Task 477: .OFF File Format
Task 477: .OFF File Format
.OFF File Format Specifications
The .OFF (Object File Format) is an ASCII-based file format primarily used for representing 3D geometric models, such as polygon meshes. It was originally developed for the Geomview software but is widely supported in 3D modeling tools. The format is text-based, human-readable, and supports polygons with an arbitrary number of vertices. It can include optional features like colors, normals, textures, and higher dimensions (e.g., 4D).
The file structure typically consists of:
- A header line starting with "OFF" (or variants like "COFF" for colors, "NOFF" for normals, "4OFF" for 4D, etc.).
- A line with counts: number of vertices (N), faces (F), and edges (E; often 0 and ignored).
- N lines of vertex data (usually 3 floats for x, y, z; more for higher dimensions or with normals/textures).
- F lines of face data: starting with the number of vertices in the face (M), followed by M vertex indices (0-based), and optional color values (e.g., 3 integers for RGB or 4 floats for RGBA).
- Comments start with "#" and can appear anywhere.
- Data is space-separated, and lines are newline-terminated.
Variants extend the base format:
- COFF: Adds color (4 floats per vertex or per face: R, G, B, A).
- NOFF: Adds normals (3 floats per vertex).
- STOFF: Adds 2D texture coordinates (2 floats per vertex).
- 4OFF: For 4D, vertices have 4 coordinates; includes an additional count for cells, with cell data after faces.
- nOFF: For n-dimensional vertices.
- Combinations like "STC4NOFF" are possible, where prefixes indicate features.
For this task, I'll focus on the standard 3D OFF with optional face colors, as it's the most common. Higher variants add complexity but follow similar parsing logic (e.g., detect prefix to determine vertex size).
List of all the properties of this file format intrinsic to its file system:
- Header Keyword: The identifier string (e.g., "OFF", "COFF", "4OFF"), indicating the format variant and features like dimensions, colors, normals, or textures.
- Dimensionality: Inferred from the header (e.g., 3 for standard, 4 for "4OFF", or n for "nOFF"); affects vertex coordinate count.
- Number of Vertices (N): Integer count of vertices in the model.
- Number of Faces (F): Integer count of polygonal faces.
- Number of Edges (E): Integer count of edges (often 0 or ignored, as edges can be derived from faces).
- Vertices: A list of N entries, each typically with 3 floating-point coordinates (x, y, z); may include additional floats for colors, normals, or textures based on header.
- Faces: A list of F entries, each starting with an integer M (number of vertices in the face), followed by M integer indices (0-based references to vertices), and optionally color values (3 integers for RGB or 4 floats for RGBA).
- Comments: Optional lines starting with "#", ignored during parsing but part of the file.
- Optional Cells (for 4D variants): If "4OFF", an additional count C for cells, followed by C lines of cell data (similar to faces but referencing face indices).
- Color Mode: Inferred from header (e.g., per-face or per-vertex colors in "COFF").
- Normal Vectors: If "NOFF", 3 floats per vertex for normals.
- Texture Coordinates: If "STOFF", 2 floats per vertex for UV textures.
These properties define the file's structure and content, with parsing starting from the header to determine how to interpret subsequent data.
Two direct download links for files of format .OFF:
- https://people.sc.fsu.edu/~jburkardt/data/off/cube.off
- https://people.sc.fsu.edu/~jburkardt/data/off/icosa.off
Ghost blog embedded HTML JavaScript that allows a user to drag n drop a file of format .OFF and it will dump to screen all these properties:
Here's an embeddable HTML snippet (suitable for a Ghost blog post or any HTML page) with JavaScript for drag-and-drop functionality. It uses the FileReader API to read the .OFF file, parses it (assuming standard 3D OFF with optional face colors; skips comments), and dumps the properties to the screen in a pre-formatted block.
