一、创建 eglContext。EGLContext 是一个比较重的对象，所以一般只创建一次。

例子代码代码主要在 EglHelper

egl = EGLContext.getEGL()
获取 eglDisplay = eglGetDisplay(EGL10.EGL_DEFAULT_DISPLAY)
获取 EGLConfig eglChooseConfig(..)
eglInitialize(mEgl, mEglDisplay, mEglConfig, eglContext)
创建 eglContext = eglCreateContext(display, config, eglContext, attrib_list))

大致的相关代码

  public EGLContext start(EGLContext eglContext) {
      if (GLThread.LOG_EGL) {
          Log.w("EglHelper", "start() tid=" + Thread.currentThread().getId());
      }
      /*
       * Get an EGL instance
       */
      mEgl = (EGL10) EGLContext.getEGL();

      /*
       * Get to the default display.
       */
      mEglDisplay = mEgl.eglGetDisplay(EGL10.EGL_DEFAULT_DISPLAY);

      if (mEglDisplay == EGL10.EGL_NO_DISPLAY) {
          throw new RuntimeException("eglGetDisplay failed");
      }

      /*
       * We can now initialize EGL for that display
       */
      int[] version = new int[2];
      if (!mEgl.eglInitialize(mEglDisplay, version)) {
          throw new RuntimeException("eglInitialize failed");
      }
      mEglConfig = eglConfigChooser.chooseConfig(mEgl, mEglDisplay);

          /*
          * Create an EGL context. We want to do this as rarely as we can, because an
          * EGL context is a somewhat heavy object.
          */
      mEglContext = eglContextFactory.createContext(mEgl, mEglDisplay, mEglConfig, eglContext);
      if (mEglContext == null || mEglContext == EGL10.EGL_NO_CONTEXT) {
          mEglContext = null;
          throwEglException("createContext");
      }
      if (GLThread.LOG_EGL) {
          Log.w("EglHelper", "createContext " + mEglContext + " tid=" + Thread.currentThread().getId());
      }

      mEglSurface = null;

      return mEglContext;
  }

二、创建 surface

eglCreateWindowSurface(display, config, nativeWindow, surfaceAttribs)
将 context 和 surface 联系起来。
eglMakeCurrent(eglDisplay, eglSurface, eglSurface, eglContext)
如果需要更换surface，就先destroy现在的
eglDestroySurface

大致的相关代码

  public boolean createSurface(Object surface) {
      if (GLThread.LOG_EGL) {
          Log.w("EglHelper", "createSurface()  tid=" + Thread.currentThread().getId());
      }
      /*
       * Check preconditions.
       */
      if (mEgl == null) {
          throw new RuntimeException("egl not initialized");
      }
      if (mEglDisplay == null) {
          throw new RuntimeException("eglDisplay not initialized");
      }
      if (mEglConfig == null) {
          throw new RuntimeException("mEglConfig not initialized");
      }

      /*
       *  The window size has changed, so we need to create a new
       *  surface.
       */
      destroySurfaceImp();

      /*
       * Create an EGL surface we can render into.
       */
      mEglSurface = eglWindowSurfaceFactory.createWindowSurface(mEgl,
              mEglDisplay, mEglConfig, surface);

      if (mEglSurface == null || mEglSurface == EGL10.EGL_NO_SURFACE) {
          int error = mEgl.eglGetError();
          if (error == EGL10.EGL_BAD_NATIVE_WINDOW) {
              Log.e("EglHelper", "createWindowSurface returned EGL_BAD_NATIVE_WINDOW.");
          }
          return false;
      }

      /*
       * Before we can issue GL commands, we need to make sure
       * the context is current and bound to a surface.
       */
      if (!mEgl.eglMakeCurrent(mEglDisplay, mEglSurface, mEglSurface, mEglContext)) {
          /*
           * Could not make the context current, probably because the underlying
           * SurfaceView surface has been destroyed.
           */
          logEglErrorAsWarning("EGLHelper", "eglMakeCurrent", mEgl.eglGetError());
          return false;
      }

      return true;
  }

三、接下来就是使用vertex数据，texture数据，vertex shader, texture shader进行绘制了，这一部分是可以随时替换数据绘制新的东西的，其中比较耗性能的部分是编译链接shader的部分，所以要对编译链接部分进行缓存以保证性能。

