# c - 3d to 2d Coordinate trouble

Im attempting to write a very basic raytracer in C from the ground up. Im having some trouble getting the 2d coordinate from my 3d point in space.

I was able to shoot off the camera rays easily and accurately enough drawing the model on screen nicely. Now I am wanting to have a defined bounding box for the model on screen so i can quickly move over areas of the image that arent relevant to the rendering process.

The closest result ive had is better when the objects are closer to the camera. When I push the camera back ( incrementing cam->pos.z ) my bounding box starts to become way way off.

Ive seen a good dozen or so different formulas on stackoverflow that ive been experimenting/applying with but havent gotten them working successfully. If anyone can look at what im doing and offer some advice/correction that would be great.

My coordinates are y-up.

functions I am using to get the ray direction (seems to be working but thought it might be relevant):

``#define FOV 55 // placeholder - should move to camera attrdouble deg2rad(double degrees){return degrees * M_PI / 180.0;}double rad2deg(double radians){return radians * 180.0 / M_PI;}void ray_fromPixel(Camera *camera, Resolution *res, Ray *ray, int x, int y) {ray->origin = camera->position;ray->orientation.x = x - (res->width/2);ray->orientation.y = (res->height / 2) - y;ray->orientation.z = -(res->height / 2) / tan(deg2rad(FOV*0.5));vector_Normalize(&ray->orientation);}``

the 3d to 2d coordinate version (not working so well):

``Coordinate coord_fromVector(Vector *v, Vector *cam_pos, int width, int height) {Coordinate screen;double hwidth = ((double)width) / 2;double hheight = ((double)height) / 2;double x_3D = v->x - cam_pos->x;double y_3D = v->y - cam_pos->y;double z_3D = v->z - cam_pos->z;screen.x = hwidth + (x_3D / z_3D) * hwidth;screen.y = hheight - (y_3D / z_3D) * hheight;/* aspect ratio is commented out as its moving the box off screen *//*double aspect = screen.resolution.width / screen.resolution.height;if (aspect > 1.0) {screen.x /= aspect;} else {screen.y *= aspect;}*/return screen;}``

Edit:Attempting to utilize some matrices...

`` void matrix_projection(double ***m, double fov,double aspect,double znear, double zfar){double xymax = znear * tan(fov * M_PI / 360);double ymin = -xymax;double xmin = -xymax;double width = xymax - xmin;double height = xymax - ymin;double depth = zfar - znear;double q = -(zfar + znear) / depth;double qn = -2 * (zfar * znear) / depth;double w = 2 * znear / width;w = w / aspect;double h = 2 * znear / height;(*m) = w; (*m) = 0; (*m) = 0; (*m) = 0;(*m) = 0; (*m) = h; (*m) = 0; (*m) = 0;(*m) = 0; (*m) = 0; (*m) = q; (*m) = qn;(*m) = 0; (*m) = 0; (*m) = -1; (*m) = 0;}//projection_matrix * modelview_matrix * world_coordinatesvoid matrix_multiply(double ***result, double ***m1, int r1,int c1, double ***m2, int c2){int i, j, k;for(i=0; i<r1; ++i)for(j=0; j<c2; ++j)for(k=0; k<c1; ++k){(*result)[i][j] += ((*m1)[i][k]) * ((*m2)[k][j]);}}void matrix_allocate(double ***matrix, int rows, int cols){int x, y;*matrix = (double**) malloc(sizeof(double *) * rows);for (x=0; x < rows; x++) {(*matrix)[x] = malloc(sizeof(double) * cols);for(y=0; y < cols; y++) {(*matrix)[x][y] = 0.0;}}}Coordinate coord_fromVector(Vector *v, Vector *cam_pos, int width, int height) {Coordinate screen;double hwidth = ((double)width) / 2;double hheight = ((double)height) / 2;double aspect = hwidth / hheight;double **proj;matrix_allocate(&proj, 4, 4);double **model;matrix_allocate(&model, 4, 4);double **vmatrix;matrix_allocate(&vmatrix, 1, 4);vmatrix = v->x;vmatrix = v->y;vmatrix = v->z;vmatrix = 1;matrix_projection(&proj, 55, aspect, 1.0, 2000.0);model = model = model = model = 1;model = cam_pos->x;model = cam_pos->y;model = cam_pos->z;double **proj_model;matrix_allocate(&proj_model, 4, 4);matrix_multiply(&proj_model, &proj, 4, 4, &model, 4) ;double **proj_model_v;matrix_allocate(&proj_model_v, 1, 4);matrix_multiply(&proj_model_v, &proj_model, 4, 4, &vmatrix, 1);screen.x = proj_model_v / proj_model_v;screen.y = proj_model_v / proj_model_v;return screen;}``

At this stage its still a long way off unfortunately.