Drag and drop a .OFF file here
Python class that can open any file of format .OFF and decode read and write and print to console all the properties from the above list:
import sys
class OFFFile:
def __init__(self, filename=None):
self.header = 'OFF'
self.dimensionality = 3
self.N = 0
self.F = 0
self.E = 0
self.vertices = []
self.faces = [] # Each: {'numVerts': int, 'indices': list[int], 'color': list[float|int]}
self.has_colors = False
self.filename = filename
if filename:
self.read()
def read(self):
with open(self.filename, 'r') as f:
lines = [line.strip() for line in f if line.strip() and not line.strip().startswith('#')]
index = 0
self.header = lines[index]
index += 1
counts = list(map(int, lines[index].split()))
self.N, self.F, self.E = counts[:3]
self.dimensionality = 4 if '4' in self.header else 3
self.has_colors = 'C' in self.header
index += 1
self.vertices = []
for _ in range(self.N):
verts = list(map(float, lines[index].split()))
self.vertices.append(verts[:self.dimensionality]) # Truncate to dim
index += 1
self.faces = []
for _ in range(self.F):
parts = list(map(float, lines[index].split())) # Float for safety
num_verts = int(parts[0])
indices = list(map(int, parts[1:1 + num_verts]))
color = parts[1 + num_verts:] if self.has_colors and len(parts) > 1 + num_verts else []
self.faces.append({'numVerts': num_verts, 'indices': indices, 'color': color})
index += 1
def write(self, filename=None):
if not filename:
filename = self.filename or 'output.off'
with open(filename, 'w') as f:
f.write(f'{self.header}\n')
f.write(f'{self.N} {self.F} {self.E}\n')
for v in self.vertices:
f.write(' '.join(map(str, v)) + '\n')
for face in self.faces:
line = f"{face['numVerts']} " + ' '.join(map(str, face['indices']))
if face['color']:
line += ' ' + ' '.join(map(str, face['color']))
f.write(line + '\n')
def print_properties(self):
print(f"Header Keyword: {self.header}")
print(f"Dimensionality: {self.dimensionality}")
print(f"Number of Vertices (N): {self.N}")
print(f"Number of Faces (F): {self.F}")
print(f"Number of Edges (E): {self.E}")
print("Vertices:")
for v in self.vertices:
print(f" {v}")
print("Faces:")
for f in self.faces:
print(f" Num Verts: {f['numVerts']}, Indices: {f['indices']}, Color: {f['color'] or 'none'}")
# Extend for other properties like normals/textures if needed
# Example usage:
# off = OFFFile('cube.off')
# off.print_properties()
# off.write('new_cube.off')
Java class that can open any file of format .OFF and decode read and write and print to console all the properties from the above list:
import java.io.*;
import java.util.*;
public class OFFFile {
private String header = "OFF";
private int dimensionality = 3;
private int N = 0;
private int F = 0;
private int E = 0;
private List<double[]> vertices = new ArrayList<>();
private List<Map<String, Object>> faces = new ArrayList<>(); // Each: numVerts:int, indices:List<Integer>, color:List<Number>
private boolean hasColors = false;
private String filename;
public OFFFile(String filename) {
this.filename = filename;
read();
}
public void read() {
try (BufferedReader br = new BufferedReader(new FileReader(filename))) {
List<String> lines = new ArrayList<>();
String line;
while ((line = br.readLine()) != null) {
