want to resize or recover some deletedpics #recover
I spent the past several days snapping photos of people I know, all of which were saved by my digital camera as JPEGs on a 1GB CompactFlash (CF) card. Unfortunately, I somehow deleted them all! Thankfully, in the computer world, "deleted" tends not to mean "deleted" so much as "forgotten." My computer insists that the CF card is now blank, but I’m pretty sure it’s lying to me.
Write in ~/workspace/pset4/jpg/recover.c a program that recovers these photos.
Even though JPEGs are more complicated than BMPs, JPEGs have "signatures," patterns of bytes that can distinguish them from other file formats. Specifically, the first three bytes of JPEGs are 0xff 0xd8 0xff from first byte to third byte, left to right. The fourth byte, meanwhile, is either 0xe0, 0xe1, 0xe2, 0xe3, 0xe4, 0xe5, 0xe6, 0xe7, 0xe8, 0xe8, 0xe9, 0xea, 0xeb, 0xec, 0xed, 0xee, of 0xef. Put another way, the fourth byte’s first four bits are 1110.
Odds are, if you find this pattern of four bytes on a disk known to store photos (e.g., my CF card), they demark the start of a JPEG.
Fortunately, digital cameras tend to store photographs contiguously on CF cards, whereby each photo is stored immediately after the previously taken photo. Accordingly, the start of a JPEG usually demarks the end of another. However, digital cameras generally initialize CF cards with a FAT file system whose "block size" is 512 bytes (B). The implication is that these cameras only write to those cards in units of 512 B. A photo that’s 1 MB (i.e., 1,048,576 B) thus takes up 1048576 ÷ 512 = 2048 "blocks" on a CF card. But so does a photo that’s, say, one byte smaller (i.e., 1,048,575 B)! The wasted space on disk is called "slack space." Forensic investigators often look at slack space for remnants of suspicious data.
The implication of all these details is that you, the investigator, can probably write a program that iterates over a copy of my CF card, looking for JPEGs' signatures. Each time you find a signature, you can open a new file for writing and start filling that file with bytes from my CF card, closing that file only once you encounter another signature. Moreover, rather than read my CF card’s bytes one at a time, you can read 512 of them at a time into a buffer for efficiency’s sake. Thanks to FAT, you can trust that JPEGs' signatures will be "block-aligned." That is, you need only look for those signatures in a block’s first four bytes.
Realize, of course, that JPEGs can span contiguous blocks. Otherwise, no JPEG could be larger than 512 B. But the last byte of a JPEG might not fall at the very end of a block. Recall the possibility of slack space. But not to worry. Because this CF card was brand-new when I started snapping photos, odds are it’d been "zeroed" (i.e., filled with 0s) by the manufacturer, in which case any slack space will be filled with 0s. It’s okay if those trailing 0s end up in the JPEGs you recover; they should still be viewable.
Now, I only have one CF card, but there are a whole lot of you! And so I’ve gone ahead and created a "forensic image" of the card, storing its contents, byte after byte, in a file called card.raw. So that you don’t waste time iterating over millions of 0s unnecessarily, I’ve only imaged the first few megabytes of the CF card. But you should ultimately find that the image contains 50 JPEGs.
For simplicity, you should hard-code "card.raw" in your program; your program need not accept any command-line arguments. When executed, though, your program should recover every one of the JPEGs from card.raw, storing each as a separate file in your current working directory. Your program should number the files it outputs by naming each ###.jpg, where ### is three-digit decimal number from 000 on up. (Befriend sprintf.) You need not try to recover the JPEGs' original names. To check whether the JPEGs your program spit out are correct, simply double-click and take a look! If each photo appears intact, your operation was likely a success!
Implement in resize.c a program called resize that resizes 24-bit uncompressed BMPs by a factor of n. Your program should accept exactly three command-line arguments, per the below usage, whereby the first (n) must be a positive integer less than or equal to 100, the second the name of the file to be resized, and the third the name of the resized version to be written.
Spend some time thinking about what it means to resize a BMP. Decide which of the fields in BITMAPFILEHEADER and BITMAPINFOHEADER you might need to modify. Consider whether or not you’ll need to add or subtract padding to scanlines.
Usage: ./resize n infile outfile
Original image: smiley.bmp
Resized image (4x zoom): bigsmiley.bmp
