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Version: v1.0.4

Objective data-oriented programming

Taichi is a data-oriented programming (DOP) language. However, simple DOP makes modularization hard.

To allow modularized code, Taichi borrow some concepts from object-oriented programming (OOP).

For convenience, let's call the hybrid scheme objective data-oriented programming (ODOP).

Data-oriented classes

Introduction

If you need to define a Taichi kernel as a Python class member function, please decorate the class with a @ti.data_oriented decorator. You can then define ti.kernels and ti.funcs in your data-oriented Python class.

note

The first argument of the function should be the class instance ("self"), unless you are defining a @staticmethod.

A brief example:

@ti.data_oriented
class TiArray:
def __init__(self, n):
self.x = ti.field(dtype=ti.i32, shape=n)

@ti.kernel
def inc(self):
for i in self.x:
self.x[i] += 1

a = TiArray(32)
a.inc()

Definitions of Taichi fields can be made not only in init functions, but also at any place of a Python-scope function in a data-oriented class. For example,

import taichi as ti

ti.init()

@ti.data_oriented
class MyClass:
@ti.kernel
def inc(self, temp: ti.template()):
for I in ti.grouped(temp):
temp[I] += 1

def call_inc(self):
self.inc(self.temp)

def allocate_temp(self, n):
self.temp = ti.field(dtype = ti.i32, shape=n)


a = MyClass()
# a.call_inc() cannot be called, since a.temp has not been allocated at this point
a.allocate_temp(4)
a.call_inc()
a.call_inc()
print(a.temp) # [2 2 2 2]
a.allocate_temp(8)
a.call_inc()
print(a.temp) # [1 1 1 1 1 1 1 1]

Another memory recycling example:

import taichi as ti

ti.init()

@ti.data_oriented
class Calc:
def __init__(self):
self.x = ti.field(dtype=ti.f32, shape=16)
self.y = ti.field(dtype=ti.f32, shape=4)

@ti.kernel
def func(self, temp: ti.template()):
for i in range(8):
temp[i] = self.x[i * 2] + self.x[i * 2 + 1]

for i in range(4):
self.y[i] = max(temp[i * 2], temp[i * 2 + 1])

def call_func(self):
fb = ti.FieldsBuilder()
temp = ti.field(dtype=ti.f32)
fb.dense(ti.i, 8).place(temp)
tree = fb.finalize()
self.func(temp)
tree.destroy()


a = Calc()
for i in range(16):
a.x[i] = i
a.call_func()
print(a.y) # [ 5. 13. 21. 29.]

Inheritance of data-oriented classes

The data-oriented property will be automatically carried beyond the Python class inheriting. This means the Taichi Kernel could be called while any of the ancestor classes are decorated by the @ti.data_oriented decorator.

An example:

import taichi as ti

ti.init(arch=ti.cuda)

class BaseClass:
def __init__(self):
self.n = 10
self.num = ti.field(dtype=ti.i32, shape=(self.n, ))

@ti.kernel
def count(self) -> ti.i32:
ret = 0
for i in range(self.n):
ret += self.num[i]
return ret

@ti.kernel
def add(self, d: ti.i32):
for i in range(self.n):
self.num[i] += d


@ti.data_oriented
class DataOrientedClass(BaseClass):
pass

class DeviatedClass(DataOrientedClass):
@ti.kernel
def sub(self, d: ti.i32):
for i in range(self.n):
self.num[i] -= d


a = DeviatedClass()
a.add(1)
a.sub(1)
print(a.count()) # 0


b = DataOrientedClass()
b.add(2)
print(b.count()) # 1

c = BaseClass()
# c.add(3)
# print(c.count())
# The two lines above will trigger a kernel define error, since class c is not decorated by @ti.data_oriented

Python built-in decorators

Common decorators that are pre-built in Python, @staticmethod1 and @classmethod2, could decorate to a Taichi kernel in data-oriented classes.

staticmethod example :

import taichi as ti

ti.init()

@ti.data_oriented
class Array2D:
def __init__(self, n, m, increment):
self.n = n
self.m = m
self.val = ti.field(ti.f32)
self.total = ti.field(ti.f32)
self.increment = float(increment)
ti.root.dense(ti.ij, (self.n, self.m)).place(self.val)
ti.root.place(self.total)

