Scale, rotate, translate w. matrices in openGl ES 2.0
I'm working with OpenGL ES 2.0 and trying to build my object class with some methods to rotate/translate/scale them.
I just set up my object in 0,0,0 and move it afterwards to the desired position on the screen. Below are my methods to move it seperately. After that i run the buildObjectModelMatrix to pass all the matrices into one objectMatrix, so i can take the vertices and multiply them with my modelMatrix/objectMatrix and render it afterwards.
What i think is right, i have to multiply my matrices in this order:
[scale]x[rotation]x[translation]
->
[temp]x[translation]
->
[objectMatrix]
I've found some literature. Maybe i get it in a few Minutes, if i will, i will update it.
Beginning Android 3D
http://gamedev.stackexchange.com
setIdentityM(scaleMatrix, 0);
setIdentityM(translateMatrix, 0);
setIdentityM(rotateMatrix, 0);
public void translate(float x, float y, float z) {
translateM(translateMatrix, 0, x, y, z);
buildObjectModelMatrix();
}
public void rotate(float angle, float x, float y, float z) {
rotateM(rotateMatrix, 0, angle, x, y, z);
buildObjectModelMatrix();
}
public void scale(float x, float y,float z) {
scaleM(scaleMatrix, 0, x, y, z);
buildObjectModelMatrix();
}
private void buildObjectModelMatrix() {
multiplyMM(tempM, 0, scaleMatrix, 0, rotateMatrix, 0);
multiplyMM(objectMatrix, 0, tempM, 0, translateMatrix, 0);
}
SOLVED: The Problem within the whole thing is, if you scale before you translate you get a difference in the distance you translate! the correct code for multiplying your matrices should be (correct me if i'm wrong)
private void buildObjectModelMatrix() {
multiplyMM(tempM, 0, translateMatrix, 0, rotateMatrix, 0);
multiplyMM(objectMatrix, 0, tempM, 0, scaleMatrix, 0);
}
with this you translate and rotate first. Afterwards you can scale the object.
Tested with multiple Objects... so i hope this helped :)
You know this is the most common issue with most people when beginning to deal with matrix operations. How matrix multiplication works is as if you were looking from the objects first person view getting some commands: For instance if you began at (0,0,0) facing toward positive X axis and up would be positive Y axis then translate (a,0,0) would mean "go forward", translate (0,0,a) would would mean "go left", rotate (a, 0, 1, 0) would mean "turn left"...
So if in your case you scaled by 3 units, rotated by 90 degrees and then translated by (2,0,0) what happens is you first enlarge yourself by scale of 3 , then turn 90 degrees so you are now facing positive Z still being quite large. Then you go forward by 2 units measured in your own coordinate system which means you will actually go to (0,0,2*3) . So you end up at (0,0,6) looking toward positive Z axis.
I believe this way is the best to be able to imagine what goes on when dealing with such operations. And might save your life when having a bug in matrix operation order.
You should know that although this kind of matrix operating is normal when beginning with a 3D scene you should try to move to a better system as soon as possible. What I mostly use is to have an object structure/class which contains 3 vectors: position , forward and up (this is much like using glLookAt but not totally the same). So when having these 3 vectors you can simply set a specific position or rotation using trigonometry or your matrix tools by multiplying the vectors with matrices instead of the matrices with matrices. Or you can work with them internally (first person) where for instance "go forward" would be done as position = position + position*forward*scale , turn left would be rotating a forward vector around the up vector. Anyway I hope can understand how to manipulate those 3 vectors to get a desired effect... So what you need to do to reconstruct the matrix from those 3 vector is need to generate another vector right which is a cross product of up and forward then the model matrix consists of:
right.x, right.y, right.z, .0
up.x, up.y, up.z, .0
forward.x, forward.y, forward.z, .0
position.x, position.y, position.z, 1.0
Just note the row-column order may change depending on what you are working with.
I hope this gives you some better understanding...
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