Problem: Resize

tl;dr

Implement a program that resizes BMPs, per the below.

$ ./resize 4 small.bmp large.bmp

Academic Honesty

This course’s philosophy on academic honesty is best stated as "be reasonable." The course recognizes that interactions with classmates and others can facilitate mastery of the course’s material. However, there remains a line between enlisting the help of another and submitting the work of another. This policy characterizes both sides of that line.

The essence of all work that you submit to this course must be your own. Collaboration on problems is not permitted (unless explicitly stated otherwise) except to the extent that you may ask classmates and others for help so long as that help does not reduce to another doing your work for you. Generally speaking, when asking for help, you may show your code or writing to others, but you may not view theirs, so long as you and they respect this policy’s other constraints. Collaboration on quizzes and tests is not permitted at all. Collaboration on the final project is permitted to the extent prescribed by its specification.

Below are rules of thumb that (inexhaustively) characterize acts that the course considers reasonable and not reasonable. If in doubt as to whether some act is reasonable, do not commit it until you solicit and receive approval in writing from your instructor. If a violation of this policy is suspected and confirmed, your instructor reserves the right to impose local sanctions on top of any disciplinary outcome that may include an unsatisfactory or failing grade for work submitted or for the course itself.

Reasonable

  • Communicating with classmates about problems in English (or some other spoken language).

  • Discussing the course’s material with others in order to understand it better.

  • Helping a classmate identify a bug in his or her code, such as by viewing, compiling, or running his or her code, even on your own computer.

  • Incorporating snippets of code that you find online or elsewhere into your own code, provided that those snippets are not themselves solutions to assigned problems and that you cite the snippets' origins.

  • Reviewing past years' quizzes, tests, and solutions thereto.

  • Sending or showing code that you’ve written to someone, possibly a classmate, so that he or she might help you identify and fix a bug.

  • Sharing snippets of your own solutions to problems online so that others might help you identify and fix a bug or other issue.

  • Turning to the web or elsewhere for instruction beyond the course’s own, for references, and for solutions to technical difficulties, but not for outright solutions to problems or your own final project.

  • Whiteboarding solutions to problems with others using diagrams or pseudocode but not actual code.

  • Working with (and even paying) a tutor to help you with the course, provided the tutor does not do your work for you.

Not Reasonable

  • Accessing a solution to some problem prior to (re-)submitting your own.

  • Asking a classmate to see his or her solution to a problem before (re-)submitting your own.

  • Decompiling, deobfuscating, or disassembling the staff’s solutions to problems.

  • Failing to cite (as with comments) the origins of code, writing, or techniques that you discover outside of the course’s own lessons and integrate into your own work, even while respecting this policy’s other constraints.

  • Giving or showing to a classmate a solution to a problem when it is he or she, and not you, who is struggling to solve it.

  • Looking at another individual’s work during a quiz or test.

  • Paying or offering to pay an individual for work that you may submit as (part of) your own.

  • Providing or making available solutions to problems to individuals who might take this course in the future.

  • Searching for, soliciting, or viewing a quiz’s questions or answers prior to taking the quiz.

  • Searching for or soliciting outright solutions to problems online or elsewhere.

  • Splitting a problem’s workload with another individual and combining your work (unless explicitly authorized by the problem itself).

  • Submitting (after possibly modifying) the work of another individual beyond allowed snippets.

  • Submitting the same or similar work to this course that you have submitted or will submit to another.

  • Using resources during a quiz beyond those explicitly allowed in the quiz’s instructions.

  • Viewing another’s solution to a problem and basing your own solution on it.

Introduction

Getting Started

Here’s how to download this problem’s "distribution code" (i.e., starter code) into your own CS50 IDE. Log into CS50 IDE and then, in a terminal window, execute each of the below.

  1. Execute cd to ensure that you’re in ~/ (i.e., your home directory, aka ~).

  2. Execute mkdir chapter3 to make (i.e., create) a directory called chapter3 in your home directory.

  3. Execute cd chapter3 to change into (i.e., open) that directory.

  4. Execute wget http://cdn.cs50.net/ap/2019/problems/resize/less/resize.zip to download a (compressed) ZIP file with this problem’s distribution.

