Polygon transformation problem

[TCH]

I just begun experimenting with vertorgraphics and i'm trying to transform a 3D polygon into a 2D one, but i get zeros for all transformed coordinates. I implemented the matrixless version of the algorithm on Wikipedia.

Here is the code:

#include <stdint.h>
#include <stdlib.h>
#include <math.h>

typedef struct
{
	float x, y, z, thx, thy, thz, costhx, costhy, costhz, sinthx, sinthy, sinthz;
} Camera;

typedef struct
{
	float x, y;
	int16_t sx, sy;
} Point2D;

typedef struct
{
	int16_t VertexCount;
	Point2D *Vertices;
} Polygon2D;

typedef struct
{
	float x, y, z;
} Point3D;

typedef struct
{
	int16_t VertexCount;
	Point3D *Vertices;
} Polygon3D;

void RecalcCameraAngles(Camera *C)
{
	C->costhx = cos(C->thx);
	C->costhy = cos(C->thy);
	C->costhz = cos(C->thz);
	C->sinthx = sin(C->thx);
	C->sinthy = sin(C->thy);
	C->sinthz = sin(C->thz);
}

void TransformPolygon3DToPolygon2D(Polygon3D *P3D, Camera *C, Polygon2D *P2D)
{
	int16_t k, vc;
	Point2D *p2D;
	Point3D *p3D;
	float x, y, z, costhx, costhy, costhz, sinthx, sinthy, sinthz, dx, dy, dz, tmp0, tmp1, px, py, Cx, Cy, Cz;
	const float ex = 0;
	const float ey = 0;
	const float ez = 500;

	costhx = C->costhx;
	costhy = C->costhy;
	costhz = C->costhz;
	sinthx = C->sinthx;
	sinthy = C->sinthy;
	sinthz = C->sinthz;
	Cx = C->x;
	Cy = C->y;
	Cz = C->z;

	p3D = &P3D->Vertices[0];
	p2D = &P2D->Vertices[0];
	for (k = 0; k < vc; ++k)
	{
		x = p3D->x - Cx;
		y = p3D->y - Cy;
		z = p3D->z - Cz;

		tmp0 = sinthz * y + costhz * x;
		dx = costhy * tmp0 - sinthy * z;
		tmp0 *= sinthy;
		tmp0 += costhy * z;
		tmp1 = costhz * y - sinthz * x;
		dy = sinthx * tmp0 + costhx * tmp1;
		dz = costhx * tmp0 - sinthx * tmp1;
	
		tmp0 = ez / dz;
		px = tmp0 * dx + ex;
		py = tmp0 * dy + ey;

		p2D->x = px;
		p2D->y = py;
		++p2D;
		++p3D;
	}
}

int main(int argc, char *argv[])
{
	Polygon2D P2D;
	Polygon3D P3D;
	Camera C;

	P3D.VertexCount = 5;
	P3D.Vertices = malloc(5 * sizeof(Point3D));
	P3D.Vertices[0].x = 0.0;
	P3D.Vertices[0].y = 0.8;
	P3D.Vertices[0].z = 0.5;
	P3D.Vertices[1].x = 0.5;
	P3D.Vertices[1].y = 0.6;
	P3D.Vertices[1].z = 0.5;
	P3D.Vertices[2].x = 0.2;
	P3D.Vertices[2].y = 0.4;
	P3D.Vertices[2].z = 0.5;
	P3D.Vertices[3].x = 0.2;
	P3D.Vertices[3].y = 0.4;
	P3D.Vertices[3].z = 0.5;
	P3D.Vertices[4].x = 0.5;
	P3D.Vertices[4].y = 0.6;
	P3D.Vertices[4].z = 0.5;

	C.x = 0.0;
	C.y = 0.0;
	C.z = 0.0;
	C.thx = 0.0;
	C.thy = 0.0;
	C.thz = 0.0;
	RecalcCameraAngles(&C);

	P2D.VertexCount = 5;
	P2D.Vertices = malloc(5 * sizeof(Point2D));

	TransformPolygon3DToPolygon2D(&P3D, &C, &P2D);
	printf("%f, %f\n", P2D.Vertices[0].x, P2D.Vertices[0].y);
	printf("%f, %f\n", P2D.Vertices[1].x, P2D.Vertices[1].y);
	printf("%f, %f\n", P2D.Vertices[2].x, P2D.Vertices[2].y);
	printf("%f, %f\n", P2D.Vertices[3].x, P2D.Vertices[3].y);
	printf("%f, %f\n", P2D.Vertices[4].x, P2D.Vertices[4].y);


	free(P2D.Vertices);
	free(P3D.Vertices);
	return 0;
}

Can anyone tell me what i am doing wrong?

[deimos]

You never set vc.

[StingRay]

Exactly. Enable warnings in your compiler, then you can spot such mistakes easily.

[TCH]

DOH! Thanks guys. In the original function which held all the parts it was set, but i missed this, when i splitted it for tests. Warnings were enabled, but not with

-Wall

, i switched to that.

[TCH]

Okay, it seems to work, but some other questions have emerged.

deimos' suggestion about setting

ex

and

ey

to 0 was correct, but giving

ez

the value of 500 yielded positionally correct but extraordinally overzoomed coordinates. However if

ez

supposed to be the distance between the eye and the selected point and here the eye is actually the camera, then isn't

ez

supposed to be the distance between the camera's current point and the currently transformed point in the vectorspace? In short:

x = p3D->x - Cx;
y = p3D->y - Cy;
z = p3D->z - Cz;
ez = sqrt((x * x) + (y * y) + (z * z));

I've tried it and i got good results, but due to the lack of experience i am unable to tell if my guess was correct or this is just coincidence.