Utility for working with BMP

Cut and rotate

The application ./hw-01_bmp is implemented, which cuts a rectangle from a BMP image, rotates this rectangle 90 degrees clockwise and saves the result in a separate file.

All images are stored in the given format:

• The general format is BMP.
• The BMP format uses the DIB format with the BITMAPINFOHEADER header (version 3).
• The value of the biHeight field (image height) is strictly greater than zero.
• Uses 24 bits of color per pixel (one byte per color channel).
• Palette (color table) is not used.
• Compression is not used.

The utility works for little endian systems and only for one specific type of BMP files.

Console application

The application is launched with the following command:

./hw-01_bmp crop-rotate ‹in-bmp› ‹out-bmp› ‹x› ‹y› ‹w› ‹h›

Used parameters:

• crop-rotate is a required parameter, indicating the action to be performed.
• in-bmp is the name of the input image file.
• out-bmp is the name of the output image file.
• x, y are the coordinates of the upper left corner of the area to be cut and rotated. Coordinates start at zero, so (0, 0) is the top left corner.
• w, h - respectively, the width and height of the area before rotation.

Thus, if we denote the width and height of the original image as W and H, respectively, for correct arguments, the following inequalities are true:

• 0 <= x < x + w <= W
• 0 <= y < y + h <= H

Shorthand

In addition to the crop-rotate command, the insert and extract commands are implemented, allowing you to hide a message inside the image (Shorthand). The insert command saves the message to the image, and extract extracts it from there.

The idea is as follows: if you change only the least significant bits in the color components, then an outside observer will not notice the distortion. This distortion can carry information.

Coding method

The initial message consists only of capital Latin letters, a space, a period and a comma. Each character is converted to a number from 0 to 28, respectively (29 different values in total), and the number is in five bits, written from the lowest to the highest. A total message of N characters is encoded using 5N Bits.

To send a message, in addition to the carrier image, you will need key - a text file, describing in which pixels the message bits are encoded. In this file, on separate lines, are written:

• The x and y coordinates (0 <= x < W, 0 <= y < H) of the pixel to save to corresponding bit.
• The letter R/G/B denoting the color channel in the least significant bit of which you want to write a bit messages.

If the key writes more bits than the message needs, the last lines are ignored.

Console application

To save the secret stitch to an image, the application is launched with the following command:

./hw-01_bmp insert ‹in-bmp› ‹out-bmp› ‹key-txt› ‹msg-txt›

To extract the secret line from an image, the application is launched with the following command:

./hw-01_bmp extract ‹in-bmp› ‹key-txt› ‹msg-txt›

Used parameters:

• in-bmp is the name of the input image file.
• out-bmp is the name of the output image file.
• key-txt is a test file with a key.
• msg-txt is a text file with a secret message.