OpenGL ES 2.0 Common Profile Specification v1.19: Differences & Implementation Details

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OpenGL ES 2.0 是一个针对移动和嵌入式设备的轻量级版本的 OpenGL，它专注于图形渲染在有限硬件资源环境下的性能和兼容性。此规格文档主要关注 OpenGL ES 2.0 的"Common Profile"，这是其核心特性集，它简化了 API，移除了对固定点数据类型的依赖，仅在顶点属性数组中支持这种类型。这样做旨在提高开发者的易用性和API一致性，同时也考虑到在移动设备上固定点运算可能带来的性能瓶颈。相比于桌面版 OpenGL 2.0，OpenGL ES 2.0在管线设计上保持了一致性，但去掉了某些不适合移动平台的特性。例如，固定点（16.16位）数据类型不再用于着色器统一变量和命令参数，这使得渲染代码更加通用，并减少了复杂性。同时，这样做也意味着开发者可以期待更简洁的编程接口和更好的性能优化。该规范详述了支持的命令和状态，区分了那些属于完整OpenGL 2.0规格但未包含在OpenGL ES 2.0中的部分。它并非独立于OpenGL 2.0规范的详细行为描述，而是侧重于对比两者之间的差异，让开发者了解如何在移动平台上正确地实现OpenGL ES 2.0的功能。此外，文档还强调了版权问题，指出只有经Khronos Group明确书面许可，才能复制、分发或使用这份规格，且不得修改或去除任何商标、版权或其他通知。Khronos Group给予其成员，如Promoter、Contributor和 Adopter，在不收取费用的前提下，可以无改动地分发和更新该规范，但不得借此生产、公开或销售基于规格描述的产品或服务。 OpenGL ES 2.0规格文档为开发者提供了一个指导，帮助他们在资源受限的移动设备上实现高性能图形渲染，同时确保遵循版权规定。它强调了与桌面版 OpenGL 2.0的区别，以及如何在移动平台上有效利用OpenGL ES 2.0 API。

8 OpenGL Operation

2.10 Rectangles

The commands for directly specifying rectangles are not supported.

OpenGL 2.0

Common

void Rect{sifd}(T x1, T y1, T x2, T y2)

–

void Rect{sifd}v(T v1[2], T v2[2])

–

n The rectangle commands are not used enough in applications to justify maintaining a redundant

mechanism for drawing a rectangle. q

2.11 Coordinate Transformations

The ﬁxed function transformation pipeline is no longer supported. The application can compute the neces-

sary matrices (can be the combined modelview and projection matrix, or an array of matrices for skinning)

and load them as uniform variables in the vertex shader. The code to compute transformed vertex will now

be executed in the vertex shader.

The Viewport command is supported since the viewport transformation happens after the programmable

vertex transform and is a ﬁxed function.

OpenGL 2.0

Common

void DepthRange(clampd n, clampd f)

–

void DepthRangef(clampf n, clampf f)



void Viewport(int x, int y, sizei w, sizei h)



void MatrixMode(enum mode)

–

void LoadMatrixf(float m[16])

–

void LoadMatrixd(double m[16])

–

void MultMatrixf(float m[16])

–

void MultMatrixd(double m[16])

–

void LoadTransposeMatrix{fd}(T m[16])

–

void MultTransposeMatrix{fd}(T m[16])

–

void LoadIdentity(void)

–

void Rotatef(float angle, float x, float y, float z)

–

void Rotated(double angle, double x, double y, double

–

void Scalef(float x, float y, float z)

–

void Scaled(double x, double y, double z)

–

void Translatef(float x, float y, float z)

–

void Translated(double x, double y, double z)

–

void Frustum(double l, double r, double b, double t,

double n, double f)

–

void Ortho(double l, double r, double b, double t,

double n, double f)

–

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

n, float f)

–

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 loading

shaders.

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 GetShaderPrecisionFormat command returns the range and precision for various precision formats

supported by the implementation.

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

shadertype must be VERTEX SHADER or FRAGMENT SHADER. precisiontype can be LOW FLOAT, MEDIUM -

FLOAT, HIGH FLOAT, LOW INT, MEDIUM INT or HIGH INT. range returns the minimum and maximum

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