stl: computing normals in the stl module by cross product now

git-svn-id: svn://anubis/misc/stl@7 bd8a9e45-a331-0410-811e-c64571078777

stl-viewer.c

@@ -101,13 +101,14 @@ GLuint	buildDL(void)
 	unsigned int i;
 	int dfltClrAct = 1;
 	GLfloat color[4];
+	float normal[3];
 	color[3] = 0.0f;
 	GLuint dl = glGenLists(1);
 	glNewList(dl, GL_COMPILE);
-	glBegin(GL_TRIANGLES);
 	glMaterialfv(GL_FRONT, GL_AMBIENT_AND_DIFFUSE, default_color);
 	for (i = 0; i < n_faces; i++)
 	{
+#if 0
 		if (stl_attr_color_valid(stl_face(stl, i).attribute))
 		{
 			color[0] = stl_attr_color_r(stl_face(stl, i).attribute) / (float) 255;
@@ -119,7 +120,13 @@ GLuint	buildDL(void)
 		else if (!dfltClrAct)
 			glMaterialfv(GL_FRONT, GL_AMBIENT_AND_DIFFUSE,
 				default_color);
-		glNormal3fv(stl_face(stl, i).normal);
+#endif
+		stl_get_face_normal(stl, i, normal);
+		printf("normal: %f, %f, %f\n",
+			normal[0],
+			normal[1],
+			normal[2]);
+		glNormal3fv(normal);
 		int j;
 		for (j = 0; j < 3; j++)
 			glVertex3fv(stl_face(stl, i).vertices[j]);

stl.c

@@ -2,6 +2,7 @@
 #include <sys/stat.h>		/* struct stat */
 #include <stdlib.h>		/* malloc() */
 #include <stdio.h>		/* fprintf() */
+#include <math.h>		/* sqrt() */
 #include "stl.h"
 
 /* load a STL file and return a pointer to the stl_t struct */
@@ -37,3 +38,32 @@ stl_t *	stl_load(const char * filename)
 	}
 	return stl;
 }
+
+/* fill vector pointed to by normal with the normal of the face */
+void	stl_get_face_normal(stl_t * stl, int face, float * normal)
+{
+	float e1[3];
+	float e2[3];
+	int i;
+	for (i = 0; i < 3; i++)
+	{
+		e1[i] = stl_face(stl, face).vertices[1][i] -
+			stl_face(stl, face).vertices[0][i];
+		e2[i] = stl_face(stl, face).vertices[2][i] -
+			stl_face(stl, face).vertices[1][i];
+	}
+
+	/* take the cross product of the first two edges */
+	normal[0] = e1[1] * e2[2] - e1[2] * e2[1];
+	normal[1] = e1[2] * e2[0] - e1[0] * e2[2];
+	normal[2] = e1[0] * e2[1] - e1[1] * e2[0];
+
+	/* then normalize */
+	double l;
+	l = sqrt(normal[0] * normal[0] +
+		 normal[1] * normal[1] +
+		 normal[2] * normal[2]);
+	normal[0] /= l;
+	normal[1] /= l;
+	normal[2] /= l;
+}

stl.h

@@ -37,6 +37,7 @@ typedef struct stl_s
 
 /* functions */
 stl_t *	stl_load(const char * filename);
+void	stl_get_face_normal(stl_t * stl, int face, float * normal);
 
 
 #ifdef __cplusplus