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Zmaker5

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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