閱讀本文前如果是初次接觸opengl可以先閱讀前文:
openGL 3D圖形和openGL簡介：
http://blog.csdn.net/pangrui201/article/details/75091501
android上opengl es基礎知識
Google在Android2.2 以後支持 opengl es 2.2,在Android 4.3以後引入opengl es 3.0。Android中使用openGL會用到GLSurfaceView控件， GLSurfaceView.Renderer，在Android studio的debug模式下我們可以清楚的看到Renderer的各個回調函數發生在非UI主線程，即渲染線程,具體渲染是在一塊稱爲”surface”(在openGL裏面稱爲ViewPort視口)的地方完成，渲染繪製完成後在將渲染結果直接在主線程顯示，實際上GLSurfaceView在View Hierarchy上”穿洞”，讓底層open gl surface顯示出來。同時，需要考慮GLSurfaceView和Activity的各個生命週期的問題。
另一方面，在Android 4.0以後提供了一個紋理視圖(TextureView)可以也可以渲染opengl，TextureView像普通view一樣不在需要”穿洞”了，但是TextureView沒有內置opengl的初始化操作。
第一個opengl es程序
如下代碼是在Android studio上創建的第一個opengl es項目:
public class MainActivity extends AppCompatActivity {
private GLSurfaceView glSurfaceView;
private boolean rendererSet;
@Override
protected void onCreate(Bundle savedInstanceState) {
super.onCreate(savedInstanceState);
glSurfaceView = new GLSurfaceView(this);
// Check if the system supports OpenGL ES 2.0.
final ActivityManager activityManager =
(ActivityManager) getSystemService(Context.ACTIVITY_SERVICE);
final ConfigurationInfo configurationInfo =
activityManager.getDeviceConfigurationInfo();
/*
final boolean supportsEs2 = configurationInfo.reqGlEsVersion >= 0x20000;
*/
// Even though the latest emulator supports OpenGL ES 2.0,
// it has a bug where it doesn't set the reqGlEsVersion so
// the above check doesn't work. The below will detect if the
// app is running on an emulator, and assume that it supports
// OpenGL ES 2.0.
final boolean supportsEs2 =
configurationInfo.reqGlEsVersion >= 0x20000
|| (Build.VERSION.SDK_INT >= Build.VERSION_CODES.ICE_CREAM_SANDWICH_MR1
&& (Build.FINGERPRINT.startsWith("generic")
|| Build.FINGERPRINT.startsWith("unknown")
|| Build.MODEL.contains("google_sdk")
|| Build.MODEL.contains("Emulator")
|| Build.MODEL.contains("Android SDK built for x86")));
if (supportsEs2) {
// Request an OpenGL ES 2.0 compatible context.
glSurfaceView.setEGLContextClientVersion(2);
// Assign our renderer.
glSurfaceView.setRenderer(new FirstOpenGLProjectRenderer());
rendererSet = true;
} else {
/*
* This is where you could create an OpenGL ES 1.x compatible
* renderer if you wanted to support both ES 1 and ES 2. Since we're
* not doing anything, the app will crash if the device doesn't
* support OpenGL ES 2.0. If we publish on the market, we should
* also add the following to AndroidManifest.xml:
*
* <uses-feature android:glEsVersion="0x00020000"
* android:required="true" />
*
* This hides our app from those devices which don't support OpenGL
* ES 2.0.
*/
Toast.makeText(this, "This device does not support OpenGL ES 2.0.",
Toast.LENGTH_LONG).show();
return;
}
setContentView(glSurfaceView);
}
@Override
protected void onPause() {
super.onPause();
if (rendererSet) {
glSurfaceView.onPause();
}
}
@Override
protected void onResume() {
super.onResume();
if (rendererSet) {
glSurfaceView.onResume();
}
}
}
supportsEs2 = configurationInfo.reqGlEsVersion >= 0x20000;是用來判斷是否支持opengl es2.0,同時在模擬器的話可能支持不是多好，所以代碼進行了判斷。 glSurfaceView.setEGLContextClientVersion(2);是來具體設置opengl版本，配置surface視圖。同時需要注意Activity的生命週期需要回調glSurfaceView對應的生命週期，這樣surface視圖才能正確處理渲染線程暫停和繼續，同時釋放和續用opengl的上下文。
下面是FirstOpenGLProjectRenderer 的代碼：
public class FirstOpenGLProjectRenderer implements Renderer {
@Override
public void onSurfaceCreated(GL10 glUnused, EGLConfig config) {
// Set the background clear color to red. The first component is
// red, the second is green, the third is blue, and the last
// component is alpha, which we don't use in this lesson.
glClearColor(1.0f, 0.0f, 0.0f, 0.0f);
}
/**
* onSurfaceChanged is called whenever the surface has changed. This is
* called at least once when the surface is initialized. Keep in mind that
* Android normally restarts an Activity on rotation, and in that case, the
* renderer will be destroyed and a new one created.
*
* @param width
* The new width, in pixels.
* @param height
* The new height, in pixels.
*/
@Override
public void onSurfaceChanged(GL10 glUnused, int width, int height) {
// Set the OpenGL viewport to fill the entire surface.
glViewport(0, 0, width, height);
}
/**
* OnDrawFrame is called whenever a new frame needs to be drawn. Normally,
* this is done at the refresh rate of the screen.
*/
@Override
public void onDrawFrame(GL10 glUnused) {
// Clear the rendering surface.
glClear(GL_COLOR_BUFFER_BIT);
}
}主要有三個回調方法，onSurfaceCreated，onSurfaceChanged，onDrawFrame。之前已經說過了，這些方法回調發生在渲染線程。onSurfaceCreated是在surface被創建時，GLSurfaceView會回調這個方法。onSurfaceChanged是在surface被創建後每次在surface尺寸發生變化的時候GLSurfaceView會回調，比如橫豎屏切換會回調這個方法。onDrawFrame是當每繪製一幀就會被GLSurfaceView回調，通常一定要在這個方法裏做點事情哪怕是設置清屏顏色，因爲這個方法調用之後會把渲染結果surface給屏幕顯示，不能說什麼都沒有，這樣的話可能會顯示花屏。另外,通常GLSurfaceView會以顯示屏的的刷新頻率來回調onDrawFrame進行渲染，當然我們可以設置GLSurfaceView的渲染模式來修改，調用
glSurfaceView.setRenderMode(GLSurfaceView.RENDERMODE_WHEN_DIRTY);
//GLSurfaceView.RENDERMODE_CONTINUOUSLY:持續渲染(默認)
//GLSurfaceView.RENDERMODE_WHEN_DIRTY:髒渲染,命令渲染；即有兩種渲染模式髒渲染和持續渲染(默認)。使用髒渲染需要和glSurfaceView.requestRender配合使用。
綜上分析，在onSurfaceCreated裏需要設置清屏色，本文爲紅色 glClearColor(1.0f, 0.0f, 0.0f, 0.0f);在onSurfaceChanged裏設置視口viewport大小即通知opengl需要渲染視口大小即surface大小。注意：視口viewport是opengl裏面的概念，surface是GLSurfaceView裏提供給底層渲染顯示的區域，故可以理解爲同一個。在onDrawFrame裏面 glClear(GL_COLOR_BUFFER_BIT);是清楚所有顏色緩衝器，這樣就會顯示清屏色即 glClearColor(1.0f, 0.0f, 0.0f, 0.0f)設置的紅色。
到此，本文章結束。主要講解基礎概念，基礎原理。本文代碼地址:
https://github.com/pangrui201/OpenGlesProject/tree/master/OpenGlesProject_lesson1