  5. Execute unzip resize.zip to uncompress that file.

  6. Execute rm resize.zip followed by yes or y to delete that ZIP file.

  7. Execute ls. You should see a directory called resize, which was inside of that ZIP file.

  8. Execute cd resize to change into that directory.

  9. Execute ls. You should see this problem’s distribution code, including bmp.h, copy.c, large.bmp, small.bmp, and smiley.bmp.

Background

First, be sure you’re familiar with the structure of 24-bit uncompressed BMPs, as introduced in Whodunit.

To reiterate a bit from that lab, 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 only expects that you support the latest version of Microsoft’s BMP format, 4.0, 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.

ffffff  ffffff  0000ff  0000ff  0000ff  0000ff  ffffff  ffffff
ffffff  0000ff  ffffff  ffffff  ffffff  ffffff  0000ff  ffffff
0000ff  ffffff  0000ff  ffffff  ffffff  0000ff  ffffff  0000ff
0000ff  ffffff  ffffff  ffffff  ffffff  ffffff  ffffff  0000ff
0000ff  ffffff  0000ff  ffffff  ffffff  0000ff  ffffff  0000ff
0000ff  ffffff  ffffff  0000ff  0000ff  ffffff  ffffff  0000ff
ffffff  0000ff  ffffff  ffffff  ffffff  ffffff  0000ff  ffffff
ffffff  ffffff  0000ff  0000ff  0000ff  0000ff  ffffff  ffffff

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.

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

Now look at the underlying bytes that compose smiley.bmp. Within CS50 IDE’s file browser, right- or control-click smiley.bmp and select Open as hexadecimal in order to view the file’s bytes in hexadecimal (i.e., base-16). In the tab that appears, change Start with byte to 54, change Bytes per row to 24, change Bytes per column to 3. Then click Set. If unable to change these values, try clicking View > Night Mode and try again. You should see the below, byte 54 onward of smiley.bmp. (Recall that a 24-bit BMP’s first 14 + 40 = 54 bytes are filled with metadata, so we’re simply ignoring that for now.) As before, we’ve highlighted in red all instances of 0000ff.

ffffff ffffff 0000ff 0000ff 0000ff 0000ff ffffff ffffff
ffffff 0000ff ffffff ffffff ffffff ffffff 0000ff ffffff
0000ff ffffff 0000ff ffffff ffffff 0000ff ffffff 0000ff
0000ff ffffff ffffff ffffff ffffff ffffff ffffff 0000ff
0000ff ffffff 0000ff ffffff ffffff 0000ff ffffff 0000ff
0000ff ffffff ffffff 0000ff 0000ff ffffff ffffff 0000ff
ffffff 0000ff ffffff ffffff ffffff ffffff 0000ff ffffff
ffffff ffffff 0000ff 0000ff 0000ff 0000ff ffffff ffffff

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, though, 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 (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.)

small.png

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.

To "see" this padding, right- or control-click small.bmp in CS50 IDE’s file browser and select Open as hexadecimal. In the tab that appears, change Start with byte to 54, change Bytes per row to 12, and change Bytes per column to 3. Then click Set. You should see output like the below; we’ve highlighted in green all instances of 00ff00.

00ff00 00ff00 00ff00 000000
00ff00 ffffff 00ff00 000000
00ff00 00ff00 00ff00 000000

For contrast, let’s next look at large.bmp, which looks identical to small.bmp but, at 12 pixels by 12 pixels, is four times as large. Right- or control-click it in CS50 IDE’s file browser, then select Open as hexadecimal. You should see output like the below; we’ve again highlighted in green all instances of 00ff00.

