# OpenGL Many questions (I promise it won't take longer to read)

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While I was reading some sites, I found some interesting things about OpenGL: 1. Is that good to use quaternions? Because I tried hard to understand its concepts, but still no clue. 2. Is it true that the resulting matrix from glGetFloatv(GL_MODELVIEW_MATRIX, mat); has all the transformations I did until this call? 3. How can I reset just rotation, or just translations I did during the drawing "phase"? 4. Some old 3D games (and all particles afaik) have images that always are facing the screen, how can I accomplish that? (Actually I'm thinking on using the answer from question 3) 5. Is that good to use gluLookAt instead of glTransform and glRotate? (because this gluLookAt is a little trickier than I thought) 6. I'm thinking on using skeletal (bone or whatever) animation someday in my game (till now they're just static models, as you would expect from a side-scrolling-spaceship-shooting-game), and I read somewhere that I should use matrices for each vertex. (can't remember if it was quaternion or just the matrix from question 2) 7. My actual particle system is a real crap, I mean, I can't even set an "initial position" to each new particle, they just go with the spaceship. (Because the only thing I could think was to place the particles drawing method 1 line below the spaceship drawing method) there are too much transformations to get into the spaceship lines, so I can't think on any way to solve that. The End! Sorry to be asking too many questions, but even after a year of 3D OpenGL learning programming, I'm still a newbie. =/ Thanks!

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It seems that your biggest hurdle is a lack of math. I'd suggest either finding some Linear Algebra tutorials online or picking up a nice textbook and working your way through it. That will clear up everything about vectors and matrices for you and make a lot of your questions pretty intuitive to solve.

Otherwise:

3. You can manage the combined matrix stack by using glPushMatrix and glPopMatrix. a call to the latter will restore the matrix stack to what it was before glPushMatrix was called

4. the technique is called billboarding

5. I find that using gluLookAt is the most intuitive way of managing a camera. Which technique you use it up to you. However take a look at an implementation of gluLookAt and you will find that it's just some simple matrix math.

6. you use a matrix for each bone, which is the total offset & rotation from the bone's parent. Each vertex is then childed to a specific bone at an offset.

-me

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Quote:
 Original post by wagner_fsoaresWhile I was reading some sites, I found some interesting things about OpenGL:1. Is that good to use quaternions? Because I tried hard to understand its concepts, but still no clue.2. Is it true that the resulting matrix from glGetFloatv(GL_MODELVIEW_MATRIX, mat); has all the transformations I did until this call?3. How can I reset just rotation, or just translations I did during the drawing "phase"?4. Some old 3D games (and all particles afaik) have images that always are facing the screen, how can I accomplish that? (Actually I'm thinking on using the answer from question 3)5. Is that good to use gluLookAt instead of glTransform and glRotate? (because this gluLookAt is a little trickier than I thought)6. I'm thinking on using skeletal (bone or whatever) animation someday in my game (till now they're just static models, as you would expect from a side-scrolling-spaceship-shooting-game), and I read somewhere that I should use matrices for each vertex. (can't remember if it was quaternion or just the matrix from question 2)7. My actual particle system is a real crap, I mean, I can't even set an "initial position" to each new particle, they just go with the spaceship. (Because the only thing I could think was to place the particles drawing method 1 line below the spaceship drawing method) there are too much transformations to get into the spaceship lines, so I can't think on any way to solve that.The End!Sorry to be asking too many questions, but even after a year of 3D OpenGL learning programming, I'm still a newbie.=/Thanks!

5. gluLookAt is good. The eye is the position where you want you to be. The look at is the position where you want to be looking (this could be 1 unit in front of the eye). The up vector is whatever you want.
You can avoid gluLookAt if you want. It is up to you.

7. For particle system, the best is to transform each one using the CPU and just send world space coordinates. You would do it like this
glLoadMatrixf(CameraMatrix);
RenderParticlesInWorldSpace();

so only the camera matrix is needed to position them.
Search for some tutorials on particle stuff. Some of them might use shaders.

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1) quite essential if objects need complete freedom in rotation
2) yes
3) if you do only rotation and translation, the 4x4 matrix basically consists of a 3x3 rotation matrix R, a 1x3 translation vector T, and padding:
R R R T
R R R T
R R R T
0 0 0 1
So, if you want to reset translation, set T = {0,0,0}. If you want to reset rotation, set R = {1,0,0, 0,1,0, 0,0,1}. Beware that gl_NormalMatrix will be wrong if you reset the rotation in this way, and thus fixed-func lighting will appear wrong

4) billboarding, as answered. 2 triangles per billboard, that are created and calculated on the cpu on every frame.
5) gluLookAt is recommended

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Sorry, I'm like 3h delayed...

OK, about linear algebra, I'm graduated from computer science, so I know the basics.
The problem is how this quaternion holds all the data and how is like working with it, the same as for GL_MODELVIEW_MATRIX.

