Transformations in EMAN2
The Transform3D Class
EMAN2 uses the [http://blake.bcm.edu/eman2/doxygen_html/classEMAN_1_1Transform3D.html Transform3D] class for storing/managing Euler angles and translations. At any time a Transform3D ($$T3D$$) object defines a group of 3 affine transformations that are applied in a specific order, namely
$$T3D \equiv T_{post} R T_{pre}$$
Where $$T_{pre} is a pre translation, $$R$$ is a rotation and $$T_{post} is a post translation. The Transform3D object stores these transformations internally in a 4x4 matrix, as is commonly the case in computer graphics applications that use homogeneous coordinate systems (i.e. OpenGL). In these approaches the 4x4 transformation matrix $$T3D$$ is constructed in this way
$$T3D = [[R,\hat{t}],[\hat{0}^T,1]]$$
Where R is a $$3x3$$ rotation matrix and $$\hat{t}=(dx,dy,dz)^T$$ is a post translation. In this approach a 3D point $$\hat{p}=(x,y,z)^T$$ as represented in homogeneous coordinates as a 4D vector $$\hat{p_{hc}}=(x,y,z,1)^T$$ and is multiplied against the matrix $$M$$ to produce the result of applying the transformation
$$ T3D \hat{p}_{hc} = ( R\hat{p} + \hat{t}, 1 )^T $$
In this way the result of applying a Transform3D to a vector is literally a rotation follow by a translation. The Transform3D allows for both pre and post translation and stores the cumulative result internally
$$T3D = T_{post} R T_{pre} = [[I,\hat{t}_{post}],[\hat{0}^T,1]] [[R,\hat{0}],[\hat{0}^T,1]] [[I,\hat{t}_{pre}],[\hat{0}^T,1]] = [[R,R\hat{t}_{pre}+\hat{t}_{post}],[\hat{0}^T,1]]$$
Constructing a Transform3D Object In Python
In Python you can construct a Transform3D object in a number of ways
1 from EMAN2 import Transform3D
2 t = Transform3D() # t is the identity
3 t = Transfrom3D(EULER_EMAN,25,45,65) # EULER_EMAN rotation convention uses the az, alt, phi
4 t = Transform3D(EULER_SPIDER,24,44,64) # EULER_SPIDER rotation convention uses the phi, theta, psi convention
5 t = Transform3D(25,45,65) # EULER_EMAN convention used by default, arguments are taken as az, alt, phi
6 t = Transform3D(Vec3f(1,2,3),25,45,65,Vec3f(4,5,6)) # Specify a pre trans, followed by EULER_EMAN convention rotations az, alt, phi, followed by the post trans
7 t = Transform3D(25,45,65,Vec3f(4,5,6)) # EULER_EMAN convention rotations az, alt, phi, followed by the post trans
8 t = Transform3D(1,0,0,0,1,0,0,0,1) # Explicitly setting the nine members of the rotation matrix, row wise.
9 s = Transform3D(t) # copy constructor
Setting Transform3D Rotations and Translation Attributes in Python
You can set the pre and post translations, as well as the rotations, directly from Python
1 from EMAN2 import Transform3D
2 t = Transform3D()
3 # setting the rotations
4 t.set_rotation(25,45,65) # EULER_EMAN convention rotations az, alt, phi
5 t.set_rotation(EULER_SPIDER,24,44,64) # EULER_SPIDER rotation convention uses the phi, theta, psi convention
6 t.set_rotation(EULER_EMAN, {"az":25,"alt":45,"phi":65}) # Optional dictionary style approach
7 t.set_rotation(1,0,0,0,1,0,0,0,1) # Explicitly set the nine members of the rotation matrix, row wise.
