OpenGL ES Common Profile 2.0规格差异详解

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本文档是OpenGL ES Common Profile Specification 2.0的一个修订版本，标题为"es_cm_spec_2.0. (Difference Specification)"，发布于2010年11月2日，由Aaftab Munshi和Jon Leech共同编辑。版权归属于Khronos Group，强调了该规格受法律保护，未经Khronos Group事先书面许可，禁止任何形式的复制、出版、分发、传输、展示、广播或商业利用其中的任何内容。 OpenGL ES（Open Graphics Library for Embedded Systems）是针对移动设备和嵌入式系统设计的轻量级图形处理API，是OpenGL家族的一部分，专注于低功耗和小内存环境下的性能。Common Profile是其标准规范之一，旨在提供跨平台兼容性，确保所有支持此规格的设备能够执行相同的基本图形功能。这个"Difference Specification"文档主要关注的是OpenGL ES 2.0与先前版本之间的差异，它详细列出了新特性、修改或删除的部分，以及可能对开发者实施影响的变化。这对于那些已熟悉旧版ES 2.0开发者来说，是一个重要的参考资源，可以帮助他们了解哪些功能已被增强、哪些被替换，以及需要注意的兼容性问题。文档中还明确指出，Khronos Group允许其Promoter（推广者）、Contributor（贡献者）和 Adopter（采用者）成员在不收取费用的情况下复制和重新分发未做修改的规格文档，但必须保持版权和商标等声明完整，且不得生产和销售基于规格的任何产品或服务。阅读和理解这份"es_cm_spec_2.0. (Difference Specification)"对于开发人员在升级到OpenGL ES 2.0的最新变化时，理解和适应这些变化，优化他们的应用程序以充分利用新的硬件能力至关重要。同时，它也是维护软件兼容性和遵循Khronos Group规定的重要文档。

OpenGL Operation 9

OpenGL 2.0

Common

void Orthof(float l, float r, float b, float t, float

n, float f)

–

void ActiveTexture(enum texture)



void PushMatrix(void)

–

void PopMatrix(void)

–

void Enable/Disable(RESCALE NORMAL)

–

void Enable/Disable(NORMALIZE)

–

void TexGen{ifd}[v](enum coord, enum pname, T param)

–

void GetTexGen{ifd}v(enum coord, enum pname, T

params)

–

void Enable/Disable(TEXTURE GEN {STRQ})

–

n Features such as texture coordinate generation, normalization and rescaling of normals etc. can

now be implemented inside a vertex shader, and are therefore not needed. q

10 OpenGL Operation

2.12 Clipping

Clipping against the viewing frustum is supported; however, separate user-speciﬁed clipping planes are not

supported.

The following modiﬁcations describes how lines and points are clipped in OpenGL ES 2.0

If the primitive is a point, then clipping discards it if it lies outside the near or far clip plane; otherwise,

it is passed unchanged. If the primitive is a line segment, and a part of it lies outside the space between the

near and the far plane, the line is clipped and new vertex coordinates are computed for one or both vertices.

A clipped line segment endpoint lies on both the original line segment and on either the near or the far

clipping plane. If the line segment lies completely between the two planes, it is passed unchanged.

OpenGL 2.0

Common

void ClipPlane(enum plane, const double

equation)

–

void GetClipPlane(enum plane, double

equation)

–

void Enable/Disable(CLIP PLANE{0-5})

–

n User-speciﬁed clipping planes are used predominately in engineering and scientiﬁc applications.

User clip planes can be emulated by calculating the dot product of the user clip plane with the vertex

position in eye space in the vertex shader. This term can be deﬁned as a varying variable. The

fragment shader can reject the pixel based on the value of this term. Depending on the ﬂoat pre-

cision types supported in a fragment shader, there may be clipping artifacts because of insufﬁcient

precision. q

2.13 Current Raster Position

The concept of the current raster position for positioning pixel rectangles and bitmaps is not supported.

Current raster state and commands for setting the raster position are not supported.

OpenGL 2.0

Common

RasterPos{2,3,4}{sifd}[v](T coords)

–

WindowPos{2,3}{sifd}[v](T coords)

–

n Bitmaps and pixel image primitives are not supported so there is no need to specify the raster

position. q

2.14 Colors and Coloring

The OpenGL 2.0 ﬁxed function lighting model is no longer supported.

OpenGL 2.0

Common

void FrontFace(enum mode)



void Enable/Disable(LIGHTING)

–

void Enable/Disable(LIGHT{0-7})

–

void Materialf[v](enum face, enum pname, T param)

–

void Materiali[v](enum face, enum pname, T param)

–

void GetMaterialfv(enum face, enum pname, T

params)

–

OpenGL Operation 11

OpenGL 2.0

Common

void GetMaterialiv(enum face, enum pname, T

params)

–

void Lightf[v](enum light, enum pname, T param)

–

void Lighti[v](enum light, enum pname, T param)

–

void GetLightfv(enum light, enum pname, T

params)

–

void GetLightiv(enum light, enum pname, T

params)

–

void LightModelf[v](enum pname, T param)

–

void LightModeli[v](enum pname, T param)

–

void Enable/Disable(COLOR MATERIAL)

–

void ColorMaterial(enum face, enum mode)

–

void ShadeModel(enum mode)

–

n The OpenGL 2.0 or any user deﬁned lighting can be implemented by writing appropriate vertex

and/or pixel shaders.