line = line.trim();
if (!line.isEmpty() && !line.startsWith("#")) {
lines.add(line);
}
}
int index = 0;
header = lines.get(index++);
String[] counts = lines.get(index++).split("\\s+");
N = Integer.parseInt(counts[0]);
F = Integer.parseInt(counts[1]);
E = Integer.parseInt(counts[2]);
dimensionality = header.contains("4") ? 4 : 3;
hasColors = header.contains("C");
vertices.clear();
for (int i = 0; i < N; i++) {
String[] parts = lines.get(index++).split("\\s+");
double[] verts = new double[dimensionality];
for (int j = 0; j < dimensionality; j++) {
verts[j] = Double.parseDouble(parts[j]);
}
vertices.add(verts);
}
faces.clear();
for (int i = 0; i < F; i++) {
String[] parts = lines.get(index++).split("\\s+");
int numVerts = Integer.parseInt(parts[0]);
List<Integer> indices = new ArrayList<>();
for (int j = 1; j < 1 + numVerts; j++) {
indices.add(Integer.parseInt(parts[j]));
}
List<Number> color = new ArrayList<>();
if (hasColors && parts.length > 1 + numVerts) {
for (int j = 1 + numVerts; j < parts.length; j++) {
try {
color.add(Integer.parseInt(parts[j]));
} catch (NumberFormatException e) {
color.add(Double.parseDouble(parts[j]));
}
}
}
Map<String, Object> face = new HashMap<>();
face.put("numVerts", numVerts);
face.put("indices", indices);
face.put("color", color);
faces.add(face);
}
} catch (IOException e) {
e.printStackTrace();
}
}
public void write(String outputFilename) {
try (PrintWriter pw = new PrintWriter(new File(outputFilename))) {
pw.println(header);
pw.println(N + " " + F + " " + E);
for (double[] v : vertices) {
for (double coord : v) {
pw.print(coord + " ");
}
pw.println();
}
for (Map<String, Object> face : faces) {
pw.print(face.get("numVerts") + " ");
@SuppressWarnings("unchecked")
List<Integer> indices = (List<Integer>) face.get("indices");
for (int idx : indices) {
pw.print(idx + " ");
}
@SuppressWarnings("unchecked")
List<Number> color = (List<Number>) face.get("color");
if (!color.isEmpty()) {
for (Number c : color) {
pw.print(c + " ");
}
}
pw.println();
}
} catch (FileNotFoundException e) {
e.printStackTrace();
}
}
public void printProperties() {
System.out.println("Header Keyword: " + header);
System.out.println("Dimensionality: " + dimensionality);
System.out.println("Number of Vertices (N): " + N);
System.out.println("Number of Faces (F): " + F);
System.out.println("Number of Edges (E): " + E);
System.out.println("Vertices:");
for (double[] v : vertices) {
System.out.println(" " + Arrays.toString(v));
}
System.out.println("Faces:");
for (Map<String, Object> f : faces) {
System.out.println(" Num Verts: " + f.get("numVerts") + ", Indices: " + f.get("indices") + ", Color: " + (f.get("color").toString().isEmpty() ? "none" : f.get("color")));
}
// Extend for other properties
}
public static void main(String[] args) {
if (args.length > 0) {
OFFFile off = new OFFFile(args[0]);
off.printProperties();
off.write("new_" + args[0]);
}
}
}
JavaScript class that can open any file of format .OFF and decode read and write and print to console all the properties from the above list:
Note: JavaScript doesn't have native file I/O in browsers; this assumes a Node.js environment (use fs module). For browser, use FileReader as in part 3.
