In 3D scenes, every frame, we keep updating the local world 4x4 matrix which can transfer the 3D object from local coordinate to the world coordinate system.
But I can't figure out the matrix which can makes the 3D object rotate itself along itself local Y axis. Because I don't know the 3D object's local up vector, I only know the local->world matrix and keep updating it. How does people deal with it?
To rotate the object, you need a rotation matrix. That should be clear. And most libraries have methods for generating such rotation matrices.
Your problem seems to be what to do with that matrix. You want to use it to modify the existing world transform. There are basically two options. In the following, I assume a column-major matrix layout as is used by OpenGL. Matrix orders for DirectX are reversed.
Option 1: Multiply on the left:
newWorld = rotationMatrix * oldWorld
This rotates the object in the global coordinate system.
Option 2: Multiply on the right:
newWorld = oldWorld * rotationMatrix
This rotates the object in its local coordinate system. This is probably what you're looking for.
Btw, if you have the world transform, you know the local y-axis, which is just the second column of the matrix.
Related
Say I have 2 objects, a camera and a cube, both on XZ plane, the cube has some arbitrary rotation, and camera is facing the cube.
now if a transformation R is applied to the camera such that it has a new rotation and position.
I want to move the cube in front of the camera using transformation R1, such that in the view it looks exactly as before R was applied, meaning relative distance, rotation and scale b/w the 2 objects remain same after both R and R1.
Following image gives a gist of the problem.
Assume that there's no scenegraph that we can use.
I've posed the problem mainly in 2D but I'm trying to solve it in 3D, so rotations can have all yaw, pitch and roll, and translations can be anywhere in 3D space.
EDIT:
I forgot to add what I have done so far.
I figured out how to maintain relative distance b/w camera and cube, I can project cube's position to get world to screen point, then unproject the screen point in new camera position to get new world position.
However for rotation, I have tried this
I thought I can apply same rotation as R in R1, this didn't work, it appears to work if rotation happens only in one axis, if rotation happens in more than one axes, it does not work.
I thought I can take delta rotation b/w camera and cube, and simply apply camera's rotation to the cube and then multiply delta rotation, this also didn't work
Let M and V be the model and view matrices before you move the camera, M2 and V2 be the matrices after you move the camera. To be clear: model matrix transforms the coordinates from object local coordinates into world coordinates; a view matrix transforms from world coordinates into clip-space camera coordinates. Consequently V*M*p transforms the position p into clip-space.
For the position on the screen to stay constant, we need V*M*p = V2*M2*p to be true for all p (that's assuming that the FOV doesn't change). Therefore V*M = V2*M2, or
M2 = inverse(V2)*V*M
If you apply the camera transformation on the right (V2 = V*R) then the above expression for M2 can be simplified:
M2 = inverse(R)*M
(that is you apply inverse(R) on the left of the model matrix to compensate).
Alternatively, ask yourself if you really need to keep the object coordinates in the world reference frame. It may be easier to not to apply the view matrix when rendering that object at all; that would effectively keep it relative to the camera at all times without any additional tweaks. That would have better numerical stability too.
I am building a camera class to look arround a scene. At the moment I have 3 cubes just spread arround to have a good impression of what is going on. I have set my scroll button on a mouse to give me translation along z-axis and when I move my mouse left or right I detect this movement and rotate arround y-axis. This is just to see what happens and play arround a bit. So I succeeded in making the camera rotate by rotating the cubes arround the origin but after I rotate by some angle, lets say 90 degrees, and try to translate along z axis to my surprise I find out that my cubes are now going from left to right and not towards me or away from me. So what is going on here? It seems that z axis is rotated also. I guess the same goes for x axis. So it seems that nothing actually moved in regard to the origin, but the whole coordinate system with all the objects was just rotated. Can anyone help me here, what is going on? How coordinate system works in opengl?
You are most likely confusing local and global rotations. Usual cheap remedy is to change(reverse) order of some of your transformation. However doing this blindly is trial&error and can be frustrating. Its better to understand the math first...
Old API OpeGL uses MVP matrix which is:
MVP = Model * View * Projection
Where Model and View are already multiplied together. What you have is most likely the same. Now the problem is that Model is direct matrix, but View is Inverse.
So if you have some transform matrix representing your camera in oder to use it to transform back you need to use its inverse...
MVP = Model * Inverse(Camera) * Projection
Then you can use the same order of transformations for both Model and Camera and also use their geometric properties like basis vectors etc ... then stuff like camera local movements or camera follow are easy. Beware some tutorials use glTranspose instead of real matrix Inverse. That is correct only if the Matrix contains only unit (or equal sized) orthogonal basis vectors without any offset so no scale,skew,offset or projections just rotation and equal scale along all axises !!!