8 # setting translations
9 t.set_pretrans(1,2,3)# pre translation dx, dy, dz
10 t.set_pretrans(Vec3f(1,2,3)) # also takes Vec3f argument
11 t.set_pretrans([1,2,3]) # also takes tuple argument
12 t.set_posttrans(4,5,6)# post translation dx, dy, dz
13 t.set_posttrans(Vec3f(4,5,6)) # also takes Vec3f argument
14 t.set_posttrans([4,5,6]) # also takes tuple argument
Getting Transform3D Rotations and Translation Attributes in Python
You can get these attributes using similar syntax to that employed for the setter methods
1 from EMAN2 import Transform3D
2 t = Transform3D(Vec3f(1,2,3),25,45,65,Vec3f(4,5,6)) # Specify a pre trans, followed by EULER_EMAN convention rotations az, alt, phi, followed by the post trans
3 # get rotations
4 dictionary = t.get_rotation(EULER_EMAN) # returns a dictionary with keys "az", "alt" and "phi"
5 dictionary = t.get_rotation(EULER_SPIDER) # returns a dictionary with keys "phi", "theta" and "psi"
6 # get translations
7 vector = t.get_pretrans() # Returns a Vec3f object containing the translation
8 vector = t.get_posttrans() # Returns a Vec3f object containing the translation
Multiplication
Transform3D Times a Transform3D
The main thing to consider when multiplying two Transform3D objects is what will be the ultimate result of asking for the pre_trans and post_trans vectors of the resulting Transform3D object ($$T3D_{rst}$$) from Python. To answer this question we look at the details
$$T3D_{rst} = T3D_{2} T3D_{1} = T_{2,post} R_{2} T_{2,pre} T_{1,post} R_{1} T_{1,pre} = T_{2,post} R_{2} T_{2,pre}[[R_{1},R_{1}\hat{t}_{1,pre}+\hat{t}_{1,post}],[\hat{0}^T,1]]$$
$$ = T_{2,post} R_{2} [[R_{1},R_{1}\hat{t}_{1,pre}+\hat{t}_{1,post}+\hat{t}_{2,pre}],[\hat{0}^T,1]]$$
The translation in right column ($$R_{1}\hat{t}_{1,pre}+\hat{t}_{1,post}+\hat{t}_{2,pre}$$) is now what will be returned when $$T3D_{rst}$$ is asked for its pre_translation vector from python (or C++). Similarly, the post translation vector of $$T3D_{2}$$ will now be returned by calling get_postrans on $$T3D_{rst}$$. To complete the details, internally the Transform3D object will look like
$$ T3D_{rst} = [[ R_{2}R_{1},R_{2}(R_{1}\hat{t}_{1,pre}+\hat{t}_{1,post}+\hat{t}_{2,pre})+\hat{t}_{2,post}],[\hat{0}^T,1]]$$
In Python the Transfrom3D x Transform3D operation can be achieved using the '*' operator
Transform3D Times a Vector (Vec3f)
If v is a three dimensional vector encapsulated as a Vec3f then one can right multiply it by a Transform3D object and this achieves the following result
$$T3D \hat{v} = [[R,R\hat{t}_{pre}+\hat{t}_{post}],[\hat{0}^T,1]] \hat{v} $$
$$T3D \hat{v} = Rv+R\hat{t}_{pre}+\hat{t}_{post} $$
The vector v is treated implicitly as though it were an homogeneous coordinate, but the last row of the matrix-vector multiplication is not performed.
In Python the Transfrom3D x Vec3f operation can be achieved using the '*' operator or by calling the Transform3D::transform(Vec3f) function
1 T = Transform3D(Vec3f(1,2,3),25,45,65,Vec3f(4,5,6)) # Specify a pre trans, followed by EULER_EMAN convention rotations az, alt, phi, followed by the post trans
2 v = Vec3f(1,2,3) # for example, pixel coordinates 1,2,3
3 v_dash = T*v
4 v_dash = T.transform(v) # Achieves the same result as calling T*v
Explicitly Rotating a Vector (Vec3f)
If a Transform3D is represented as
$$T3D = [[R,\hat{t}],[\hat{0}^T,1]]$$
One can calculate
$$\hat{v}_R = R \hat{v}$$
in Python by doing
Transformations and projections
Say the data model is a 3D map denoted M(x,y,z) and a projection is to be generated in a particular direction. The model may also be pre and/or post translated as part of the projection process. The translation information along with the direction of the projection is to be stored in a Transform3D object T, and the projection is to be generated according to or equivalently to the following
$$p(x,y) = int_z T3D M(x,y,z) dz$$
That is, the projection is generated in an equivalent fashion to first transforming the 3D map M by the Transform3D object, and followed by taking line integrals along z. In EMAN2 real space projection applies the inverse of the Transform3D object to the coordinate system to achieve the same result, and this is to avoid having to interpolate multiple times.
Transformations and recontructors
In order to insert a projection as generated in the conventional way (above) into a 3D volume in the correct orientation, one must invert the Transform3D object that was used to generate the projection prior to slice insertion.