代码主要在GLES20Canvas

vertex 数据

glGenBuffers(...)
glBindBuffer(GLES20.GL_ARRAY_BUFFER, bufferId)
glBufferData(GLES20.GL_ARRAY_BUFFER, buffer.capacity() * elementSize, buffer, GLES20.GL_STATIC_DRAW)

大致的相关代码

  private int uploadBuffer(Buffer buffer, int elementSize) {
      mGLId.glGenBuffers(1, mTempIntArray, 0);
      checkError();
      int bufferId = mTempIntArray[0];
      GLES20.glBindBuffer(GLES20.GL_ARRAY_BUFFER, bufferId);
      checkError();
      GLES20.glBufferData(GLES20.GL_ARRAY_BUFFER, buffer.capacity() * elementSize, buffer,
              GLES20.GL_STATIC_DRAW);
      checkError();
      return bufferId;
  }

编译，链接shader等

以两个 String 对象作为参数，分别是 vertex shader 和 fragment shader

编译
glCreateShader
glShaderSource
glCompileShader

  private static int loadShader(int type, String shaderCode) {
      // create a vertex shader type (GLES20.GL_VERTEX_SHADER)
      // or a fragment shader type (GLES20.GL_FRAGMENT_SHADER)
      int shader = GLES20.glCreateShader(type);

      // add the source code to the shader and compile it
      GLES20.glShaderSource(shader, shaderCode);
      checkError();
      GLES20.glCompileShader(shader);
      checkError();

      return shader;
  }

链接
glCreateProgram
glAttachShader vertex
glAttachShader fragment
glLinkProgram
glGetProgramiv

  private int assembleProgram(int vertexShader, int fragmentShader, ShaderParameter[] params) {
      int program = GLES20.glCreateProgram();
      checkError();
      if (program == 0) {
          throw new RuntimeException("Cannot create GL program: " + GLES20.glGetError());
      }
      GLES20.glAttachShader(program, vertexShader);
      checkError();
      GLES20.glAttachShader(program, fragmentShader);
      checkError();
      GLES20.glLinkProgram(program);
      checkError();
      int[] mLinkStatus = mTempIntArray;
      GLES20.glGetProgramiv(program, GLES20.GL_LINK_STATUS, mLinkStatus, 0);
      if (mLinkStatus[0] != GLES20.GL_TRUE) {
          Log.e(TAG, "Could not link program: ");
          Log.e(TAG, GLES20.glGetProgramInfoLog(program));
          GLES20.glDeleteProgram(program);
          program = 0;
      }
      loadHandles(params, program);
      return program;
  }

设定绘制区域

glViewport(width, height)
此时就可以利用 width 和 height 来对 model view projection 矩阵进行一些初始设置了

    @Override
    public void setSize(int width, int height) {
        mWidth = width;
        mHeight = height;
        GLES20.glViewport(0, 0, mWidth, mHeight);
        checkError();
        Matrix.setIdentityM(mMatrices, mCurrentMatrixIndex);
        Matrix.orthoM(mProjectionMatrix, 0, 0, width, 0, height, -1, 1);
        if (getTargetTexture() == null) {
            mScreenWidth = width;
            mScreenHeight = height;
            Matrix.translateM(mMatrices, mCurrentMatrixIndex, 0, height, 0);
            Matrix.scaleM(mMatrices, mCurrentMatrixIndex, 1, -1, 1);
        }
    }

对model-view矩阵进行处理(也可以不处理，看需求)

可以是平移，旋转，放缩之类的。

传入 texture, 一个带有宽高和图像数据的对象。

glUseProgram(programId) 使用之前编译链接好的的id
将texture数据传入到OpenGL里的gl操作
glActiveTexture(GLES20.GL_TEXTURE0)

    private void prepareTexture(BasicTexture texture, int program, ShaderParameter[] params) {
        GLES20.glUseProgram(program);
        checkError();
        enableBlending(!texture.isOpaque() || getAlpha() < OPAQUE_ALPHA);
        GLES20.glActiveTexture(GLES20.GL_TEXTURE0);
        checkError();
        texture.onBind(this);
        GLES20.glBindTexture(texture.getTarget(), texture.getId());
        checkError();
        GLES20.glUniform1i(params[INDEX_TEXTURE_SAMPLER].handle, 0);
        checkError();
        GLES20.glUniform1f(params[INDEX_ALPHA].handle, getAlpha());
        checkError();
    }

