Bottom Up

Learning Goals

  • Practice working with images
  • Learn about metadata
  • Learn more about how structs can be useful

Background

Imagine an image you need to present to your boss for a major presentation somehow got corrupted! Upon investigating, you find that the image is mostly intact. Except, when you view the image, it now appears to be upside down!

Recall that a digital image 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. 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.

Your job is to edit the metadata programmatically so that the bitmap’s top row is first and bottom row last. Best to avoid editing the pixels directly, lest you further corrupt the file!

  • Hints
    • Be sure to look carefully at the members of the BITMAPINFOHEADER struct in bmp.h.
    • If you read the documentation for each of these members, which should you modify? How so?

Getting Started

  1. Log into code.cs50.io using your GitHub account.
  2. Click inside the terminal window and execute cd.
  3. Execute wget https://cdn.cs50.net/2022/fall/labs/4/bottomup.zip followed by Enter in order to download a zip called bottomup.zip in your codespace. Take care not to overlook the space between wget and the following URL, or any other character for that matter!
  4. Now execute unzip bottomup.zip to create a folder called bottomup.
  5. You no longer need the ZIP file, so you can execute rm bottomup.zip and respond with “y” followed by Enter at the prompt.

Demo

Implementation Details

Go ahead and pull up the URLs to which BITMAPFILEHEADER and BITMAPINFOHEADER are attributed, per the comments in bmp.h. Take a close look at the members of the BITMAPINFOHEADER struct. Use that information to write a bit of code in bottomup.c to change the image from bottom-up to top-down. The code in question needn’t be very complicated, particularly if you know what you’re doing!

Thought Question

  • Why do image files need metadata?

How to Test Your Code

Your program should behave per the examples below.

bottomup/ $ ./bottomup harvard_bottomup.bmp harvard_topdown.bmp

When your program is working correctly, you should see a new file, harvard_topdown.bmp in your bottomup directory. Open it up and see if the orientation of the image is correct.

You can check your code using check50, a program that CS50 will use to test your code when you submit, by typing in the following at the $ prompt. But be sure to test it yourself as well!

check50 cs50/labs/2023/x/bottomup

To evaluate that the style of your code (indentations and spacing) is correct, type in the following at the $ prompt.

style50 bottomup.c

How to Submit

No need to submit! This is a practice problem.