How do I detect if an object has reached the screen boundary in a 3D game?
Author
I am implementing a starfield into my 3D space shooter. The scene is viewed from a slightly downward angle and the stars move towards the player. What I want to do is, when the stars reach the lower boundary of the screen, they will move back to the upper boundary. However, I can''t simply compare the position on the y-axis (the y-axis goes into and out of the screen because of the view angle) because the stars lower down will visibly just disappear. I need to detect using actual screen coordinates. Any idea how to do this? I get the feeling it is something to do with multiplying or dividing some view matrices. I hope I''ve explained this clearly.
[projection matrix] * [model matrix] * [point position] = [screen pos]
Or if you''re using OpenGL, gluProject(), D3D has something similar in D3DX I''m sure.
Or if you''re using OpenGL, gluProject(), D3D has something similar in D3DX I''m sure.
Author
Thanks. I'm using OpenGL so will use gluProject(). Is this efficient when using this for e.g. 100 stars in the starfield?
[edited by - Billy Lee on March 24, 2004 4:22:35 AM]
[edited by - Billy Lee on March 24, 2004 4:22:35 AM]
Author
I''ve done some work on it. The game starts up i.e. goes to the title screen, but when I start the game, it crashes. I''ve narrowed it down to the bit in the for loop which compares the screen position of the star to a certain value:
if (*screen_pos[1] <= 10.0f)
Can anyone tell what is wrong? I realise that I''m comparing a double to a float but it doesn''t seem to complain. I tried different ways to do it too and they compile but have the same result.
Also, I get a warning saying warning C4700: local variable ''screen_pos'' used without having been initialized . How do you initialise this because I thought I did it by doing double* screen_pos[3] = {0,0,0}; ?
if (*screen_pos[1] <= 10.0f)
Can anyone tell what is wrong? I realise that I''m comparing a double to a float but it doesn''t seem to complain. I tried different ways to do it too and they compile but have the same result.
Also, I get a warning saying warning C4700: local variable ''screen_pos'' used without having been initialized . How do you initialise this because I thought I did it by doing double* screen_pos[3] = {0,0,0}; ?
// Check if star has reached bottom of screenvoid Starfield::checkOutOfBounds(){ // Get modelview matrix double modelview[16]; glGetDoublev (GL_MODELVIEW_MATRIX, modelview); // Get projection matrix double projection[16]; glGetDoublev (GL_PROJECTION_MATRIX, projection); // Get viewport int viewport[4]; glGetIntegerv (GL_VIEWPORT, viewport); double* screen_pos[3] = {0,0,0}; // Use to store screen coordinates of a star // For each star for (int loop = 0; loop < num_particles; loop++) { gluProject (particles[loop].x, particles[loop].y, particles[loop].z, modelview, projection, viewport, screen_pos[0], screen_pos[1], screen_pos[3]); if (*screen_pos[1] <= 10.0f) // If star has reached lower boundary { // Reset y position to top of screen particles[loop].y = screen_top; } }} // End of method checkOutOfBoundsThis topic is closed to new replies.
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