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.

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. Hidden among this file’s red "noise" is a drawing of 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 whodunit .

Okay, stop! Don’t proceed further until you’re sure you understand why 0000ff represents a red pixel in a 24-bit BMP file.

To be clear, recall that a hexadecimal digit represents 4 bits. Accordingly, ffffff in hexadecimal actually signifies 111111111111111111111111 in binary.

Because we’ve presented these bits from left to right, top to bottom, in 8 columns, you can actually see the red smiley if you take a step back.

Now let’s get more technical. Recall that a file is just a sequence of bits, arranged in some fashion. A 24-bit BMP file, then, is essentially just a sequence of bits, (almost) every 24 of which happen to represent some pixel’s color. But a BMP file also contains some "metadata," information like an image’s height and width. That metadata is stored at the beginning of the file in the form of two data structures generally referred to as "headers" (not to be confused with C’s header files). (Incidentally, these headers have evolved over time. This problem set only expects that you support version 4.0 (the latest) of Microsoft’s BMP format, which debuted with Windows 95.) The first of these headers, called BITMAPFILEHEADER , is 14 bytes long. (Recall that 1 byte equals 8 bits.) The second of these headers, called BITMAPINFOHEADER , is 40 bytes long. Immediately following these headers is the actual bitmap: an array of bytes, triples of which represent a pixel’s color. (In 1-, 4-, and 16-bit BMPs, but not 24- or 32-, there’s an additional header right after BITMAPINFOHEADER called RGBQUAD , an array that defines "intensity values" for each of the colors in a device’s palette.) However, BMP stores these triples backwards (i.e., as BGR), with 8 bits for blue, followed by 8 bits for green, followed by 8 bits for red. (Some BMPs also store the entire bitmap backwards, with an image’s top row at the end of the BMP file. But we’ve stored this problem set’s BMPs as described herein, with each bitmap’s top row first and bottom row last.) In other words, were we to convert the 1-bit smiley above to a 24-bit smiley, substituting red for black, a 24-bit BMP would store this bitmap as follows, where 0000ff signifies red and ffffff signifies white; we’ve highlighted in red all instances of 0000ff .

Incidentally, HTML and CSS (languages in which webpages can be written) model colors in this same way. If curious, see http://en.wikipedia.org/wiki/Web_colors for more details.

If the R, G, and B values of some pixel in a BMP are, say, 0xff, 0x00, and 0x00 in hexadecimal, that pixel is purely red, as 0xff (otherwise known as 255 in decimal) implies "a lot of red," while 0x00 and 0x00 imply "no green" and "no blue," respectively. Given how red Mr. Boddy’s BMP is, it clearly has a lot of pixels with those RGB values. But it also has a few with other values.

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 color, and 8 bits to signify the amount of blue in a pixel’s color. If you’ve ever heard of RGB color, well, there you have it: red, green, blue.

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. (BMP actually supports 1-, 4-, 8-, 16-, 24-, and 32-bit color.)

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. (Image adapted from http://www.brackeen.com/vga/bitmaps.html .)

If smiley.bmp actually contained ASCII characters, you’d see them in `xxd’s rightmost column instead of all of those dots.

In the leftmost column above are addresses within the file or, equivalently, offsets from the file’s first byte, all of them given in hex. Note that 00000036 in hexadecimal is 54 in decimal. You’re thus looking at byte 54 onward of smiley.bmp . Recall that a 24-bit BMP’s first 14 + 40 = 54 bytes are filled with metadata. If you really want to see that metadata in addition to the bitmap, execute the command below.

You should see the below; we’ve again highlighted in red all instances of 0000ff .

