Lab 4: Smiley

Learning Goals

  • Learn how to work with images
  • Practice manipulating pixels



You’ve seen in lecture a bit about how images are stored on a computer. In this lab, you’ll practice working with a BMP file, actually the smiley face pictured here, and change all the black pixels to a color of your choosing.

However, the smiley face you’ll be working with is not just made of of 0’s and 1’s, or black and white pixels, but consists of 24 bits per pixel. It uses eight bits to represent red values, eight bits for green and eight bits for blue. Since each color uses eight bits or one byte, we can use a number in the range of 0 to 255 to represent its color value. In hexadecimal, this is represented by 0x00 to 0xff. By mixing together these red, green and blue values, we can create millions of possible colors.

If you look at bmp.h, one of the the helper files in the distribution code, you’ll see how each RGB triple is represented by a struct like:

typedef struct
    BYTE rgbtBlue;
    BYTE rgbtGreen;
    BYTE rgbtRed;

where BYTE is defined as an 8-bit integer.

You’ll notice several files provided in the distribution code to handle the reading and writing of an image file, as well as handling the image’s metadata or “headers”. You’ll be completing the function colorize in helpers.c, which already has as input parameters, the image’s height, width, and a two-dimensional array of RGBTRIPLE’s which create the image itself.

  • Hints
    • If we were to save the first pixel as RGBTRIPLE pixel = image[0][0] we could then access each of the individual colors of pixel as pixel.rgbtBlue, pixel.rgbtGreen, and pixel.rgbtRed.


Getting Started

Open VS Code.

Start by clicking inside your terminal window, then execute cd by itself. You should find that its “prompt” resembles the below.


Click inside of that terminal window and then execute


followed by Enter in order to download a ZIP called in your codespace. Take care not to overlook the space between wget and the following URL, or any other character for that matter!

Now execute


to create a folder called smiley. You no longer need the ZIP file, so you can execute


and respond with “y” followed by Enter at the prompt to remove the ZIP file you downloaded.

Now type

cd smiley

followed by Enter to move yourself into (i.e., open) that directory. Your prompt should now resemble the below.

smiley/ $

If all was successful, you should execute


and you should see bmp.h, colorize.c, helpers.c, helpers.h, Makefile, and smiley.bmp.

If you run into any trouble, follow these same steps again and see if you can determine where you went wrong!

Implementation Details

Open up helpers.c and notice that the colorize function is incomplete. Note that the image’s height, width and a two-dimensional array of pixels is set up as the input parameters for this function. You are to implement this function to change all the black pixels in the image to a color of your choosing.

You can compile your code by simply typing make at the $ prompt.

You then execute the program by typing:

./colorize smiley.bmp outfile.bmp

where outfile.bmp is the name of the new bmp you are creating.

Thought Question

  • How do you think you represent a black pixel when using a 24-bit color BMP file?
  • Is this the same or different when mixing paints to repesent various colors?

How to Test Your Code

Your program should behave per the examples below.

smiley/ $ ./colorize smiley.bmp smiley_out.bmp

When your program is working correctly, you should see a new file, smiley_out.bmp in your smiley directory. Open it up and see if the black pixels are now the color you’ve specified.

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/smiley

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

style50 helpers.c

How to Submit

In your terminal, execute the below to submit your work.

submit50 cs50/labs/2023/x/smiley
Want to see the staff's solution?
#include "helpers.h"

void colorize(int height, int width, RGBTRIPLE image[height][width])
    for (int i = 0; i < height; i++)
        for (int j = 0; j < width; j++)
            // Make black pixels turn red
            if (image[i][j].rgbtRed == 0x00 && image[i][j].rgbtGreen == 0x00 && image[i][j].rgbtBlue == 0x00)
                image[i][j].rgbtRed = 0xff;