concept
OpenGL (Open Graphics Library) is a cross-platform, high-performance 3D rendering API, OpenGL ES 2.0 (Open Graphics Library Embedded System 2.0) is its embedded platform version 2.0.
Simple implementation
Activity in the inside, creating a GLSurfaceView object, and then as the layout of the Activity and achieve a Renderer interface upon their GLSurfaceView, and make them simple configuration.
const val TAG = "HelloPoints"
class HelloPoints : Activity() {
private lateinit var glSurfaceView: GLSurfaceView
override fun onCreate(savedInstanceState: Bundle?) {
super.onCreate(savedInstanceState)
title = "Play with Points"
glSurfaceView = GLSurfaceView(this)
//GLSurfaceView作为布局
setContentView(glSurfaceView)
glSurfaceView.setEGLContextClientVersion(2)//OpenGL 2.0版本
//Renderer接口设置进入
glSurfaceView.setRenderer(PointsRender)
glSurfaceView.renderMode = GLSurfaceView.RENDERMODE_CONTINUOUSLY
}
override fun onResume() {
super.onResume()
glSurfaceView.onResume()
}
override fun onPause() {
super.onPause()
glSurfaceView.onPause()
}
companion object PointsRender : GLSurfaceView.Renderer {
/**
* shader语言跟C语言很像，它有一个主函数，也叫void main(){}。
* gl_Position是一个内置变量，用于指定顶点，它是一个点，三维空间的点，所以用一个四维向量来赋值。
* vec4是四维向量的类型，vec4()是它的构造方法。等等，三维空间，不是（x, y, z）三个吗？为什么用
* *vec4层呢？
* 四维是叫做齐次坐标，它的几何意义仍是三维，对于2D的话，第四位永远传1.0就可以了。
* 这里，是指定原点(0, 0, 0)作为顶点，就是说想在原点位置画一个点。gl_PointSize是另外一个内置变量，用于指定点的大小。
*
* 这个shader就是想在原点画一个尺寸为20的点。
*/
private const val VERTEX_SHADER =
"void main() {\n" +
"gl_Position = vec4(0.0, 0.0, 0.0, 1.0);\n" +
"gl_PointSize = 20.0;\n" +
"}\n"
/**
* gl_FragColor是fragment shader的内置变量，
* 用于指定当前顶点的颜色，四个分量（r, g, b, a）。
* 这里是想指定为红色，不透明。
*/
private const val FRAGMENT_SHADER =
"void main() {\n" +
"gl_FragColor = vec4(1.0, 0.0, 0.0, 1.0);\n" +
"}\n"
private var mGLProgram: Int = -1
/**
* 所有的GL API的调用都要在GLSurfaceView.Renderer的三个方法里面来call，
* 就是方法的调用栈必须从这几个方法开始
*/
//初始化操作
override fun onSurfaceCreated(p0: GL10?, p1: EGLConfig?) {
GLES20.glClearColor(1f, 0f, 0f, 1f)//把背景，或者叫作画布，画成黑色，不透明
//编译和链接shader程序
val vsh = GLES20.glCreateShader(GLES20.GL_VERTEX_SHADER)
// 告诉OpenGL，这一坨字串里面是vertex shader的源码。
GLES20.glShaderSource(vsh, VERTEX_SHADER)
GLES20.glCompileShader(vsh) // 编译vertex shader
val fsh = GLES20.glCreateShader(GLES20.GL_FRAGMENT_SHADER)
GLES20.glShaderSource(fsh, FRAGMENT_SHADER)
GLES20.glCompileShader(fsh)
mGLProgram = GLES20.glCreateProgram() // 创建shader program句柄
GLES20.glAttachShader(mGLProgram, vsh) // 把vertex shader添加到program
GLES20.glAttachShader(mGLProgram, fsh) // 把fragment shader添加到program
// 做链接，可以理解为把两种shader进行融合，做好投入使用的最后准备工作
GLES20.glLinkProgram(mGLProgram)
// 让OpenGL来验证一下我们的shader program，并获取验证的状态
GLES20.glValidateProgram(mGLProgram)
val status = IntArray(1)
GLES20.glGetProgramiv(mGLProgram, GLES20.GL_VALIDATE_STATUS, status, 0) // 获取验证的状态
Log.d(TAG, "validate shader program: " + GLES20.glGetProgramInfoLog(mGLProgram))
}
//surface发生改变时调用，通常是size发生变化
override fun onSurfaceChanged(p0: GL10?, p1: Int, p2: Int) {
GLES20.glViewport(0, 0, p1, p2) // 参数是left, top, width, height
}
//而此方法是用来绘制每帧的，所以每次刷新都会被调一次，所有的绘制都发生在这里。
override fun onDrawFrame(p0: GL10?) {
// 清除颜色缓冲区，因为我们要开始新一帧的绘制了，所以先清理，以免有脏数据。
GLES20.glClear(GLES20.GL_COLOR_BUFFER_BIT)
// 告诉OpenGL，使用在onSurfaceCreated里面准备好了的shader program来渲染
GLES20.glUseProgram(mGLProgram)
//开始渲染，发送渲染点的指令， 第二个参数是offset，第三个参数是点的个数。
//目前只有一个点，所以是1。
GLES20.glDrawArrays(GLES20.GL_POINTS, 0, 1)
}
}
}
Some explanations:
Description 1
All the GL API calls should be in GLSurfaceView.Renderer inside three methods to call, is the call stack method must start from several methods. Call elsewhere is not effective:
onSurfaceCreated
onSurfaceChanged
onDrawFrame
DESCRIPTION 2
the OpenGL coordinate system is a so-called right-handed
Description. 3: Shader
GL ES 2.0 put rendering operations associated with a specific called shading language programs expressed, full name as OpenGL ES Shading language, it is a programming language, C language is very similar **, matrix and vector operations can be direct, run on dedicated graphics rendering GPU **. It is divided into two, called vertex shader (vertex shader), the geometry of vertices specified location and size , the other is called the fragment shader (fragment shader), which specifies each vertex shader . Each GL program must have a vertex shader and a fragment shader, and they correspond to each other. (Correspond to each other, meaning that there must be a vertex shader fragment shader, and vice versa, but not necessarily one to one). Of course, also can be reused, such as the same vertex shader, a plurality of fragment shader may express different colored solution.
Description 4: the coordinate values and the color values of
the coordinates of the normal range of -1 to 1, and a float.
Color value is 0 to 1, is of type float, 0 is empty (no meaning, such as black or transparent), is 1 (full meaning, such as white, or opaque).
Description 5
Renderer instance, rendering mode (render mode) is divided into two, one is GLSurfaceView initiative to refresh (Continuously) , kept callback Renderer of onDrawFrame, another is called passive refresh (the when Dirty) , that is, when requested refresh tune once onDrawFrame.
