Frontal Surface Area STL

main.go

package main

import (
	"bufio"
	"fmt"
	"math"
	"os"
	"strings"
)

func calculateTriangleArea(v1, v2, v3 [3]float64) float64 {
	a := math.Sqrt(math.Pow(v2[0]-v1[0], 2) + math.Pow(v2[1]-v1[1], 2) + math.Pow(v2[2]-v1[2], 2))
	b := math.Sqrt(math.Pow(v3[0]-v2[0], 2) + math.Pow(v3[1]-v2[1], 2) + math.Pow(v3[2]-v2[2], 2))
	c := math.Sqrt(math.Pow(v1[0]-v3[0], 2) + math.Pow(v1[1]-v3[1], 2) + math.Pow(v1[2]-v3[2], 2))
	s := (a + b + c) / 2
	return math.Sqrt(s * (s - a) * (s - b) * (s - c))
}

var (
	// Z-axis (default)
	directionZ = [3]float64{0, 0, 1}
	// Y-axis
	directionY = [3]float64{0, 1, 0}
	// X-axis
	directionX = [3]float64{1, 0, 0}

	// Define a threshold angle (in radians) to determine if a triangle is frontal
	//
	// 45 degrees
	thresholdAngle = math.Pi / 4.0
)

func isFrontalTriangle(normal, incomingAirflowDirection [3]float64) bool {
	// Calculate the angle between the normal vector and the incoming (airflow) direction
	angle := math.Acos(normal[0]*incomingAirflowDirection[0] + normal[1]*incomingAirflowDirection[1] + normal[2]*incomingAirflowDirection[2])

	return angle <= thresholdAngle
}

func main() {
	if os.Args[1] == "-h" || os.Args[1] == "--help" {
		fmt.Println("Usage: go run main.go [path_to_your_stl_file.stl]")
		return
	}

	filePath := os.Args[1]

	if !strings.HasSuffix(filePath, ".stl") {
		fmt.Println("Error: The file must be an STL file (and end with '.stl').")
		return
	}

	frontalArea := 0.0

	file, err := os.Open(filePath)
	if err != nil {
		fmt.Println("Error opening file:", err)
		return
	}
	defer file.Close()

	var (
		v1, v2, v3 [3]float64
		normal     [3]float64
	)

	scanner := bufio.NewScanner(file)

	for scanner.Scan() {
		line := strings.TrimSpace(scanner.Text())

		// Identify facet normal lines
		if strings.HasPrefix(line, "facet normal") {
			// Extract the normal vector
			_, err := fmt.Sscanf(line, "facet normal %f %f %f", &normal[0], &normal[1], &normal[2])
			if err != nil {
				fmt.Printf("Error reading normal vector %q: %v\n", line, err)
				return
			}

			if isFrontalTriangle(normal, directionZ) {
				// Identify vertex lines
				for i := 0; i < 3; i++ {
					scanner.Scan()
					line := strings.TrimSpace(scanner.Text())

					// Extract the vertex coordinates
					var x, y, z float64
					_, err := fmt.Sscanf(line, "vertex %f %f %f", &x, &y, &z)
					if err != nil {
						fmt.Printf("Error reading vertex %q: %v\n", line, err)
						return
					}

					switch i {
					case 0:
						v1 = [3]float64{x, y, z}
					case 1:
						v2 = [3]float64{x, y, z}
					case 2:
						v3 = [3]float64{x, y, z}
					}
				}
				triangleArea := calculateTriangleArea(v1, v2, v3)
				frontalArea += triangleArea
			} else {
				// discard the next 3 lines
				for i := 0; i < 3; i++ {
					scanner.Scan()
				}
			}
		}
	}

	if err := scanner.Err(); err != nil {
		fmt.Println("Error reading file:", err)
		return
	}

	fmt.Printf("Frontal Area: %.6f\n", frontalArea)
}