iOS --- OpenGLES之初步认识

iOS上绘制图形的方式很多，UIKit，CoreGraphics，SpriteKit，OpenGL ES，Metal等。OpenGL ES是一套非常底层但使用非常广泛的C语言API，专为移动设备定制，可在不同的手机系统或浏览器上使用，渲染效果非常好。

OpenGLES

援引Apple的OpenGLES_ProrogrammingGuide中的一段简介：

The Open Graphics Library (OpenGL) is used for visualizing 2D and 3D data. It is a multipurpose open-standard graphics library that supports applications for 2D and 3D digital content creation, mechanical and architectural design, virtual prototyping, flight simulation, video games, and more. You use OpenGL to configure a 3D graphics pipeline and submit data to it. Vertices are transformed and lit, assembled into primitives, and rasterized to create a 2D image. OpenGL is designed to translate function calls into graphics commands that can be sent to underlying graphics hardware. Because this underlying hardware is dedicated to processing graphics commands, OpenGL drawing is typically very fast.
OpenGL for Embedded Systems (OpenGL ES) is a simplified version of OpenGL that eliminates redundant functionality to provide a library that is both easier to learn and easier to implement in mobile graphics hardware.
OpenGL ES allows an app to harness the power of the underlying graphics processor. The GPU on iOS devices can perform sophisticated 2D and 3D drawing, as well as complex shading calculations on every pixel in the final image. You should use OpenGL ES if the design requirements of your app call for the most direct and comprehensive access possible to GPU hardware. Typical clients for OpenGL ES include video games and simulations that present 3D graphics.

OpenGL可用于渲染2D和3D图像，是一个多用途的开源图形库。OpenGL设计用来将函数命令转换成图形命令，发送到GPU中。GPU正是被设计用来处理图形命令的，所以OpenGL的绘制非常高效。
OpenGLES是OpenGL的简化版本，抛弃了冗余的文件及命令，使之专用于嵌入式设备。OpenGLES使得移动APP能充分利用GPU的强大运算能力。iOS设备上的GPU能执行更精确的2D和3D绘制，以及更加复杂的针对每个像素的图形脚本（shader）计算。

使用OpenGL ES需要在项目的Build Phases中引入QuartzCore和OpenGLES框架。其基本的使用步骤都是大致固定的。
OpenGLES是一个状态机，可看到很多如glEnable*，glBind*等的函数，使用方式大致类似于切换场景、设置状态、使用命令等。
建立工程后，在implementation文件中添加如下实例变量：

EAGLContext *_eaglContext; //