of the same size, so

we want show.getWidth and show.getHeight.

Show is one of the parameters to combine the image we want to show up and

hide is the other image which we want to hide.

We're going to assume they're the same size right now.

That's not very robust.

You'll write some code to crop the images to be of the same size,

then you can work with any images and for

each pixel in answer.values, we want to get the x and y of that pixel so

var x = px.get x(), you should be pretty familiar with that by now,

and var y = px.get y(), again, a familiar thing.

We want to get the same pixel, showPixel we'll call it,

from the show image .getPixel with x,y.

And we want the same hidePixel hide.getPixel.

The same pixel from the hide image, and now we want to set px as red

to the sum of showPixels red plus a hidePixels red.

Remember, that's going to combine the two together.

One of them is going to have the most significant four bit set and

the other's zero.

The other's going to have the least significant four bit set, and the other 0,

so when they get added together we'll get a whole eight bit number.

And then we're going to do the same thing for

green and for blue.

And after doing that for each pixel, our answer is going to be the image answer.

So now I'm going to come down here.

I'm going to uncomment these last two lines.

Get rid of the extra one I wrote.

Now we get this image of Hussein Bolt that looks like what we expected.

Are we sure it's right?

Not really, we'd want to investigate a little more.

We could have just messed up and returned our start image somewhere or

something like that.

We're not going to test terrible rigorously in this video.

So that would take more time, we'd leave that up to you.

So with that, I'll leave you with one final important question.

What would happen if we hid an image of a stegosaurus inside of another image?

It would be stegasaurusonography.

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