Cg语言规格详解：GPU数据流编程与Unity Shader实战指南

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"Cg语言规格文档"是一份针对学习计算机图形学(CG)编程和Unity Shader开发的重要参考资料。它基于ANSI C，借鉴了C++和Java的部分特性，以及RenderMan和Stanford shading language的传统，同时引入了适应GPU流处理架构的新理念。Cg语言的特点包括支持编译时指定的profile，允许开发者在性能和功能上进行选择，如控制循环结构和计算精度。在语言规范的概述部分，Cg强调了数据流处理的特性，这使得它特别适合在GPU等并行计算环境中运行。然而，尽管大部分改动是对ANSI C的扩展或补充，但也存在潜在的静默不兼容性。这些不兼容性主要表现在对常量类型提升规则的处理上：当未明确使用类型转换或后缀指定类型时，常量与其他变量之间的运算规则可能与标准C有所不同，这可能导致原本无误编译的程序行为发生改变。学习者在使用Cg时需要注意这些细节，确保代码在Cg环境中的正确性和预期性能。此外，理解并掌握这些新特性和潜在的不兼容性对于避免潜在问题、优化性能以及充分利用Cg语言的强大功能至关重要。这份文档不仅适合初学者系统地学习Cg语言，也是进阶开发者调试和改进现有Shader的必备工具。通过深入阅读和实践，开发者能够更好地利用Cg语言为Unity游戏开发提供丰富的视觉效果和高性能渲染。

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NVIDIA

Cg Language Toolkit

The declaration and use of arrays of arrays is in the same style as in C. That is, if

the 2D array A is declared as

then, the following statements are true:

! The array is indexed as A[row][column].

! The array can be built with a constructor using

! A[0] is equivalent to {A[0][0], A[0][1], A[0][2], A[0][3]}.

Support must be provided for any

struct containing arrays.

Minimum Array Requirements

Profiles are required to provide partial support for certain kinds of arrays. This

partial support is designed to support vectors and matrices in all profiles. For

vertex profiles, it is additionally designed to support arrays of light state

(indexed by light number) passed as uniform parameters, and arrays of skinning

matrices passed as uniform parameters.

Profiles must support subscripting, copying, and swizzling of vectors and

matrices. However, subscripting with run-time computed indices is not required

to be supported.

Vertex profiles must support the following operations for any non-packed array

that is a uniform parameter to the program, or is an element of a structure that

is a uniform parameter to the program. This requirement also applies when the

array is indirectly a uniform program parameter (that is, it and or the structure

containing it has been passed via a chain of

in function parameters). The two

operations that must be supported are

! Rvalue subscripting by a run-time computed value or a compile-time value

! Passing the entire array as a parameter to a function, where the

corresponding formal function parameter is declared as

The following operations are explicitly not required to be supported:

! Lvalue subscripting

! Copying

! Other operators, including multiply, add, compare, and so on

float A[4][4];

A = { {A[0][0], A[0][1], A[0][2], A[0][3]},

{A[1][0], A[1][1], A[1][2], A[1][3]},

{A[2][0], A[2][1], A[2][2], A[2][3]},

{A[3][0], A[3][1], A[3][2], A[3][3]} };

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NVIDIA

Appendix A Cg Language Specification

Note that when the array is rvalue subscripted, the result is an expression, and

this expression is no longer considered to be a uniform program parameter.

Therefore, if this expression is an array, its subsequent use must conform to the

standard rules for array usage.

These rules are not limited to arrays of numeric types, and thus imply support

for arrays of

struct, arrays of matrices, and arrays of vectors when the array is

a uniform program parameter. Maximum array sizes may be limited by the

number of available registers or other resource limits, and compilers are

permitted to issue error messages in these cases. However, profiles must

support sizes of at least

float arr[8], float4 arr[8], and float4x4

arr[4][4].

Fragment profiles are not required to support any operations on arbitrarily sized

arrays; only support for vectors and matrices is required.

Function Overloading

Multiple functions may be defined with the same name, as long as the

definitions can be distinguished by unqualified parameter types and do not have

an open-profile conflict (see “Overloading of Functions by Profile” on

page 170).

Function-matching rules:

1. Add all visible functions with a matching name in the calling scope to the

set of function candidates.

2. Eliminate functions whose profile conflicts with the current compilation

profile.

3. Eliminate functions with the wrong number of formal parameters. If a

candidate function has excess formal parameters, and each of the excess

parameters has a default value, do not eliminate the function.

4. If the set is empty, fail.

5. For each actual parameter expression in sequence, perform the following:

a. If the type of the actual parameter matches the unqualified type of the

corresponding formal parameter in any function in the set, remove all

functions whose corresponding parameter does not match exactly.

b. If there is a defined promotion for the type of the actual parameter to

the unqualified type of the formal parameter of any function, remove

all functions for which this is not true from the set.

c. If there is a valid implicit cast that converts the type of the actual

parameter to the unqualified type of the formal parameter of any

function, remove all functions without this cast.

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NVIDIA

Appendix A Cg Language Specification

Overview of Binding Semantics

In stream-processing architectures, data packets flow between different

programmable units. On a GPU, for example, packets of vertex data flow from

the application to the vertex program.

Because packets are produced by one program (the application, in this case),

and consumed by another (the vertex program), there must be some method

for defining the interface between the two. Cg allows the user to choose

between two different approaches to defining these interfaces.

The first approach is to associate a binding semantic with each element of the

packet. This approach is a bind-by-name approach. For example, an output with

the binding semantic

FOO is fed to an input with the binding semantic FOO.

Profiles may allow the user to define arbitrary identifiers in this “semantic

namespace,” or they may restrict the allowed identifiers to a predefined set.

Often, these predefined names correspond to the names of hardware registers

or API resources.

In some cases, predefined names may control non-programmable parts of the

hardware. For example, vertex programs normally compute a position that is

fed to the rasterizer, and this position is stored in an output with the binding

semantic

POSITION.

For any profile, there are two namespaces for predefined binding semantics—

the namespace used for

in variables and the namespace used for out variables.

The primary implication of having two namespaces is that the binding semantic

cannot be used to implicitly specify whether a variable is

in or out.

The second approach to defining data packets is to describe the data that is

present in a packet and allow the compiler decide how to store it. In Cg, the

user can describe the contents of a data packet by placing all of its contents into

struct. When a struct is used in this manner, we refer to it as a connector.

The two approaches are not mutually exclusive, as is discussed later. The

connector approach allows the user to rely on a combination of user-specified

semantic bindings and compiler-determined bindings.

Binding Semantics

A binding semantic may be associated with an input to a top-level function in

one of three ways:

! The binding semantic is specified in the formal parameter declaration for

the function. The syntax for formal parameters to a function is

[

const] [in | out | inout]

<type> <identifier> [ : <binding-semantic>][= <initializer>]

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