That means when you rotate Model and View in the same way the result is opposite. So in old code there is usual to have something like this:
// view part of matrix
glMatrixMode(GL_MODELVIEW);
glLoadIdentity();
glRotate3f(view_c,0,0,1); // ugly euler angles
glRotate3f(view_b,0,1,0); // ugly euler angles
glRotate3f(view_a,1,0,0); // ugly euler angles
glTranslatef(view_pos); // set camera position
// model part of matrix
for (i=0;i<objs;i++)
{
glMatrixMode(GL_MODELVIEW);
glPushMatrix();
glTranslatef(obj_pos[i]); // set camera position
glRotate3f(obj_a[i],1,0,0); // ugly euler angles
glRotate3f(obj_b[i],0,1,0); // ugly euler angles
glRotate3f(obj_c[i],0,0,1); // ugly euler angles
//here render obj[i]
glMatrixMode(GL_MODELVIEW);
glPopMatrix();
}
note the order of transforms is opposite (I just wrote it here in editor so its not tested and can be opposite to native GL notation ... I do not use Euler angles) ... The order must match your convention... To know more about these (including examples) not using useless Euler angles see:
Understanding 4x4 homogenous transform matrices
Here is 4D version of what your 3D camera class should look like (just shrink the matrices to 4x4 and have just 3 rotations instead of 6):
reper4D
pay attention to difference between local lrot_?? and global grot_?? functions. Also note rotations are defined by plane not axis vector as axis vector is just human abstraction that does not really work except 2D and 3D ... planes work from 2D to ND
PS. its a good idea to have the distortions (scale,skew) separated from model and keep transform matrices representing coordinate systems orthonormal. It will ease up a lot of things latter on once you got to do advanced math on them. Resulting in:
MVP = Model * Model_distortion * Inverse(Camera) * Projection
I have a basic cube generated in my 3D world. I can rotate correctly around the camera, but when I translate after rotating, the translations are not correct.
For example, if I rotate 90 degrees and translate into the Z axis, it would move as if translating in the X axis.
glLoadIdentity();
glRotatef(angle,0,1,0); //Rotate around the camera.
glTranslatef(movX,movY,movZ); //Translate after rotating around the camera.
glCallList(cubes[0]);
I need some help with this. Also, I tried translating before rotating, but the rotation is not at the camera. It is at the edge of the cube.
Keep in mind that in OpenGL that the transformation is applied to the camera, not the objects rendered; so you observe the inverse of the transformation you expected.
Also in OpenGL the Y and Z axes are flipped (Y is vertical), so you observe a horizontal translation instead of a vertical one.
Also, because the object is rotated though 90 degrees about Y, the X and Z axes replace each other (one of them is reversed).
willywonkadailyblah's answer is half correct. Because you are using the old OpenGL, you're using the old matrix stack. You are modifying the modelview matrix when you're doing your glRotatef and glTranslatef calls. The modelview matrix is actually the model's matrix and the camera's view matrix precombined (already multiplied together). These matrices are what determine where your object is in 3D space and where your viewing position/direction of the world is. So you can think of your calls as moving the camera, but it's probably easier to think of them as moving and rotating the world.
These rotate and translation calls are linear transformations. This has a precise definition, but for our purposes it means that you can represent the transformation as a matrix and you multiply it with the point's coordinates to apply the transformation to a point. Now matrix multiplication is not commutative, meaning AB != BA. All this to say that when you rotate, then translate it is different than translating and rotating, which I think you know. But then when you translate, rotate, and translate again, it might be a little more difficult to follow what you're actually doing. Worse even if you throw in some scaling in there. So I would suggest learning how linear transformations work and maintaining your own matrices for the objects and camera if you're serious about learning OpenGL.
learnopengl.org is an excellent website, but it teaches you Modern OpenGL, not what you're currently using. But the lesson on transformations and on coordinate systems are probably generally helpful, even without exact code for you to follow
I have a simple 3D LookAt vector, and I wish to rotate the player model (a simple cube) to show where the player/cube is looking at.
For sideways camera movement I've managed to figure it out and do the following:
glTranslatef(position.x, position.y, position.z);
glRotatef(atan2(lookAt.z, lookAt.x) * 180 / PI, 0, 1, 0);
Now I know that to get up-down camera movement to map to the rendered cube model, I need to rotate the cube around it's x and z axes as well, but I can't seem to figure out what formula to use for those two.
OpenGL will rotate the whole coordinate system (whole space, not only a cube) so after first rotation you just need to rotate only around z axis.
// first rotation
glRotatef(-atan2(lookAt.z, lookAt.x) * 180 / PI, 0, 1, 0);
// second rotation
float d = sqrt(pow(lookAt.x,2) + pow(lookAt.z,2));
float pitch = atan2(lookAt.y, d);
glRotatef(pitch * 180 / PI, 0, 0, 1);
First and second rotation:
I assume your model is looking along x axis (red arrow). I also assume lookAt is given relative to the position of the model.
If you're familiar with matrix math, matrices are an easier way to think about it. If you're not familiar with matrices, this series explains how to use them to solve common game development problems: https://www.youtube.com/playlist?list=PLW3Zl3wyJwWNQjMz941uyOIq3Nw6bcDYC Getting good with matrices is a good idea if you want to be a 3D game programmer.
