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Billy Lee

Problem when billboarding particles

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My code is based on NeHe''s lesson 19 particle system. I''m trying to billboard the particles so that they always face the camera but it''s only drawing one triangle per particle, so only half of the particle is drawn.
        // Variables for billboarding

	float modelview[16];
	glGetFloatv (GL_MODELVIEW_MATRIX, modelview);
	float up[3] = {modelview[0], modelview[4], modelview[8]};
	float right[3] = {modelview[1], modelview[5], modelview[9]};

	for (loop = 0; loop < MAX_PARTICLES; loop++)
	{
		if (particle[loop].active)									// If particle is active

		{
			// Get particle position

			float x = particle[loop].x;
			float y = particle[loop].y;
			float z = particle[loop].z + zoom;

			// Colour the particle using RGB values. Fade the particle based on its life

			glColor4f (particle[loop].r, particle[loop].g, particle[loop].b, particle[loop].life);

			// Draw particle

			glBegin (GL_TRIANGLE_STRIP);
				glTexCoord2d(1,1); glVertex3f(x+(up[0]+right[0]), y+(up[1]+right[1]), z+(up[2]+right[2]));	// Top right

				glTexCoord2d(0,1); glVertex3f(x+(up[0]-right[0]), y+(up[1]-right[1]), z+(up[2]-right[2]));	// Top left

				glTexCoord2d(1,0); glVertex3f(x+(right[0]-up[0]), y+(right[1]-up[1]), z+(right[2]-up[2]));	// Bottom right

				glTexCoord2d(0,0); glVertex3f(x+(right[0]+up[0]), y+(right[1]+up[1]), z+(right[2]+up[2]));	// Bottom left

			glEnd();
...
What am I doing wrong? I have a suspicion that I am supposed to add the whole ''up'' and ''right'' vectors instead of e.g. up[0]+right[0] but when I do that, I get an error saying "cannot add two pointers".

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Well you are using the exact same coords for the first and last vertex and therefor you draw 2 identical triangles.

You should be subtracting both values instead of adding them to get the coords right.

Eggert Jóhannesson
Kálfhól 1, 801 Selfoss
ICELAND

Örhugi Software


[edited by - Eggert on June 19, 2003 4:23:23 AM]

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