EP01_10 — 📄 Metadata: Reading a PGM File

1.18.13 EP01_10 📄 Metadata: Reading a PGM File

In this activity, you must read an image file in PGM (Portable Gray Map) format and extract its dimensions from the header.

The PGM (P2) format is a plain text (ASCII) file that stores grayscale images.
The file has a header structured as follows:

Version: The identifier P2.
Comments: Optional lines starting with # (should be ignored).
Dimensions: Two integers representing Width and Height.
Maximum: An integer representing the maximum intensity (usually 255).

After the header, the pixel data follows.

📌 Important:

File Reading: You must open the file indicated in the example using Python’s open() function.
Output Order: Contrary to the order present in the file, the expected output must be in tuple format: (Height, Width, Channels).
Since PGM files are grayscale, the number of Channels is always 1.
See an interactive simulator for this question at Figure 1.20 (adjust the dimensions to see how the ASCII header is generated).

1.18.13.1 🧠 Understanding the PGM Format

The PGM format is one of the simplest for image processing. Because it is plain text, it allows you to view the metadata and even the pixel values by opening the file in a notepad:

Component	Example	Meaning
Magic Number	`P2`	Identifies that it is a PGM in text format (ASCII).
Comment	`# CREATOR...`	Informational line ignored by the processor.
Dimensions	`397 343`	397 columns (Width) and 343 rows (Height).
Intensity	`255`	Defines the value of pure white (scale from 0 to 255).

1.18.13.2 📋 Task (VPL specification)

Input:

No keyboard input. The program must read the file "aula01fig03b.pgm" present in the execution directory.

Output:

A tuple containing (Height, Width, 1).

1.18.13.3 📌 Examples

File Name	Expected Output	Note
“aula01fig03b.pgm”	(343, 397, 1)	Note the order inversion: Height first

📄 Simulator EP01_10: PGM File Structure ASCII P2 & Format (H, W, C)

Change the width (W) and height (H) dimensions to observe the dynamic assembly of the PGM header and the tuple format of the resulting array in Python (Rows × Columns × Channels).

IMAGE DEFINITIONS

Width (W - Columns):

Height (H - Rows):

💡 Note the convention: The PGM header declares first W H (Width × Height), while the array in Python/NumPy reports the tuple as (H, W, C) (Rows × Columns × Channels).

FILE CONTENT (.PGM)
P2
# CREATOR: UFABC PDI / EP01_10

397
343
255
120 134 210 0 85 255 ...
Output of Function mm.readImg (Array Format):
(343, 397, 1)

Figure 1.20: Simulator EP01_10: PGM File Structure (ASCII P2 Header and Mapping to Python Tuple)

📝 Note

The file required for this EP will be automatically downloaded from the repository using the following code:

import os, urllib.request

BASE_URL = "https://raw.githubusercontent.com/fzampirolli/pdi-vc/master/all/cap01/imagens"
file = "aula01fig03b.pgm"

opener = urllib.request.build_opener()
opener.addheaders = [('User-agent', 'Mozilla/5.0')]
urllib.request.install_opener(opener)

for f in [file]:
    if not os.path.exists(f):
        url = f"{BASE_URL}/{f}"
        try:
            urllib.request.urlretrieve(url, f)
            print(f"✅ File downloaded: {f}")
        except urllib.error.HTTPError as e:
            print(f"❌ Error {e.code}: Not found at {url}")

# your solution

TestSuite("EP01_10.py").run()

✔️ EP01_10.cases already exists in casos/
📋 1 case(s) loaded from casos/EP01_10.cases
💥 File EP01_10.py not found.