Okay, go ahead and return your attention to a terminal window, and navigate your way to ~/Dropbox/pset5/bmp . (Remember how?) Let’s look at the underlying bytes that compose smiley.bmp using xxd , a command-line "hex editor." Execute:

Okay, let’s transition from theory to practice. Double-click Home on John Harvard’s desktop and you should find yourself in John Harvard’s home directory. Double-click pset5 , double-click bmp , and then double-click smiley.bmp therein. You should see a tiny smiley face that’s only 8 pixels by 8 pixels. Select View > Zoom > Zoom Fit , and you should see a larger, albeit blurrier, version. (So much for "enhance," huh?) Actually, this particular image shouldn’t really be blurry, even when enlarged. The program that launched when you double-clicked smiley.bmp (called Ristretto Image Viewer) is simply trying to be helpful (CSI-style) by "dithering" the image (i.e., by smoothing out its edges). Below’s what the smiley looks like if you zoom in without dithering. At this zoom level, you can really see the image’s pixels (as big squares).

Knowing all this has got to be useful!

Worthy of note is that this BMP lacks padding! After all, (12 pixels) × (3 bytes per pixel) = 36 bytes is indeed a multiple of 4.

You should see output like the below; we’ve again highlighted in green all instances of 00ff00

For contrast, let’s use xxd on large.bmp , which looks identical to small.bmp but, at 12 pixels by 12 pixels, is four times as large. Go ahead and execute the below; you may need to widen your window to avoid wrapping.

Note that we’re using a different value for -c than we did for smiley.bmp so that xxd outputs only 4 columns this time (3 for the green box and 1 for the padding). You should see output like the below; we’ve highlighted in green all instances of 00ff00 .

To "see" this padding, go ahead and run the below.

Each scanline in small.bmp thus takes up (3 pixels) ×(3 bytes per pixel) = 9 bytes, which is not a multiple of 4. And so the scanline is "padded" with as many zeroes as it takes to extend the scanline’s length to a multiple of 4. In other words, between 0 and 3 bytes of padding are needed for each scanline in a 24-bit BMP. (Understand why?) In the case of small.bmp, 3 bytes' worth of zeroes are needed, since (3 pixels) × (3 bytes per pixel) + (3 bytes of padding) = 12 bytes, which is indeed a multiple of 4.

So, smiley.bmp is 8 pixels wide by 8 pixels tall, and it’s a 24-bit BMP (each of whose pixels is represented with 24 ÷ 8 = 3 bytes). Each row (aka "scanline") thus takes up (8 pixels) × (3 bytes per pixel) = 24 bytes, which happens to be a multiple of 4. It turns out that BMPs are stored a bit differently if the number of bytes in a scanline is not, in fact, a multiple of 4. In small.bmp , for instance, is another 24-bit BMP, a green box that’s 3 pixels wide by 3 pixels wide. If you view it with Ristretto Image Viewer (as by double-clicking it), you’ll see that it resembles the below, albeit much smaller. (Indeed, you might need to zoom in again to see it.)

Okay, xxd only showed you the bytes in these BMPs. How do we actually get at them programmatically? Well, in copy.c is a program whose sole purpose in life is to create a copy of a BMP, piece by piece. Of course, you could just use cp for that. But cp isn’t going to help Mr. Boddy. Let’s hope that copy.c does!

Go ahead and compile copy.c into a program called copy using make . (Remember how?) Then execute a command like the below.

./copy smiley.bmp copy.bmp

If you then execute ls (with the appropriate switch), you should see that smiley.bmp and copy.bmp are indeed the same size. Let’s double-check that they’re actually the same! Execute the below.

diff smiley.bmp copy.bmp

If that command tells you nothing, the files are indeed identical. (Note that some programs, like Photoshop, include trailing zeroes at the ends of some BMPs. Our version of copy throws those away, so don’t be too worried if you try to copy a BMP that you’ve downloaded or made only to find that the copy is actually a few bytes smaller than the original.) Feel free to open both files in Ristretto Image Viewer (as by double-clicking each) to confirm as much visually. But diff does a byte-by-byte comparison, so its eye is probably sharper than yours!

