z-sorted list of triangles

This commit is contained in:
Trammell hudson 2017-09-27 08:19:21 -04:00
parent a3815d7c4e
commit 36aba4db78
Failed to extract signature

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@ -35,6 +35,17 @@ typedef struct
} __attribute__((__packed__))
stl_face_t;
typedef struct _tri_t tri_t;
struct _tri_t
{
v3_t p[3];
float area;
float depth;
tri_t * next;
tri_t ** prev;
};
#if 0
typedef struct face face_t;
typedef struct poly poly_t;
@ -714,6 +725,117 @@ stl2faces(
}
#endif
/*
s = 1/(2*Area)*(p0y*p2x - p0x*p2y + (p2y - p0y)*px + (p0x - p2x)*py);
t = 1/(2*Area)*(p0x*p1y - p0y*p1x + (p0y - p1y)*px + (p1x - p0x)*py);
where Area is the (signed) area of the triangle:
Area = 0.5 *(-p1y*p2x + p0y*(-p1x + p2x) + p0x*(p1y - p2y) + p1x*p2y);
Just evaluate s, t and 1-s-t. The point p is inside the triangle if and only if they are all positive.
*/
int inside_triangle(
const v3_t * const p,
const v3_t * const t0,
const v3_t * const t1,
const v3_t * const t2
)
{
const float p0x = t0->p[0];
const float p0y = t0->p[1];
const float p1x = t1->p[0];
const float p1y = t1->p[1];
const float p2x = t2->p[0];
const float p2y = t2->p[1];
const float px = p->p[0];
const float py = p->p[1];
const float s = p0y*p2x - p0x*p2y + (p2y - p0y)*px + (p0x - p2x)*py;
const float t = p0x*p1y - p0y*p1x + (p0y - p1y)*px + (p1x - p0x)*py;
if (s <= 0 || t <= 0)
return 0;
// maybe inside; check for sure
const float area = 0.5 *(-p1y*p2x + p0y*(-p1x + p2x) + p0x*(p1y - p2y) + p1x*p2y);
if (s + t <= 2 * area)
return 0;
// inside!
return 1;
}
tri_t *
tri_new(
const v3_t * p
)
{
tri_t * const t = calloc(1, sizeof(*t));
if (!t)
return NULL;
for(int i = 0 ; i < 3 ; i++)
t->p[i] = p[i];
// precompute the area
const float p0x = t->p[0].p[0];
const float p0y = t->p[0].p[1];
const float p1x = t->p[1].p[0];
const float p1y = t->p[1].p[1];
const float p2x = t->p[2].p[0];
const float p2y = t->p[2].p[1];
t->area = 0.5 *(-p1y*p2x + p0y*(-p1x + p2x) + p0x*(p1y - p2y) + p1x*p2y);
// compute an average z-depth
// this isn't exactly right, but close enough
t->depth = (t->p[0].p[2] + t->p[1].p[2] + t->p[2].p[2]) / 3;
// should we pre-compute the normal?
return t;
}
// insert a triangle into our z-sorted list
void
tri_insert(
tri_t ** zlist,
tri_t * t
)
{
while(1)
{
tri_t * const iter = *zlist;
if (!iter)
break;
if(iter->depth > t->depth)
break;
zlist = &(iter->next);
}
// either we reached the end of the list,
// or we have found where our new triangle is sorted
t->next = *zlist;
*zlist = t;
if (t->next)
t->next->prev = &t->next;
}
void
tri_delete(tri_t * t)
{
if (t->next)
t->next->prev = t->prev;
*(t->prev) = t->next;
free(t);
}
int main(
int argc,
@ -753,11 +875,12 @@ int main(
printf("<g transform=\"translate(%f %f)\">\n", off_x, off_y);
int rejected = 0;
tri_t * zlist = NULL;
// transform the stl in place by the camera projection
for (int i = 0 ; i < num_triangles ; i++)
{
const stl_face_t * const stl = &stl_faces[i];
int reject = 0;
v3_t s[3];
@ -767,7 +890,7 @@ int main(
// reject this face if any of them are behind us
for(int j = 0 ; j < 3 ; j++)
if (s[j].p[2] <= 0)
reject = 1;
goto reject;
// do a back-face cull to determine if this triangle
// is not facing us. we have to determine the orientation
@ -778,22 +901,34 @@ int main(
);
if (normal.p[2] <= 0)
reject = 1;
goto reject;
if (reject)
{
rejected++;
continue;
}
// draw each of the three lines
for(int j = 0 ; j < 3 ; j++)
svg_line("#FF0000", s[j].p, s[(j+1) % 3].p, 0);
// it passes the first tests, so insert it into the list
tri_t * const tri = tri_new(s);
tri_insert(&zlist, tri);
continue;
reject:
rejected++;
}
if (debug)
fprintf(stderr, "Rejected %d triangles\n", rejected);
// we now have a z-sorted list of triangles
for( tri_t * t = zlist ; t ; t = t->next )
{
// check to see if this triangle is entirely occluded
// by another triangle
// draw each of the three lines
for(int j = 0 ; j < 3 ; j++)
svg_line("#FF0000", t->p[j].p, t->p[(j+1) % 3].p, 0);
}
#if 0
face_t * const faces = stl2faces(stl_faces, num_triangles);