World coordinates to screen coordinates

Shawn Halayka · 2020-10-28T17:59:02

I'm calling a function: // Get pixel coordinates vertex_3 p = get_screen_coords_from_world_coords(transformed_vertex, camera_pos, projection_modelview_mat, win_x, win_y); // Detect region GLint y_viewport_pos = -1; if (p.y < 0) y_viewport_pos = ABOVE_VIEWPORT; else if (p.y < win_y) y_viewport_pos = IN_VIEWPORT; else y_viewport_pos = BELOW_VIEWPORT; GLint x_viewport_pos = -1; if (p.x < 0) x_viewport_pos = LEFT_OF_VIEWPORT; else if (p.x < win_x) x_viewport_pos = IN_VIEWPORT; else x_viewport_pos = RIGHT_OF_VIEWPORT; where vertex_3 get_screen_coords_from_world_coords(const vertex_3 p, const vertex_3 c, const float(&mat)[16], const int width, const int height) { vertex_3 v = p - c; v.x /= v.z; v.y /= v.z; v.z /= v.z; float point[4]; point[0] = c.x + v.x; point[1] = c.y + v.y; point[2] = c.z + v.z; point[3] = 0; float out[4]; multiply_4x4_matrix_by_4_vector(mat, point, out); return vertex_3( round(((1.0f + out[0]) * 0.5f) * (float)width), round(((1.0f - out[1]) * 0.5f) * (float)height), 0.0f); } This function tells the 2D pixel coordinate of a 3D vertex. It works great for 8 of 9 cases, except for the last case, This last case is where the x location lies on screen and the y location lies below the screen. Instead of detecting these last case locations, it detects that the location is actually above the screen – that is, the value of p.y < 0, where it should be p.y > win_y. Any idea why? Thanks for your time. P.S. void init_perspective_camera(float fovy, float aspect, float znear, float zfar, float eyex, float eyey, float eyez, float centrex, float centrey, float centrez, float upx, float upy, float upz, float (&projection_modelview_mat)[16]) { float projection_mat[16]; get_perspective_matrix(fovy, aspect, znear, zfar, projection_mat); float modelview_mat[16]; get_look_at_matrix(eyex, eyey, eyez, // Eye position. centrex, centrey, centrez, // Look at position (not direction). upx, upy, upz, // Up direction vector. modelview_mat); multiply_4x4_matrices(projection_mat, modelview_mat, projection_modelview_mat); } and void multiply_4x4_matrix_by_4_vector(const float(&in_a)[16], const float(&in_b)[4], float(&out)[4]) { float temp[4]; for (int i = 0; i < 4; i++) { temp[i] = 0.0; for (int j = 0; j < 4; j++) { temp[i] += in_a[i*4 + j] * in_b[j]; } } for (size_t i = 0; i < 4; i++) out[i] = temp[i]; } void multiply_4x4_matrices(float (&in_a)[16], float (&in_b)[16], float (&out)[16]) { /* matrix layout: [0 4 8 12] [1 5 9 13] [2 6 10 14] [3 7 11 15] */ out[0] = in_a[0] * in_b[0] + in_a[4] * in_b[1] + in_a[8] * in_b[2] + in_a[12] * in_b[3]; out[1] = in_a[1] * in_b[0] + in_a[5] * in_b[1] + in_a[9] * in_b[2] + in_a[13] * in_b[3]; out[2] = in_a[2] * in_b[0] + in_a[6] * in_b[1] + in_a[10] * in_b[2] + in_a[14] * in_b[3]; out[3] = in_a[3] * in_b[0] + in_a[7] * in_b[1] + in_a[11] * in_b[2] + in_a[15] * in_b[3]; out[4] = in_a[0] * in_b[4] + in_a[4] * in_b[5] + in_a[8] * in_b[6] + in_a[12] * in_b[7]; out[5] = in_a[1] * in_b[4] + in_a[5] * in_b[5] + in_a[9] * in_b[6] + in_a[13] * in_b[7]; out[6] = in_a[2] * in_b[4] + in_a[6] * in_b[5] + in_a[10] * in_b[6] + in_a[14] * in_b[7]; out[7] = in_a[3] * in_b[4] + in_a[7] * in_b[5] + in_a[11] * in_b[6] + in_a[15] * in_b[7]; out[8] = in_a[0] * in_b[8] + in_a[4] * in_b[9] + in_a[8] * in_b[10] + in_a[12] * in_b[11]; out[9] = in_a[1] * in_b[8] + in_a[5] * in_b[9] + in_a[9] * in_b[10] + in_a[13] * in_b[11]; out[10] = in_a[2] * in_b[8] + in_a[6] * in_b[9] + in_a[10] * in_b[10] + in_a[14] * in_b[11]; out[11] = in_a[3] * in_b[8] + in_a[7] * in_b[9] + in_a[11] * in_b[10] + in_a[15] * in_b[11]; out[12] = in_a[0] * in_b[12] + in_a[4] * in_b[13] + in_a[8] * in_b[14] + in_a[12] * in_b[15]; out[13] = in_a[1] * in_b[12] + in_a[5] * in_b[13] + in_a[9] * in_b[14] + in_a[13] * in_b[15]; out[14] = in_a[2] * in_b[12] + in_a[6] * in_b[13] + in_a[10] * in_b[14] + in_a[14] * in_b[15]; out[15] = in_a[3] * in_b[12] + in_a[7] * in_b[13] + in_a[11] * in_b[14] + in_a[15] * in_b[15]; }

Graphics and GPU Programming Programming OpenGLC++

Started by taby October 27, 2020 08:25 PM

12 comments, last by taby 4 years, 3 months ago

taby

Author

1,557

October 28, 2020 12:41 AM

Yes, let me show you a mock-up. If the piece lies off-screen, there is an arrow indicating its location. The arrows in the toolbar rotate the playing field (and all the game pieces, etc). Thank you so much for your help!

taby

Author

1,557

October 28, 2020 12:44 AM

P.S.

    if (v.z != 0)
    {
        v.x /= v.z;
        v.y /= v.z;
        v.z /= v.z;
    }

JoeJ

4,405

October 28, 2020 01:01 AM

Never did such HUD, but i think this would work:

If point is behind front clip plane, project it to it (in world space).
Clamp the resulting screenspace position inside the visible screen rectangle.

taby

Author

1,557

October 28, 2020 05:59 PM

The hack that I use relies on a stationary camera. The geometry to draw is rotated instead.

const vertex_3 camera_pos(0, 0, -1); // Camera position.
const vertex_3 look_at_pos(0, 0, 0); // Look at position.

Thanks again for your help, you guys!

World coordinates to screen coordinates

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World coordinates to screen coordinates

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