For your problem, you want to make a translation/rotation matrix which will transform the box to the proper place for you. You can make a translation matrix and a rotation matrix individually, and then at the end take the product of the two. I'll try to break that down.
The translation matrix is simple, if your position is then your matrix will be
To construct a rotation matrix, you need to rotate the standard basis vectors the way you want. Then when you create a matrix from those rotated basis vectors, the matrix will rotate other vectors in the same way. As an example of that, take the standard basis vectors:
Now I'm going to rotate and around by 90 degrees clockwise:
Now put them into a matrix:
and you have R is a matrix that rotates things around by 90 degrees.
In your case you want to rotate stuff such that it faces a vector that you provide. That makes things easy, we can calculate our basis vectors from that vector. If your vector is then and we can solve for the other two basis vectors using cross products. You know that the character won't ever roll their view (right?) so we can use the global up vector as well. I'll call the global up vector . In your case you're using y as the "up" dimension so the global up vector will be
Then:
In the first line you do a cross product between the view vector and the up vector to get a vector orthogonal to both - this will serve as the third basis vector after it is normalized, which is the second line. In the third line another cross product generates the second basis vector. These three vectors represent what happens when the standard basis vectors are rotated the way you want them to be. Use them as the columns in a matrix like so:
Now the last step in the math is to make a final matrix that will do both translation and rotation, and this step is easy:
Then load that matrix into OpenGL with glLoadMatrix:
glLoadMatrixf(&M);
All of this gets explained in the video series I linked as well :)
I am trying to achieve the same rotational effect like Maya in my project.
I have some knowledge on quaternions and the trackball example.
Unfortunately I am still unable to wrap my head around the concept of using the quaternions to get the desired effect.
Basically I am still getting the same issue I had before with the 3d trackball. After flipping the object upside down, and then trying to rotate to the right, the object will rotate to the left. Well actually its my camera rotating around the focus point in the opposite direction.
The problem is that I am using the screen coordinates & trackball to get the old / new vectors and getting the angle of rotation from those two vectors. I will always get the wrong axis of rotation this way.
How should I go about solving this issue?
I don't know Maya so I can only guess that its rotation is like this: if you rotate left-right, it feels natural. Then if you rotate the object up-down 180 degrees, then rotate left-right again, it still feels natural.
If you are familiar with the concept of using a matrix to do transformations (like rotate, scale and translate), well a quaternion is just the same concept but it only allows rotations, so you might want to use it to constrain your transforms to just rotations. In practice, you can use either a matrix or a quaternion to do the same thing.
What you need to do is remember the current quaternion state for the object, then when the next frame of rotation occurs, multiply the new rotation with the old quaternion (in that order) to give you the next frame's quaternion. That will ensure that no matter what orientation the object is in, the next frame's rotation will be applied from the viewer's viewpoint. This is as opposed to some naive rotation where you just say "user is scrolling up/down, therefore alter the object's X-axis rotation", which causes that flipping.
Remember, like matrices, quaternions need to be multiplied in reverse order that the actions are actually applied, which is why I said to multiply the new operation by the existing quaternion.
To finish with an example. Let's say the user is going to perform 2 actions:
On frame 1, the user rotates the object 180 degrees about the X axis (up/down rotation).
On frame 2, the user rotates the object 90 degrees about the Y axis (left/right rotation).
Lets say the object has a quaternion Q. Every frame, you will reset the object to its default coordinates and apply the quaternion Q to rotate it. Now you might initialise it with the identity quaternion, but let's just say the initial quaternion is called Q0.
On frame 1, create a new quaternion R1 which is a "rotate 180 degrees about the X axis" quaternion (you can find some maths to compute such a quaternion). Pre-multiply the new operation by the existing quaternion: Q1 = R1 * Q0.
On frame 2, create a new quaternion R2 which is a "rotate 90 degrees about the Y axis" quaternion. Pre-multiply the new operation by the existing quaternion: Q2 = R2 * Q1.
On frame 1 you will use Q1 to display the object, and on frame 2 you will use Q2. You can simply keep applying any subsequent user actions to the quaternion and it will always be rotated in the viewer's frame of reference.
I think you have problems with changing coordinate system.
Suppose, you want to rotate object in X Axis, then in Y Axis, and then move it and scale. So, you should multiply your transformation maxtrix (at the beginning it equals to itentity matrix) to the rotation matrix (firstly to X, then to Y), then to translation matrix and at the end to scaling matrix. So, when your current matrix multiplies to the resulting matrix, your coordinate systems changes.
To avoid this problem you can use 2 methods:
1) to accumulate your resultig matrix as product of all previous matrices.
2) to use stack, where in the top will be the matrix, which equals to product of all matrices in the bottom of this matrix (in the stack).
P.S. I'm not sure, that it helps you. I never used quaternions in my projects.