So how now did that copy get made? It turns out that copy.c relies on bmp.h . Let’s take a look. Open up bmp.h (as with gedit ), and you’ll see actual definitions of those headers we’ve mentioned, adapted from Microsoft’s own implementations thereof. In addition, that file defines BYTE , DWORD , LONG , and WORD , data types normally found in the world of Win32 (i.e., Windows) programming. Notice how they’re just aliases for primitives with which you are (hopefully) already familiar. It appears that BITMAPFILEHEADER and BITMAPINFOHEADER make use of these types. This file also defines a struct called RGBTRIPLE that, quite simply, "encapsulates" three bytes: one blue, one green, and one red (the order, recall, in which we expect to find RGB triples actually on disk).

Why are these struct s useful? Well, recall that a file is just a sequence of bytes (or, ultimately, bits) on disk. But those bytes are generally ordered in such a way that the first few represent something, the next few represent something else, and so on. "File formats" exist because the world has standardized what bytes mean what. Now, we could just read a file from disk into RAM as one big array of bytes. And we could just remember that the byte at location [i] represents one thing, while the byte at location [j] represents another. But why not give some of those bytes names so that we can retrieve them from memory more easily? That’s precisely what the struct`s in `bmp.h allow us to do. Rather than think of some file as one long sequence of bytes, we can instead think of it as a sequence of `struct`s.

Recall that smiley.bmp is 8 by 8 pixels, and so it should take up 14 + 40 + 8 ⋅ 8 ⋅ 3 = 246 bytes on disk. (Confirm as much if you’d like using ls .) Here’s what it thus looks like on disk according to Microsoft:

As this figure suggests, order does matter when it comes to struct`s' members. Byte 57 is `rgbtBlue (and not, say, rgbtRed ), because rgbtBlue is defined first in RGBTRIPLE . Our use, incidentally, of the attribute called packed ensures that clang does not try to "word-align" members (whereby the address of each member’s first byte is a multiple of 4), lest we end up with "gaps" in our `struct`s that don’t actually exist on disk.

Now go ahead and pull up the URLs to which BITMAPFILEHEADER and BITMAPINFOHEADER are attributed, per the comments in bmp.h . You’re about to start using MSDN (Microsoft Developer Network)!

Rather than hold your hand further on a stroll through copy.c , we’re instead going to ask you some questions and let you teach yourself how the code therein works. As always, man is your friend, and so, now, is MSDN. If not sure on first glance how to answer some question, do some quick research and figure it out! You might want to turn to http://www.cs50.net/resources/cppreference.com/stdio/ as well.

Allow us to suggest that you also run copy within gdb while answering these questions. Set a breakpoint at main and walk through the program. Recall that you can tell gdb to start running the program with a command like the below at `gdb’s prompt.

run smiley.bmp copy.bmp

If you tell gdb to print the values of bf and bi (once read in from disk), you’ll see output like the below, which we daresay you’ll find quite useful.

{bfType = 19778, bfSize = 246, bfReserved1 = 0, bfReserved2 = 0, bfOffBits = 54} {biSize = 40, biWidth = 8, biHeight = -8, biPlanes = 1, biBitCount = 24, biCompression = 0, biSizeImage = 192, biXPelsPerMeter = 2834, biYPelsPerMeter = 2834, biClrUsed = 0, biClrImportant = 0}

In ~/Dropbox/pset5/questions.txt , answer each of the following questions in a sentence or more.

What’s stdint.h ? What’s the point of using uint8_t , uint32_t , int32_t , and uint16_t in a program? How many bytes is a BYTE , a DWORD , a LONG , and a WORD , respectively? (Assume a 32-bit architecture like the CS50 Appliance.) What (in ASCII, decimal, or hexadecimal) must the first two bytes of any BMP file be? (Leading bytes used to identify file formats (with high probability) are generally called "magic numbers.)" What’s the difference between bfSize and biSize ? What does it mean if biHeight is negative? What field in BITMAPINFOHEADER specifies the BMP’s color depth (i.e., bits per pixel)? Why might fopen return NULL in copy.c:37 ? Why is the third argument to fread always 1 in our code? What value does copy.c:70 assign padding if bi.biWidth is 3 ? What does fseek do? What is SEEK_CUR ?