The BMP file format is among the simplest image representation formats; possibly the simplest. There are various versions of the format, and the format is able to support several kinds of color values and compression. But the simplest and most common bitmap files are not compressed.

For our purposes, assume that a BMP file consists of 24-bit RGB pixels and is composed of the following “structures” (i.e., segments of data) in the following order:

The Wikipedia article linked above has a lot more information on the format. However, here is a thorough description of what you need to know about each structure for this assignment:

This structure consists of 14 bytes, as follows:

This structure immediately follows the BMP header and consists of a variable number of bytes, as follows:

This structure contains component values for every pixel in the image.

1. The pixel array begins at the file offset specified in the BMP file header.
2. Each pixel value is composed of 3 bytes, which represent a 24-bit RGB color, i.e. 3 8-bit unsigned integers.
3. Pixel values are presented row-by-row starting with the bottom row¹⁾, i.e. the entire bottom row of pixels, then the row second from the bottom, etc.
4. Each row is padded to a multiple of 4 bytes in size, if necessary.

A very small (2×2-pixel) BMP file is available at /srv/datasets/simple.bmp, or via HTTP. (This is the same image described in Example 1 of Wikipedia's article.)

Here it is, surrounded by a 10-pixel padded bordering box (you probably can't see the image very easily!):

And here is how it would look if it were resized to 200×200 pixels:

On the command line, we could inspect the contents of this file as 1-byte hexadecimal values using the hexdump utility:

hexdump -C /srv/datasets/simple.bmp

hexdump gives the following output, in which each line displays the file offset in hexadecimal, followed by sixteen space-separated, two column, hexadecimal bytes, followed by the same sixteen bytes presented as printable characters (or periods if not printable), enclosed in | characters.:

00000000  42 4d 46 00 00 00 00 00  00 00 36 00 00 00 28 00  |BMF.......6...(.|
00000010  00 00 02 00 00 00 02 00  00 00 01 00 18 00 00 00  |................|
00000020  00 00 10 00 00 00 00 00  00 00 00 00 00 00 00 00  |................|
00000030  00 00 00 00 00 00 00 00  ff ff ff ff 00 00 ff 00  |................|
00000040  00 00 ff 00 00 00                                 |......|
00000046

The first 14 bytes constitute the BMP File Header: 42 4d 46 00 00 00 00 00 00 00 36 00 00 00

Note the bytes 42 4d initially, i.e. the characters B and M. The next 4 bytes (46 00 00 00) are the file size. While this looks to be (and is) the value 0x46000000, note that this does not mean the file is size 1174405120, which is how we would normally interpret 0x46000000. This is because BMP files (and most modern CPUs) represent integers in "little-endian" format, meaning that the bytes are ordered from least significant to most significant, unlike how we usually write integers for human benefit. This means the way we would normally write that value in hexadecimal is 0x00000046, which is 70 in decimal. The size of this file is indeed 70 bytes:

$ stat -c '%s' /srv/datasets/simple.bmp
70

The next 40 bytes constitute the DIB Header: 28 00 00 00 02 00 00 00 02 00 00 00 01 00 18 00 00 00 00 00 10 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00

Note that 0x28000000 is the header size (little-endian 40), and the next 8 bytes are the image width and height, both 2 in this case: 0x02000000 0x02000000.

From offset 0x36 to the end of the file is the pixel array: 00 00 ff ff ff ff 00 00 ff 00 00 00 ff 00 00 00

The first row of pixels: 00 00 ff ff ff ff 00 00

The second row of pixels: ff 00 00 00 ff 00 00 00

Note that first-row pixel values 00 00 ff and ff ff ff are the colors red (255, 0, 0) and white (255, 255, 255), respectively, and that even pixel values are stored as little-endian integers, i.e. BGR instead of RGB.

executable input_file output_file modification

-brighten increases the intensity of each pixel component by a factor given as a subsequent command-line argument, e.g in the following that increases the intensity by 25%:

cs19_bmp_mod /srv/datasets/cabrillo-logo.bmp cabrillo-logo-brighten-25.bmp -brighten 25

-darken decreases the intensity of each pixel component by a factor given as a subsequent command-line argument, e.g in the following that decreases the intensity by 25%:

cs19_bmp_mod /srv/datasets/cabrillo-logo.bmp cabrillo-logo-darken-25.bmp -darken 25

-desaturate converts all colors in the image to corresponding shades of gray, where each component in each pixel is assigned the average of all of the pixel's original components, e.g in the following:

cs19_bmp_mod /srv/datasets/cabrillo-logo.bmp cabrillo-logo-desaturate.bmp -desaturate

-invert inverts all pixel components, e.g in the following:

cs19_bmp_mod /srv/datasets/cabrillo-logo.bmp cabrillo-logo-invert.bmp -invert

-hflip flips the image horizontally.

cs19_bmp_mod /srv/datasets/cabrillo-logo.bmp cabrillo-logo-hflip.bmp -hflip

-vflip flips the image vertically.

cs19_bmp_mod /srv/datasets/cabrillo-logo.bmp cabrillo-logo-vflip.bmp -vflip

As submissions are received, this leaderboard will be updated with the top-performing fully/nearly functional solutions, with regard to execution speed.

Submit your source-code file(s) via turnin.