Introduction
A tiny print studio needs a postcard made from a grid of colors. You will first run a complete four-by-four sample, give it a warmer ink, then finish a C program that turns a rectangular character pattern into a real image file.
You need C variables, conditions, loops, arrays, functions and basic terminal navigation. Two-dimensional indexing and image output are explained here. The prepared browser environment includes a Workspace terminal and Postcard Preview. The picture is a P3 PPM file: readable numbers, not a PNG. Estimated reading and practice time is an author estimate, not a measured beginner completion time.
Print and Recolor a Small Sample
In this step, you will see a complete C sample become a picture, then change one ink color.
Open the Workspace Tab. Its terminal is in /home/labex/project/postcards; the explicit cd below also lets you recover after changing directories. pwd prints your current directory and ls lists its files.
cd /home/labex/project/postcards
pwd
ls
The ready files are sample.c, unfinished card.c, and supplied art.h. Change only sample.c and card.c; keep art.h unchanged. cat prints a file without editing it:
cat sample.c
The sample uses a grid with four rows and four columns. In grid[row][col], the first index chooses a row from top to bottom; the second chooses a column from left to right. Both start at zero. Each entry is either 0 or 1, selecting one RGB triple in color. RGB means red, green and blue; each component is an integer from 0 through 255. The second index in color[c][0] selects red, [1] green and [2] blue.
printf writes text to standard output, normally this terminal. \n starts a new line. P3 files begin with P3, width and height, then the largest component value, 255. After that come RGB triples, read left to right across each row, from the top row to the bottom. Whitespace separates numbers, so one RGB triple per line is valid. P3 also allows # comments.
Compile the sample. gcc is the C compiler; -std=c11 selects C11, -Wall -Wextra enable useful warnings, and -o sample names the executable sample.
gcc -std=c11 -Wall -Wextra sample.c -o sample
./sample runs the executable in this directory. > redirects standard output into a file instead of the terminal. The shell opens or replaces that file before the program runs. Use the new file name shown here:
./sample > sample.ppm
Switch to the Postcard Preview Tab. Click Refresh files, select sample.ppm under Postcard file, then click Open postcard. The status reads 4 × 4 pixels · sample.ppm. You should see a dark, asymmetric mark on a cream background. The preview displays the file you just created; it does not fill in missing pixels.
Return to Workspace. Predict the edit first: the grid entries selecting color 1 should change, while color 0 and the grid dimensions stay the same. You will change only the dark ink {30, 76, 64} to the warm color {170, 70, 45} in sample.c.
Use nano, the terminal text editor. Keep keyboard focus inside Workspace. In the instructions below, Ctrl+W means hold Control, press W, then release both; do not type those characters. nano's bottom help bar writes Control as ^, so ^O means Ctrl+O. Arrow keys move the cursor without changing text. First preserve the supplied sample with cp, which copies the first named file to the second; this backup name is separate from the file you will edit.
cp sample.c sample-before-edit.c
nano sample.c
In nano, press Ctrl+W, type 30, 76, 64, then press Enter. The cursor moves to the matching text. Backspace deletes the character immediately before the cursor. Use the arrow keys to place the cursor immediately after 30, before its following comma. Press Backspace twice to remove 30, then type 170. Move the cursor immediately after 76, before its following comma, press Backspace twice, then type 70. Move the cursor immediately after 64, before the following space or closing brace, press Backspace twice, then type 45. Leave the existing commas, spaces, braces and surrounding array declaration untouched. Read the finished entry: it should be {170, 70, 45}. This changes the palette entry rather than painting new grid cells.
Press Ctrl+O to write the file, then Enter to confirm sample.c. Press Ctrl+X to exit to the shell. Saving source does not rebuild the executable or update an existing image. Check the saved source before compiling:
cat sample.c
If you opened the wrong file or made a change you want to abandon before saving, press Ctrl+X, then N when asked whether to save. If no changes were made, nano exits without that question. Reopen the correct name from the workspace. If you already saved an unwanted sample edit, inspect your backup with cat sample-before-edit.c, then reopen sample.c and restore only the affected text; do not rerun setup or replace unrelated files.
Recompile after a source edit; the old executable does not change by itself.
gcc -std=c11 -Wall -Wextra sample.c -o sample
./sample > sample.ppm
Switch to Postcard Preview, keep sample.ppm selected, and click Open postcard again. Only the foreground color should change; the dimensions and cream pixels stay the same.
If the terminal reports a missing file, use the taught cd and ls to return to the workspace. If the preview says no file exists, return to Workspace and check that compilation succeeded before running ./sample > sample.ppm. A compiler error means no new executable was built. Read the file name and line number in GCC's message, reopen nano sample.c, use the arrow keys to reach that line, and correct the missing punctuation or mistyped number. Save and exit as above, then compile again. Do not run the old sample after a failed compilation: it would regenerate the previous picture and hide your unsaved or invalid edit.
