# OpenGL Moving objects towards and away from the camera

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I'm not sure why this is eluding me. I have some cubes and I can rotate in all directions, move them in the x and y directions, but I can't figure out how to cause them to move towards and away from the camera. I am using glFrustrum, so the problem isn't that I'm drawing them orthographically, but it seems any translation of the camera in the z direction causes no change. I've spent hours looking in books and tutorials and I have no idea why this simple action is eluding me though 2D actions in opengl seem common, even when displaying 3D objects all I can find are 2D movements or rotations around a point, nothing about moving things closer or farther away from the current view.

I had assumed that if I draw an object, then translate, then draw the next object based on their stored local positions they would be drawn at their respective distances.

Any hints? I know I probably seem completely ignorant but I'm going out of my mind with the repeated suggestions of gluLookAt which I cannot use due to the lack of glut on the platform I'm working in.

Greatly appreciate any guidance.

overall DRAWVIEW call
[EAGLContext setCurrentContext:context];        glBindFramebufferOES(GL_FRAMEBUFFER_OES, viewFramebuffer);        glClearColor(0.0f, 0.0f, 0.0f, 1.0f);	glClearDepthf(1.0f);    glClear(GL_COLOR_BUFFER_BIT | GL_DEPTH_BUFFER_BIT);	glEnableClientState(GL_VERTEX_ARRAY);    GLfloat triVertices[40*3*2];		// draw static grid as background	int index = 0;	int lCount = 0;	for ( float i = -0.8f; i <= 0.9f; i += 0.1f)	{				triVertices[index] = -0.9f;		triVertices[index + 1] = i;		triVertices[index + 2] = 0.0f;				triVertices[index + 3] = 0.9f;		triVertices[index + 4] = i;		triVertices[index + 5] = 0.0f;				triVertices[index + 6] = i;		triVertices[index + 7] = -0.9f;		triVertices[index + 8] = 0.0f;				triVertices[index + 9] = i;		triVertices[index + 10] = 0.9f;		triVertices[index + 11] = 0.0f;				index += 12;		lCount += 4;	}		glVertexPointer(3, GL_FLOAT, 0, triVertices);	glDrawArrays(GL_LINES, 0, lCount);		// draw 3D objects		glEnable(GL_DEPTH_TEST);	glMatrixMode(GL_PROJECTION);        glLoadIdentity();	glFrustumf(-1.6f, 1.6, -2.4, 2.4, -10, 10);	glMatrixMode(GL_MODELVIEW);		// enable GL states for textures	glEnableClientState(GL_VERTEX_ARRAY);	glEnableClientState(GL_TEXTURE_COORD_ARRAY);	glEnable(GL_TEXTURE_2D);		glEnable(GL_BLEND); // cubes have slightly transparent texture	glBlendFunc(GL_ONE, GL_ONE_MINUS_SRC_ALPHA);	[self drawCubes];	glDisable(GL_BLEND);		// draw buttons	glMatrixMode(GL_PROJECTION);         glLoadIdentity();        glOrthof(-1.0f, 1.0f, -1.5f, 1.5f, -10.0f, 10.0f);	glMatrixMode(GL_MODELVIEW);		glEnable(GL_BLEND); // buttons are one texture with transparent center	glBlendFunc(GL_ONE, GL_ONE_MINUS_SRC_ALPHA);	[self drawButtons];	glDisable(GL_BLEND);		//disable GL states for texturing	glDisableClientState(GL_VERTEX_ARRAY);	glDisableClientState(GL_TEXTURE_COORD_ARRAY);	glDisable(GL_TEXTURE_2D);    glBindRenderbufferOES(GL_RENDERBUFFER_OES, viewRenderbuffer);    [context presentRenderbuffer:GL_RENDERBUFFER_OES];

drawCubes
{		const GLfloat cubeTexCoords[8] =        {		1, 0,  1, 1,  0, 0,  0, 1,	};		glBindTexture(GL_TEXTURE_2D, texture[0]);		glPushMatrix();	glLoadIdentity();	//glTranslatef( playerShape.x, playerShape.y, playerShape.z );  this seems to be causing problems, perhaps done in wrong order	glScalef(kCubeScale, kCubeScale, kCubeScale);		glTexCoordPointer(2, GL_FLOAT, 0, cubeTexCoords);	for (int f = 0; f < playerShape.num_outer_sides/12; f++) {		//glColor4f(cubeColors[f%6][0], cubeColors[f%6][1], cubeColors[f%6][2], cubeColors[f%6][3]);				glColor4f(cubeColors[1][0], cubeColors[1][1], cubeColors[1][2], cubeColors[1][3]);		glVertexPointer(3, GL_FLOAT, 0, cubeVertices[f]);		glDrawArrays(GL_TRIANGLE_STRIP, 0, 4);	}		glPopMatrix();		glBindTexture(GL_TEXTURE_2D, 0);}

[Edited by - Zmaker5 on October 10, 2010 5:12:35 PM]

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Hey I'm definitely still a ameteur in OpenGL, but I can share my method. I usually start by making a Class of any object I'm going to draw. Then I give every object a center point. And all rendering is done based off the size of the object and its center. Like in the example of a cube you know each Face is going to be size/2 away from the Center. So you draw everything in a formula based off the center. Translate to center then draw off the formula. When I want to move an object I can just change the center cartesian coordinates(3 set x, y, z) and the object will automaticly move where I tell its center to go.

for(int a = 0; a < 20; a++){  Cube.SetCenter(Cube.CenterX(), Cube.CenterY(), Cube.CenterZ()+1);}

This would move it 20 forward from its original position. If you know your classes and geomotry this should be a snap. I apologize if this isn't an applicable solution due to my lack of understanding of all OpenGL formats, but I figured I'd take a stab at it since no one had responded yet.

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I assume you mean you're updating the vertices at each step, but that seems like a lot of unneeded computation though I'd considered it. I didn't think I would have to change all the vertices values to move objects, I figure there's a way for something to be placed at a location rather than actually defining the vertices themselves around the location.

Thanks for your help though I hope to avoid having to implement using that method. From various tutorials I had assumed there was some way to use glTranslate or something similar to handle drawing objects in their correct location on screen.

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My method does use GL translate to draw onto the screen at the "correct" position. The translate translates to the center of the object. The example I gave was simple, but I use this method so the physics engine can manipulate the objects location. Shouldn't every object be movable unless it is a wall or some sort of stationary object? I'm sorry I seem to be confused. Are you just wanting to move the camera rather than the object?

I also made a little play around particle engine using this method with a couple std::vector that did a pushback to increase the number of particles. The computer I'm using with 2gig ram was able to handle around 500k particles (which each contained 4 triangles so around 2 million triangles on the screen moving their own centers) before it even started to slow down. I have recently read since then that the physics engine should operate calculations at half the speed of your renderer to increase speed so I'm guessing using that tactic I could probably nearly double the number of triangles on the screen.

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