const fs = require('fs');
class OFFFile {
constructor(filename) {
this.header = 'OFF';
this.dimensionality = 3;
this.N = 0;
this.F = 0;
this.E = 0;
this.vertices = [];
this.faces = []; // Each: {numVerts: number, indices: number[], color: number[]}
this.hasColors = false;
this.filename = filename;
if (filename) {
this.read();
}
}
read() {
const content = fs.readFileSync(this.filename, 'utf8');
const lines = content.split('\n').map(l => l.trim()).filter(l => l && !l.startsWith('#'));
let index = 0;
this.header = lines[index++];
const counts = lines[index++].split(/\s+/).map(Number);
this.N = counts[0];
this.F = counts[1];
this.E = counts[2];
this.dimensionality = this.header.includes('4') ? 4 : 3;
this.hasColors = this.header.includes('C');
this.vertices = [];
for (let i = 0; i < this.N; i++) {
const verts = lines[index++].split(/\s+/).map(parseFloat).slice(0, this.dimensionality);
this.vertices.push(verts);
}
this.faces = [];
for (let i = 0; i < this.F; i++) {
const parts = lines[index++].split(/\s+/).map(Number);
const numVerts = parts[0];
const indices = parts.slice(1, 1 + numVerts);
const color = this.hasColors && parts.length > 1 + numVerts ? parts.slice(1 + numVerts) : [];
this.faces.push({ numVerts, indices, color });
}
}
write(filename = null) {
if (!filename) filename = this.filename || 'output.off';
let output = `${this.header}\n`;
output += `${this.N} ${this.F} ${this.E}\n`;
this.vertices.forEach(v => {
output += v.join(' ') + '\n';
});
this.faces.forEach(f => {
let line = `${f.numVerts} ${f.indices.join(' ')}`;
if (f.color.length) line += ` ${f.color.join(' ')}`;
output += line + '\n';
});
fs.writeFileSync(filename, output);
}
printProperties() {
console.log(`Header Keyword: ${this.header}`);
console.log(`Dimensionality: ${this.dimensionality}`);
console.log(`Number of Vertices (N): ${this.N}`);
console.log(`Number of Faces (F): ${this.F}`);
console.log(`Number of Edges (E): ${this.E}`);
console.log('Vertices:');
this.vertices.forEach(v => console.log(` [${v.join(', ')}]`));
console.log('Faces:');
this.faces.forEach(f => console.log(` Num Verts: ${f.numVerts}, Indices: [${f.indices.join(', ')}], Color: [${f.color.join(', ') || 'none'}]`));
}
}
// Example usage:
// const off = new OFFFile('cube.off');
// off.printProperties();
// off.write('new_cube.off');
C class that can open any file of format .OFF and decode read and write and print to console all the properties from the above list:
In C, we use a struct with functions (no classes). Here's a implementation using dynamic allocation.
#include <stdio.h>
#include <stdlib.h>
#include <string.h>
#include <ctype.h>
#define MAX_LINE 1024
typedef struct {
char *header;
int dimensionality;
int N, F, E;
double **vertices; // N x dimensionality
struct Face *faces; // F faces
int has_colors;
} OFFFile;
typedef struct Face {
int num_verts;
int *indices;
double *color; // NULL if none, else 3 or 4 elems
int color_count;
} Face;
OFFFile* off_create(const char *filename) {
OFFFile *off = malloc(sizeof(OFFFile));
off->header = strdup("OFF");
off->dimensionality = 3;
off->N = off->F = off->E = 0;
off->vertices = NULL;
off->faces = NULL;
off->has_colors = 0;
if (filename) {
off_read(off, filename);
}
return off;
}
void off_destroy(OFFFile *off) {
free(off->header);
if (off->vertices) {
for (int i = 0; i < off->N; i++) free(off->vertices[i]);
free(off->vertices);
}
if (off->faces) {
for (int i = 0; i < off->F; i++) {
free(off->faces[i].indices);
if (off->faces[i].color) free(off->faces[i].color);
}
free(off->faces);
}
free(off);
}
void off_read(OFFFile *off, const char *filename) {
FILE *fp = fopen(filename, "r");