Map a Rectangular Pattern to RGB Pixels
In this step, you will complete the postcard generator and open its actual output.
Start in Workspace, in /home/labex/project/postcards. Read the supplied artwork and unfinished generator:
cd /home/labex/project/postcards
cat art.h
cat card.c
#include "art.h" makes these prepared definitions available when C compiles card.c. #define WIDTH 5 and #define HEIGHT 3 give names to the dimensions. pattern has three rows of five digit characters. Each string row reserves WIDTH + 1 characters because C stores a zero-valued terminator after the visible characters; the loops use only the five visible positions.
The first row, "10000", begins with palette color 1, then four pixels of color 0. Character '2' is not the integer 2. Subtracting the character '0' converts one of the supplied digit characters to its palette index: '2' - '0' is 2. This artwork contains only digits 0 through 3, matching the four palette rows. Keep its pattern, dimensions and colors unchanged. Write the loops using WIDTH and HEIGHT so recompilation also works for supplied rectangular patterns from 1 through 16 columns and rows, using the same palette indexes 0 through 3.
The incomplete generator writes a header but no pixels. A file header alone is not a picture. Complete it with an outer loop over y (rows) and an inner loop over x (columns). pattern[y][x] chooses the character; palette[c][0..2] chooses its RGB values. The %d placeholders in printf print integer arguments in their listed order.
Before editing, trace two positions in art.h. At (x=0,y=0), pattern[0][0] is '1', so the RGB value must come from palette row 1: 30 76 64. At (x=2,y=1), read the second pattern row and third character: '2' selects 242 158 56. Predict the total number of RGB triples by multiplying the width and height. There must be 15; a header cannot supply any of them.
Open the existing generator:
nano card.c
Use Ctrl+W to find printf, then Enter. Leave the existing P3 header and return 0; in place. Move to the line after the header and insert your pixel traversal before return 0;. You are extending main, not replacing the file or changing art.h.
Build the traversal from the row order: initialize integer y to zero, continue while it is less than HEIGHT, and increment it after each row. Inside that loop, do the same for integer x from zero up to (but excluding) WIDTH. This visits the entire top row before moving down. Reversing the bounds would make the five-by-three rectangle incomplete or read outside a row.
Inside the inner loop, calculate int c = pattern[y][x] - '0';. That converts the current digit to the palette row. Then output exactly one triple for this pixel:
printf("%d %d %d\n", palette[c][0], palette[c][1], palette[c][2]);
Write the two for statements yourself using those bounds. Put braces around each loop body; the inner loop's closing brace comes before the outer loop's. Only the color lookup and pixel printf belong inside the inner loop. The existing header must stay before both loops, and return 0; must stay after them. Compare your traversal with the already working row/column loops in sample.c if you need a syntax model. Use the symbolic dimensions, not fixed limits 5 and 3.
Save with Ctrl+O, Enter, then exit with Ctrl+X. Read back card.c and mentally follow your first three pixels: palette rows 1, 0, 0. Check your (2,1) prediction too.
cat card.c
Compile and generate a new output file:
gcc -std=c11 -Wall -Wextra card.c -o card
./card > original.ppm
The provided inspection tool only decodes a file; it cannot generate or repair your postcard. Run it with the file name as its argument:
python3 /home/labex/.local/share/pixel-postcards/inspect.py original.ppm
Expected output:
5 x 3 pixels; 15 RGB triples; max value 255
Why are these numbers useful? A square could hide swapped dimensions. This five-column, three-row pattern must be wider than it is tall, with its dark strip on the left. WIDTH controls the inner loop, HEIGHT the outer loop; exchanging them can access outside a row or leave out pixels.
Switch to Postcard Preview. Click Refresh files, select original.ppm, then Open postcard. Check the 5 × 3 status and compare the rows to art.h. Click Download PPM to save the real image file. A browser may download PPM rather than display it directly; use this preview to view it. Return to Workspace for the lab check.
The screenshot shows original.ppm selected, its saved 5 × 3 dimensions, and Download PPM below the image. Compare the left dark strip and all three rows.
Keep card.c, the executable card, art.h, and original.ppm in this workspace.
If the preview reports a pixel-count error, a file exists but its content is incomplete: check both loop limits, save card.c, recompile and generate original.ppm again. The preview's screen enlargement does not alter the saved dimensions. In the next lab you will make enlarged image files yourself.
Summary
You compiled and recolored a small sample, then mapped a rectangular character array to RGB triples. Your C program created a real P3 file that you decoded, viewed and downloaded. You practiced row-first indexing and standard-output redirection.
