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Unformatted text preview: 0/fall/psets/5/garfinkel.pdf Though somewhat technical, you should find the article’s language quite accessible. Once you’ve read the article, answer each of the following questions in a sentence or more in ~/pset5/questions.txt. 4. 5. What happens, technically speaking, when a file is deleted on a FAT file system? What can someone like you do to ensure (with high probability) that files you delete cannot be recovered? 1 Look back at Problem Set 4 if your memory needs to be jogged! For this question, you’re welcome to consult How Computers Work, Google, Wikipedia, a friend, or anyone else, so long as your words are ultimately your own! 2 2 < 16 This is CS50. Harvard College Fall 2010 Whodunit. Welcome to Tudor Mansion. Your host, Mr. John Boddy, has met an untimely end—he’s the victim of foul play. To win this game, you must determine the answer to these three questions: Who done it? Where? And with what weapon? Unfortunately for you (though even more unfortunately for Mr. Boddy), the only evidence you have is a 24 ­bit BMP file called clue.bmp, pictured below, that Mr. Boddy whipped up on his computer in his final moments.3 Hidden among this file’s red “noise” is a message from him to you. You long ago threw away that piece of red plastic from childhood that would solve this mystery for you, and so you must attack it as a computer scientist instead. But, first, some background. 3 Realize that this BMP is in color even though you might have printed this document in black and white. 3 < 16 This is CS50. Harvard College Fall 2010 Perhaps the simplest way to represent an image is with a grid of pixels (i.e., dots), each of which can be of a different color. For black ­and ­white images, we thus need 1 bit per pixel, as 0 could represent black and 1 could represent white, as in the below.4 In this sense, then, is an image just a bitmap (i.e., a map of bits). For more colorful images, you simply need more bits per pixel. A file format (like GIF) that supports “8 ­bit color” uses 8 bits per pixel. A file format (like BMP, JPEG, or PNG) that supports “24 ­bit color” uses 24 bits per pixel.5 A 24 ­bit BMP like Mr. Boddy’s uses 8 bits to signify the amount of red in a pixel’s color, 8 bits to signify the amount of green in a pixel’s co...
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