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OpenGL Particles system integration between different view volumes

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MajinMusashi    182
USING: Visual C++ 2005, SDL, OpenGL CONTEXT: I'm trying to increase the visual appeal of an old Pong clone by adding fancy particles effects to it (both game and particles system are fully functional). PROBLEM: The game is rendered using orthogonal projection (origin at bottom-left), and the particles system uses perspective projection (origin at the center of the screen) to render its particles. Got it? I don't know how to place the particles emitter on a given place (in "game coordinates"). I'll give a very simplified example: EXAMPLE:
// emitter.cpp
void Emitter::draw() {

  glDisable( GL_DEPTH_TEST );
  glBlendFunc( GL_SRC_ALPHA, GL_ONE );
  // Draws particles
  glEnable( GL_DEPTH_TEST );

// game.cpp
void Pong::gameLoop() {
  Emitter fireEmitter( "data/texture.tga" );

  while ( true ) {
    // Updates objects
    // This is absolutelly wrong since the leftPaddle's coordinates are expressed in an orthogonal projection
    //   and the emitter's in a perspective projection
   fireEmitter.setPosition( leftPaddle.x, leftPaddle.y );

    // Draws objects

    // Sets perspective view so that the rotating cube can be rendered

        glTranslatef( 0.0f, 0.0f, -50.0f );

    // Returns to the previous view (orthogonal)

QUESTION: Am I being clear enough? The particles emitter is being drawn, but in the wrong place. Can you show me how to convert the coordinates from a perspective view volume to an orthogonal view volume? Thanks a lot!

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zedzeek    529
youre not to clear.
u typically draw everything in the same space (nornmally a perspective projective matrix), this includes the particles etc.
after youve finished though u often setup a 2d orthogonal projection matrix to draw the HUD for the score/lives etc

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JoshG    127
Well, i'm not sure entirely, cos I havn't thought this through too much yet,

First off, usually you do want to draw in the same perspective, but I can understand wanting to make pong look cool over the top of some awesome perspective view style background. SOOOO

Hopefully you have a good grasp of Matrix Maths and can figure out exactly how to do what you want to do with this basic idea...

The projection matricies that you setup are actual 4x4 matricies that convert the co-ordinates you give ogl in glVertex, into actual screen positions. Soo... if you were to get the projection matricies from OGL, you should be able to figure out a matrix that will map a perspective point into an ortho point, and vice-versa...

You'd prolly need to look at the OGL Redbook for how OGL uses the projection matricies, but you should be able to work out a mapping based on those...

Otherwise, for a project like this, Trial and Error seems to be pretty good!

Just remember that with perspective view, (0,0) is in the center of the screen, and that you need to use a z value for the things you draw as well, (as Z=0 is Actually ON the screen, and often won't be drawn) so try using z=-10, or z=10 to see if you can get something to appear :)

Good luck


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