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Matrix Transformations

A lot of calculations in Unigine are performed by using matrices. Actually, matrix transformations are one of the main concepts of 3D engines. This article contains an explanation of matrix transformations with usage examples.

See Also

  • An article about mat4 and dmat4 Unigine data types.
  • The description of Math Matrix Functions.

Transformations

In simple terms, a matrix in 3D graphics is an array of numbers arranged in rows and columns:

Usually, 4x4 matrices are used. Such size (4x4) of matrices is caused by the translation state in 3D space. When you put a new node into the world, it has a 4x4 world transform matrix that defines the position of the node in the world.

In Unigine, matrices are column major (column-oriented). Hence, the first column of the transform matrix represent the X vector v1, the second represent Y vector v2, the third represent Z vector v3, and the fourth represent the translation vector t. First three columns show directions of axes and the scale of the origin. The last column contains the translation of the local origin relatively to the world origin.

Identity Matrix

The world origin has the following matrix:

This matrix is called identity matrix, a matrix with ones on the main diagonal, and zeros elsewhere. If a matrix will be multiplied by the identity matrix, it won't change anything: the resulting matrix will be the same as it was before multiplying.

If the local origin has the identity matrix, it means the local origin and the world origin are coincident.

Rotation

To change the orientation of the local origin, the first three columns of the matrix should be changed.

To rotate the origin along different axes, you should use proper matrices:

In the matrices given above, α is a rotation angle along the axis.

The next matrix shows the rotation of the local origin along the Y axis at 45 degrees:

Translation

The last column of the transform matrix shows the position of the local origin in the world relatively to the world origin. The next matrix shows the translation of the origin. The translation vector t is (3, 0, 2).

Scaling

The length of the vector shows the scale coefficient along the axis.

To calculate the vector length (also known as magnitude), you should find a square root of the sum of the squares of vector components. The formula is the following:

Vector Length
|vector length| = √(x² + y² + z²)

The following matrix scales the local origin up to 2 units along all axes.

Cumulating Transformations

The order of matrix transformations (scaling, rotation and translation) is very important.

The order of cumulating transformations is the following:

  1. Translation
  2. Rotation
  3. Scaling

This is the formula of the transformation order:

Transfomation order
TransformedVector = TranslationMatrix * RotationMatrix * ScaleMatrix * Vector

Here is an example which demonstrates different positions of the local origin related to the world origin.

Translation * Rotation order
Rotation * Translation order

On the left picture, the local origin was translated first and then rotated; on the right picture, the local origin was rotated first and then translated. All values (rotation angle, translation vector) are the same, but the result is different.

Example

This example shows what happens if you choose another order of matrix transformations.

The code example below adds an ObjectMeshStatic object to the world. In the first case we use the translation * rotation order, in the second case we use the rotation * translation order.

Source code(UnigineScript)
// declare ObjectMeshStatic
ObjectMeshStatic mesh;

/* add a mesh as a node to the editor
 */
Node add_editor(Node node) {
	engine.editor.addNode(node);
	return node_remove(node);
}

/* the init() function of the world script file
 */
int init() {
	/* create a camera and add it to the world
	 */
	Player player = new PlayerSpectator();
	player.setDirection(Vec3(0.755f,-1.0f,0.25f));
	engine.game.setPlayer(player);

	// add the mesh to the editor and set the material
	mesh = add_editor(new ObjectMeshStatic("matrix_project/meshes/statue.mesh"));
	mesh.setMaterial("mesh_base", "*");

	// create the matrix of rotation at 270 degrees angle around Z axis
	mat4 rotation = rotateZ(-90);
	log.message("rotation matrix is: %s\n", typeinfo(rotation));

	// create the translation matrix
	mat4 translation = translate(vec3(0,7,0));
	log.message("translation matrix is: %s\n", typeinfo(translation));

	// cumulate transformations by using the current transformation matrix
	mat4 transform = box.getTransform() * translation * rotation;
	mesh.setTransform(transform);
	log.message("transformation matrix is:%s\n", typeinfo(mesh.getTransform()));

	return 1;
}

To change the order, just change the line of cumulating transformations:

Source code(UnigineScript)
mat4 transform = box.getTransform() * rotation * translation;

The result will be different. The pictures below show the difference (camera is located at the same place).