if (!fp) {
perror("Failed to open file");
return;
}
char line[MAX_LINE];
int line_count = 0;
char **lines = NULL;
while (fgets(line, MAX_LINE, fp)) {
char *trimmed = line;
while (isspace(*trimmed)) trimmed++;
if (*trimmed && *trimmed != '#') {
lines = realloc(lines, (line_count + 1) * sizeof(char*));
lines[line_count++] = strdup(trimmed);
}
}
fclose(fp);
int index = 0;
off->header = strdup(lines[index++]);
char *counts_str = lines[index++];
sscanf(counts_str, "%d %d %d", &off->N, &off->F, &off->E);
off->dimensionality = strstr(off->header, "4") ? 4 : 3;
off->has_colors = strstr(off->header, "C") ? 1 : 0;
off->vertices = malloc(off->N * sizeof(double*));
for (int i = 0; i < off->N; i++) {
off->vertices[i] = malloc(off->dimensionality * sizeof(double));
char *v_str = lines[index++];
char *token = strtok(v_str, " ");
for (int j = 0; j < off->dimensionality; j++) {
off->vertices[i][j] = atof(token);
token = strtok(NULL, " ");
}
}
off->faces = malloc(off->F * sizeof(Face));
for (int i = 0; i < off->F; i++) {
char *f_str = lines[index++];
char *token = strtok(f_str, " ");
off->faces[i].num_verts = atoi(token);
off->faces[i].indices = malloc(off->faces[i].num_verts * sizeof(int));
token = strtok(NULL, " ");
for (int j = 0; j < off->faces[i].num_verts; j++) {
off->faces[i].indices[j] = atoi(token);
token = strtok(NULL, " ");
}
off->faces[i].color = NULL;
off->faces[i].color_count = 0;
if (off->has_colors && token) {
int color_max = 4; // RGBA max
off->faces[i].color = malloc(color_max * sizeof(double));
int c = 0;
while (token && c < color_max) {
off->faces[i].color[c++] = atof(token);
token = strtok(NULL, " ");
}
off->faces[i].color_count = c;
}
}
for (int i = 0; i < line_count; i++) free(lines[i]);
free(lines);
}
void off_write(OFFFile *off, const char *filename) {
FILE *fp = fopen(filename, "w");
if (!fp) {
perror("Failed to write file");
return;
}
fprintf(fp, "%s\n", off->header);
fprintf(fp, "%d %d %d\n", off->N, off->F, off->E);
for (int i = 0; i < off->N; i++) {
for (int j = 0; j < off->dimensionality; j++) {
fprintf(fp, "%f ", off->vertices[i][j]);
}
fprintf(fp, "\n");
}
for (int i = 0; i < off->F; i++) {
fprintf(fp, "%d ", off->faces[i].num_verts);
for (int j = 0; j < off->faces[i].num_verts; j++) {
fprintf(fp, "%d ", off->faces[i].indices[j]);
}
for (int j = 0; j < off->faces[i].color_count; j++) {
fprintf(fp, "%f ", off->faces[i].color[j]);
}
fprintf(fp, "\n");
}
fclose(fp);
}
void off_print_properties(OFFFile *off) {
printf("Header Keyword: %s\n", off->header);
printf("Dimensionality: %d\n", off->dimensionality);
printf("Number of Vertices (N): %d\n", off->N);
printf("Number of Faces (F): %d\n", off->F);
printf("Number of Edges (E): %d\n", off->E);
printf("Vertices:\n");
for (int i = 0; i < off->N; i++) {
printf(" [");
for (int j = 0; j < off->dimensionality; j++) {
printf("%f%s", off->vertices[i][j], j < off->dimensionality - 1 ? ", " : "");
}
printf("]\n");
}
printf("Faces:\n");
for (int i = 0; i < off->F; i++) {
printf(" Num Verts: %d, Indices: [", off->faces[i].num_verts);
for (int j = 0; j < off->faces[i].num_verts; j++) {
printf("%d%s", off->faces[i].indices[j], j < off->faces[i].num_verts - 1 ? ", " : "");
}
printf("], Color: [");
if (off->faces[i].color_count > 0) {
for (int j = 0; j < off->faces[i].color_count; j++) {
printf("%f%s", off->faces[i].color[j], j < off->faces[i].color_count - 1 ? ", " : "");
}
} else {
printf("none");
}
printf("]\n");
}
}
int main(int argc, char **argv) {
if (argc > 1) {
OFFFile *off = off_create(argv[1]);
off_print_properties(off);
off_write(off, "new.off");
off_destroy(off);
}
return 0;
}