@staticmethod
@ti.func
def clamp(x): # Clamp to [0, 1)
return max(0., min(1 - 1e-6, x))

@ti.kernel
def inc(self):
for i, j in self.val:
ti.atomic_add(self.val[i, j], self.increment)

@ti.kernel
def inc2(self, increment: ti.i32):
for i, j in self.val:
ti.atomic_add(self.val[i, j], increment)

@ti.kernel
def reduce(self):
for i, j in self.val:
ti.atomic_add(self.total[None], self.val[i, j] * 4)

arr = Array2D(2, 2, 3)

double_total = ti.field(ti.f32, shape=())

ti.root.lazy_grad()

arr.inc()
arr.inc.grad()
print(arr.val[0, 0]) # 3
arr.inc2(4)
print(arr.val[0, 0]) # 7

with ti.Tape(loss=arr.total):
arr.reduce()

for i in range(arr.n):
for j in range(arr.m):
print(arr.val.grad[i, j]) # 4

@ti.kernel
def double():
double_total[None] = 2 * arr.total[None]

with ti.Tape(loss=double_total):
arr.reduce()
double()

for i in range(arr.n):
for j in range(arr.m):
print(arr.val.grad[i, j]) # 8

classmethod example:

import taichi as ti

ti.init(arch=ti.cuda)

@ti.data_oriented
class Counter:
num_ = ti.field(dtype=ti.i32, shape=(32, ))
def __init__(self, data_range):
self.range = data_range
self.add(data_range[0], data_range[1], 1)

@classmethod
@ti.kernel
def add(cls, l: ti.i32, r: ti.i32, d: ti.i32):
for i in range(l, r):
cls.num_[i] += d

@ti.kernel
def num(self) -> ti.i32:
ret = 0
for i in range(self.range[0], self.range[1]):
ret += self.num_[i]
return ret

a = Counter((0, 5))
print(a.num()) # 5
b = Counter((4, 10))
print(a.num()) # 6
print(b.num()) # 7

Python classes as Taichi struct types

Taichi provides custom struct types for developers to associate pieces of data together. However, it is often convenient to have:

  1. A Python representation of the struct type which is more object oriented.
  2. Functions associated with a struct type. (C++ style structs)

To achieve these two points, developers can use the @ti.struct_class decorator on a Python class. This is heavily inspired by the Python dataclass feature, which uses class fields with annotations to create data types.

Creating a struct from a Python class

Here is an example of how we could create a Taichi struct type from a Python class:

@ti.struct_class
class Sphere:
center: vec3
radius: ti.f32

This will create the exact same type as doing:

Sphere = ti.types.struct(center=vec3, radius=ti.f32)

Using the @ti.struct_class decorator will convert the annotated fields in the Python class to members in the resulting struct type. In both of the above examples you would create a field of the struct the same way.

sphere_field = Sphere.field(shape=(n,))

Associating functions with the struct type

Python classes can have functions attached to them, as can Taichi struct types. Building from the above example, here is how one would add functions to the struct.

@ti.struct_class
class Sphere:
center: vec3
radius: ti.f32

@ti.func
def area(self):
# a function to run in taichi scope
return 4 * math.pi * self.radius * self.radius

def is_zero_sized(self):
# a python scope function
return self.radius == 0.0

Functions associated with structs follow the same scope rules as normal functions, in that they can be in Taichi or Python scope. Each instance of the Sphere struct type now will have the above functions added to them. The functions can be called such as:

a_python_struct = Sphere(center=vec3(0.0), radius=1.0)
# calls a python scope function from python
a_python_struct.is_zero_sized() # False

@ti.kernel
def get_area() -> ti.f32:
a_taichi_struct = Sphere(center=vec3(0.0), radius=4.0)
# return the area of the sphere, a taichi scope function
return a_taichi_struct.area()
get_area() # 201.062...

Notes on struct classes

  • Inheritance of struct classes is not implemented.
  • While functions attached to a struct with the @ti.struct_class decorator is convenient and encouraged, it is actually possible to associate a function to structs with the older method of defining structs. As mentioned above, the two methods for defining a struct type are identical in their output. To do this, use the __struct_methods argument with the ti.types.struct call:
@ti.func
def area(self):
# a function to run in taichi scope
return 4 * math.pi * self.radius * self.radius

Sphere = ti.types.struct(center=vec3, radius=ti.f32,
__struct_methods={'area': area})