ShadeModel is no longer supported as ﬂat vs. gouraud shading only applied to the predeﬁned color

vertex attribute. Predeﬁned vertex attributes are not supported by OpenGL ES 2.0. q

2.15 Vertex Shaders

OpenGL 2.0 supports the ﬁxed function vertex pipeline and a programmable vertex pipeline using vertex

shaders. OpenGL ES 2.0 supports the programmable vertex pipeline only. OpenGL ES 2.0 allows applica-

tions to describe operations that occur on vertex values and their associated data by using a vertex shader.

OpenGL ES 2.0 provides interfaces to directly load pre-compiled shader binaries, or to load the shader

sources and compile them. An OpenGL ES implementation must support one of these methods for load-

ing shaders. A query of boolean value SHADER COMPILER can be used to determine if the OpenGL ES

implementation supports a shader compiler.

2.15.1 Loading and Compiling Shader Sources

The ShaderSource command loads source code into a vertex or a fragment shader object. Once the source

code for a shader has been loaded, a shader object can be compiled using the CompileShader command. A

string that contains information about the last compilation attempt on a shader object, called the info log,

can be obtained with the GetShaderInfoLog command. The GetShaderSource command returns the shader

source for the speciﬁed shader object.

The ReleaseShaderCompiler command allows the OpenGL ES implementation to release the resources

allocated by the shader compiler. This is a hint from the application and is no indicator that the compiler

will not be used in the future. If shader sources are loaded and compiled after ReleaseShaderCompiler has

been called, the CompileShader call is supposed to successfully compile the shaders provided there are no

errors in the shader source(s).

The command

void GetShaderPrecisionFormat(enum shadertype, enum precisiontype, int *range, int *precision)

returns the range and precision for different numeric formats supported by the shader compiler. shadertype

must be VERTEX SHADER or FRAGMENT SHADER. precisiontype must be one of LOW FLOAT, MEDIUM -

12 OpenGL Operation

FLOAT, HIGH FLOAT, LOW INT, MEDIUM INT or HIGH INT. range points to an array of two integers in

which encodings of the format’s numeric range are returned. If min and max are the smallest and largest

values representable in the format, then the values returned are deﬁned to be

range[0] = blog

(|min|)c

range[1] = blog

(|max|)c

precision points to an integer in which the log

value of the number of bits of precision of the format is

returned. If the smallest representable value greater than 1 is 1 + , then *precision will contain b−log

()c,

and every value in the range

[−2

range[0]

, 2

range[1]

]

can be represented to at least one part in 2

∗precision

. For example, an IEEE single-precision ﬂoating-point

format would return range[0] = 127, range[1] = 127, and ∗precision = 23, while a 32-bit twos-

complement integer format would return range[0] = 31, range[1] = 30, and ∗precision = 0.

The minimum required precision and range for formats corresponding to the different values of preci-

siontype are described in section 4.5 of the OpenGL ES Shading Language speciﬁcation.

If high precision ﬂoating-point is not supported in fragment shaders, calling GetShaderPrecisionFormat

with a precisiontype of HIGH FLOAT will return zero for range[0], range[1], and *precision.

If the value of SHADER COMPILER is not TRUE, then the error INVALID OPERATION is generated.

2.15.2 Shader Binaries

The ShaderBinary command can be used to load precompiled shader binaries.

void ShaderBinary(sizei count, const uint *shaders, enum binaryformat, const void *binary, sizei length)

This call takes a list of count shader handles described by shaders. Each shader handle refers to a unique

shader type i.e. a vertex shader or a fragment shader. The binary argument points to length bytes of pre-

compiled binary code. This provides the ability to individually load binary vertex, or fragment shaders or

load an executable binary that contains the optimized pair of vertex and fragment shaders stored in the same

binary.

The binary image will be decoded according to the speciﬁcation deﬁning the binaryformat token. A

binary data that does not match the speciﬁed binaryformat will result in an INVALID VALUE error. The bits

that represent the binary is implementation speciﬁc. An INVALID OPERATION error is generated if any of

the handles in shaders is not a valid shader object created with CreateShader, or if more than one of the

handles refers to the same type of shader (vertex or fragment shader.) If ShaderBinary failed, GetError can

be used to return the appropriate error. A failed binary load does not restore the old state of shaders for

which the binary was being loaded.

Queries of values NUM

SHADER BINARY FORMATS and SHADER BINARY FORMATS return the number

of shader binary formats and the list of shader binary format values supported by an OpenGL ES implemen-

tation

Note that if shader binary interfaces are supported, then an OpenGL ES implementation may require

that an optimized pair of vertex and fragment shader binaries that were compiled together be speciﬁed to

LinkProgram. Not specifying an optimized pair may result in the LinkProgram call to fail.

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