So using or not gluLookAt hasn't any effect at all huh!
I'd some problems understanding and using it.
I still didn't do with gluLookAt what these 2 lines did:
glTranslatef(0.0f, 0.0f, -30.0f);glRotatef(27 - (m_scroll * 2), -1.0f, 0.0f, 0.0f);glTranslatef(0.0f, m_scroll / 2, 0.0f);

Just place all the objects far from the "view" and rotate and get near as the spaceship goes.

And about particles, it won't work, cuz I want these particles to be behind the spaceship as it moves on the screen and at the same time, keep all particles facing the screen.

Palidine, about reseting rotation and translation, yeah you're right, it's time for me to be more organized. =)
About answer 4, I didn't know it had a name. =P
Gonna start reading about this billboarding.

What about quaternions, do you guys use it?
Can I still be happy without using it?

Thanks for the answers!

[EDIT]
Sorry idinev, I didn't see your post, I think we've posted at the same time. =P
Whatever.
Gonna start learning (again) this quaternion thing, do you have a good source so I can learn?
And about billboard, what do you think about this one: http://nehe.gamedev.net/data/articles/article.asp?article=19 ? (I was pretty sure everything I ever code but "gl" functions, was calculated on the CPU)

[Edited by - wagner_fsoares on December 28, 2009 9:03:59 PM]

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Ok, I've some news:

I'm now working with gluLookAt, and I got the logic behind quaternions.
But I'm still stuck with billboards, I've following this article: http://nehe.gamedev.net/data/articles/article.asp?article=19 , but I think I'm not getting the right values for the object's world coordinate.
I'm using this logic: http://www.lighthouse3d.com/opengl/billboarding/index.php?billWhere to get the object's world position, but I think I'm missing something.

here's my code for billboarding:

inline QVector3D objWorldPos(QVector3D &camPos) {    GLfloat modelview[16];    glGetFloatv(GL_MODELVIEW_MATRIX, modelview);    QVector3D worldPos;    worldPos.setX(camPos.x() + (modelview[12] * modelview[0]) + (modelview[13] * modelview[1]) + (modelview[14] * modelview[2]));    worldPos.setY(camPos.y() + (modelview[12] * modelview[4]) + (modelview[13] * modelview[5]) + (modelview[14] * modelview[6]));    worldPos.setZ(camPos.z() + (modelview[12] * modelview[8]) + (modelview[13] * modelview[9]) + (modelview[14] * modelview[10]));    return worldPos;}inline void billboardSphericalBegin(QVector3D camPos, QVector3D objPos) {    QVector3D objLook, objUp, objRight;    objLook = camPos - objPos;    objLook.normalize();    QVector3D camUp(0, 1, 0);    objRight = QVector3D::crossProduct(camUp, objLook);    objUp = QVector3D::crossProduct(objLook, objRight);    GLfloat mat[] = { objRight.x(),  objRight.y(),   objRight.z(),   0,                      objUp.x(),     objUp.y(),      objUp.z(),      0,                      objLook.x(),   objLook.y(),    objLook.z(),    0,                      objPos.x(),    objPos.y(),     objPos.z(),     1                    };    glPushMatrix();    glMultMatrixf(mat);}//the drawglClear(GL_COLOR_BUFFER_BIT | GL_DEPTH_BUFFER_BIT);glLoadIdentity();m_modelView.setToIdentity();//the rest of the cam values are 0m_camEye.setZ(10);m_camLook.setX(m_camMove.x());m_camLook.setY(m_camMove.y());m_camUp.setY(1);m_modelView.lookAt(m_camEye, m_camLook, m_camUp);qMultMatrix(m_modelView);m_modelView.setToIdentity();m_modelView.translate(-1, -1, -10);qMultMatrix(m_modelView);QVector3D objWPos = objWorldPos(m_camEye);billboardSphericalBegin(m_camEye, objWPos);glBindTexture(GL_TEXTURE_2D, m_tex_grass);glBegin(GL_TRIANGLE_FAN);glVertex3f(10, -12, 0);glVertex3f(10, 12, 0);glVertex3f(-10, 12, 0);glVertex3f(-10, -12, 0);glEnd();billboardEnd();

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Quote:
 1) quite essential if objects need complete freedom in rotation
That's actually not true. Quaternions have the same fundamental behavior with respect to rotations as do matrices; in other words, if you can do it with a quaternion, you can do it with a matrix.

@The OP: Quaternions do some things more elegantly and/or efficiently than matrices and therefore it can sometimes be advantageous to use them, but if you're fairly new to 3-d math I'd suggest just sticking with matrices for the time being.

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Thanks for the correction. I was also doubting my use of "essential".

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I've got billboarding to work like this:

GLfloat matrix[16];glGetFloatv(GL_MODELVIEW_MATRIX, matrix);matrix[0] = 1.0f; matrix[1] = 0.0f; matrix[2] = 0.0f;matrix[4] = 0.0f; matrix[5] = 1.0f; matrix[6] = 0.0f;glPushMatrix();glLoadMatrixf(matrix);

I don't know if it's the right thing to do but... oh hell it's working!

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