Translation * Rotation order
Rotation * Translation order

The pictures above show the position of the meshes related to the world origin.

Matrix Hierarchy

Another important concept is a matrix hierarchy. When a node is added into the world as a child of another node, it has a transform matrix that is related to the parent node. That is why the Node class has different functions: getTransform(), setTransform() and getWorldTransform(), setWorldTransform() that return the local and the world transformation matrices respectively.

Notice
If the added node has no parent, this node uses the World transformation matrix.

What is the reason of using matrix hierarchy? To move a node relatively to another node. And when you move a parent node, child nodes will also be moved, that is the point.

Parent origin is the same with the world origin
Parent origin has been moved and the child origin has also been moved

Pictures above show the main point of the matrix hierarchy. When the parent origin (node) is moved, the chlld origin will also be moved and the local transformation matrix of the child would not be changed. But the world transformation matrix of the child will be changed. If you need the world transformation matrix of the child related to the world origin, you should use the getWorldTransform(), setWorldTransform() functions; in case, when you need the local transformation matrix of the child related to the parent, you should use the getTransform(), setTransform() functions.

Example

Consider the following example that shows the difference between the local and world transformation matrices.

This code is from the UnigineScript world script file. It creates two nodes (child and parent) by using the box.mesh file.

Source code(UnigineScript)
/* declare ObjectMeshStatic objects for nodes 
*/
ObjectMeshStatic box_1;
ObjectMeshStatic box_2;

/* add a mesh as a node to the editor
 */
Node add_editor(Node node) {
	engine.editor.addNode(node);
	return node_remove(node);
}

/* the init() function of the world script file
 */
int init() {
	/* create a camera and add it to the world
	 */
	Player player = new PlayerSpectator();
	player.setDirection(Vec3(0.755f,-1.0f,0.25f));
	engine.game.setPlayer(player);

	// add the box mesh to the editor
	box_1 = add_editor(new ObjectMeshStatic("matrix_project/meshes/box.mesh"));

	// set the transformation and the material for the first box mesh
	mat4 transform = translate(vec3(0.0f,0.0f,0.0f));
	box_1.setWorldTransform(mat4(transform));
	box_1.setMaterial("mesh_base", "*");

	// add the second box mesh to the editor
	box_2 = add_editor(new ObjectMeshStatic("matrix_project/meshes/box.mesh"));
	
	// set the transformation and the material for the second box mesh
	mat4 transform1 = translate(vec3(0.0f,2.0f,1.0f));
	box_2.setWorldTransform(Mat4(transform1));
	box_2.setMaterial("mesh_base", "*");
	
	// add the second box mesh as a child to the first box mesh
	box_1.addChild(box_2);

	// show the transformation matrix and the world transformation matrix of the child node
	log.message("transformation matrix of the child node: %s\n", typeinfo(box_2.getTransform()));
	log.message("world transformation matrix of the child node: %s\n", typeinfo(box_2.getWorldTransform()));

	return 1;
}

After running this code, we get the following result:

Output
transformation matrix of the child node: dmat4: (1 0 0 0) (0 1 0 0) (0 0 1 0) (0 2 1 1)
world transformation matrix of the child node: dmat4 (1 0 0 0) (0 1 0 0) (0 0 1 0) (0 2 1 1)

The matrices are the same, since the parent box_1 node has the zero transformation (0,0,0). It means its origin and the world origin are coincident. If we change the transformation of the first mesh to another, for example:

Source code(UnigineScript)
mat4 transform = translate(vec3(2.0f,2.0f,2.0f));

We will get another result:

Output
transformation matrix of the child node: dmat4: (1 0 0 0) (0 1 0 0) (0 0 1 0) (0 2 1 1)
world transformation matrix of the child node: dmat4 (1 0 0 0) (0 1 0 0) (0 0 1 0) (2 4 3 1)

As you can see, the local transformation matrix remains the same but the world transformation matrix has been changed. This means that the second node has the same position relatively to the first, but it has another position relatively to the world origin because the position of the parent node has been changed.

Parent node has the same origin with the world origin
Parent node has been moved and the child node has also been moved automatically
Last update: 2017-07-03