00ff00 00ff00 00ff00 00ff00 00ff00 00ff00 00ff00 00ff00 00ff00 00ff00 00ff00 00ff00
00ff00 00ff00 00ff00 00ff00 00ff00 00ff00 00ff00 00ff00 00ff00 00ff00 00ff00 00ff00
00ff00 00ff00 00ff00 00ff00 00ff00 00ff00 00ff00 00ff00 00ff00 00ff00 00ff00 00ff00
00ff00 00ff00 00ff00 00ff00 00ff00 00ff00 00ff00 00ff00 00ff00 00ff00 00ff00 00ff00
00ff00 00ff00 00ff00 00ff00 ffffff ffffff ffffff ffffff 00ff00 00ff00 00ff00 00ff00
00ff00 00ff00 00ff00 00ff00 ffffff ffffff ffffff ffffff 00ff00 00ff00 00ff00 00ff00
00ff00 00ff00 00ff00 00ff00 ffffff ffffff ffffff ffffff 00ff00 00ff00 00ff00 00ff00
00ff00 00ff00 00ff00 00ff00 ffffff ffffff ffffff ffffff 00ff00 00ff00 00ff00 00ff00
00ff00 00ff00 00ff00 00ff00 00ff00 00ff00 00ff00 00ff00 00ff00 00ff00 00ff00 00ff00
00ff00 00ff00 00ff00 00ff00 00ff00 00ff00 00ff00 00ff00 00ff00 00ff00 00ff00 00ff00
00ff00 00ff00 00ff00 00ff00 00ff00 00ff00 00ff00 00ff00 00ff00 00ff00 00ff00 00ff00
00ff00 00ff00 00ff00 00ff00 00ff00 00ff00 00ff00 00ff00 00ff00 00ff00 00ff00 00ff00

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.

Knowing all this has got to be useful!

Specification

Implement a program called resize that resizes (i.e., enlarges) 24-bit uncompressed BMPs by a factor of n.

  • Implement your program in a file called resize.c inside of ~/chapter3/resize/less/.

  • Your program should accept exactly three command-line arguments, whereby

    • the first (n) must be a positive integer less than or equal to 100,

    • the second must be the name of a BMP to be resized, and

    • the third must be the name of the resized version to be written.

    + If your program is not executed with such, it should remind the user of correct usage, as with printf, and main should return 1.

  • Your program, if it uses malloc, must not leak any memory. Be sure to call free.

Usage

Your program should behave per the examples below. Assumed that the underlined text is what some user has typed.

$ ./resize
Usage: ./resize n infile outfile
$ echo $?
1
$ ./resize 2 small.bmp larger.bmp
$ echo $?
0

Hints

With a program like this, we could have created large.bmp out of small.bmp by resizing the latter by a factor of 4 (i.e., by multiplying both its width and its height by 4), per the below.

./resize 4 small.bmp large.bmp

You’re welcome to get started by copying (yet again) copy.c and naming the copy resize.c. But spend some time thinking about what it means to resize a BMP. (You may assume that n times the size of infile will not exceed 232 - 1.) 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. And do be sure to support a value of 1 for n, the result of which should be an outfile with dimensions identical to infile's.

If you happen to use malloc, be sure to use free so as not to leak memory. Try using valgrind to check for any leaks!

Testing

If you’d like to peek at, e.g., large.bmp's headers (in a more user-friendly way than xxd allows), you may execute the below.

~cs50/2019/ap/chapter3/less/peek large.bmp

Better yet, if you’d like to compare your outfile’s headers against those from the staff’s solution, you might want to execute commands like the below. (Think about what each is doing.)

./resize 4 small.bmp student.bmp
~cs50/2019/ap/chapter3/less/resize 4 small.bmp staff.bmp
~cs50/2019/ap/chapter3/less/peek student.bmp staff.bmp

check50

Here’s how to evaluate the correctness of your code using check50. But be sure to compile and test it yourself as well!

check50 cs50/problems/2019/ap/resize/less

style50

Here’s how to evaluate the style of your code using style50.

style50 resize.c

Staff’s Solution

If you’d like to play with the staff’s own implementation of resize, you may execute the below.

./resize

How to Submit

Step 1 of 2

Head back to the ide.cs50.io[CS50 IDE] and ensure that resize.c is in ~/chapter3/resize/less, as with:

cd ~/chapter3/resize/less
ls

If resize.c is not in ~/chapter3/resize/less, move it into that directory, as via mv (or via CS50 IDE’s lefthand file browser).

Step 2 of 2

  • To submit resize, execute

    cd ~/chapter3/resize/less
    submit50 cs50/problems/2019/ap/resize/less

    inputting your GitHub username and GitHub password as prompted.

If you run into any trouble, email sysadmins@cs50.harvard.edu!

You may resubmit any problem as many times as you’d like.

Your submission should be graded for correctness within 2 minutes, at which point your score will appear at submit.cs50.io!

This was Resize.