glGenTextures
glBindTexture
glTexParameteri 设定相关的texture params

    @Override
    public void setTextureParameters(BasicTexture texture) {
        int target = texture.getTarget();
        GLES20.glBindTexture(target, texture.getId());
        checkError();
        GLES20.glTexParameteri(target, GLES20.GL_TEXTURE_WRAP_S, GLES20.GL_CLAMP_TO_EDGE);
        GLES20.glTexParameteri(target, GLES20.GL_TEXTURE_WRAP_T, GLES20.GL_CLAMP_TO_EDGE);
        GLES20.glTexParameterf(target, GLES20.GL_TEXTURE_MIN_FILTER, GLES20.GL_LINEAR);
        GLES20.glTexParameterf(target, GLES20.GL_TEXTURE_MAG_FILTER, GLES20.GL_LINEAR);
    }

如果bitmap的宽高是2的倍数
glBindTexture
glTexImage2D

    @Override
    public void initializeTexture(BasicTexture texture, Bitmap bitmap) {
        int target = texture.getTarget();
        GLES20.glBindTexture(target, texture.getId());
        checkError();
        GLUtils.texImage2D(target, 0, bitmap, 0);
    }

如果不是2的倍数
glBindTexture
GLUtils.texSubImage2D

    @Override
    public void texSubImage2D(BasicTexture texture, int xOffset, int yOffset, Bitmap bitmap,
            int format, int type) {
        int target = texture.getTarget();
        GLES20.glBindTexture(target, texture.getId());
        checkError();
        GLUtils.texSubImage2D(target, 0, xOffset, yOffset, bitmap, format, type);
    }

glUniform1i(...) // 对shader里面的textureSampler传值，因为ActiveTexture的是Texture0，所以传0就可以了。

    GLES20.glUniform1i(params[INDEX_TEXTURE_SAMPLER].handle, 0);

将vertex的值传入OpenGL

glBindBuffer
glVertexAttribPointer
glBindBuffer 0 清理

    private void setPosition(ShaderParameter[] params, int offset) {
        GLES20.glBindBuffer(GLES20.GL_ARRAY_BUFFER, mBoxCoordinates);
        checkError();
        GLES20.glVertexAttribPointer(params[INDEX_POSITION].handle, COORDS_PER_VERTEX,
                GLES20.GL_FLOAT, false, VERTEX_STRIDE, offset * VERTEX_STRIDE);
        checkError();
        GLES20.glBindBuffer(GLES20.GL_ARRAY_BUFFER, 0);
        checkError();
    }

texture Matrix

利用texture的宽高设定 textureMatrix。 textureMatrix 是shader 里的一个uniform对象，用来映射从texture对应位置获取的颜色到绘制区域。例如在不进行平移旋转等情况下，第一个像素点就绘制在第一个位置。

        TextureMatrixTransformer.copyTextureCoordinates(...);
        TextureMatrixTransformer.convertCoordinate(...);
        TextureMatrixTransformer.setTextureMatrix(...);

model view projection matrix 将这个matrix传入到OpenGL里

translateM
scaleM
multiplyMM
glUniformMatrix4fv(...)

    private void setMatrix(ShaderParameter[] params, float x, float y, float width, float height) {
        Matrix.translateM(mTempMatrix, 0, mMatrices, mCurrentMatrixIndex, x, y, 0f);
        Matrix.scaleM(mTempMatrix, 0, width, height, 1f);
        Matrix.multiplyMM(mTempMatrix, MATRIX_SIZE, mProjectionMatrix, 0, mTempMatrix, 0);
        GLES20.glUniformMatrix4fv(params[INDEX_MATRIX].handle, 1, false, mTempMatrix, MATRIX_SIZE);
        checkError();
    }

最后就是绘制了

glEnableVertexAttribArray positionHandle
glDrawArrays
glDisableVertexAttribArray positionHandle

    private void draw(ShaderParameter[] params, int type, int count, float x, float y, float width,
            float height) {
        setMatrix(params, x, y, width, height);
        int positionHandle = params[INDEX_POSITION].handle;
        GLES20.glEnableVertexAttribArray(positionHandle);
        checkError();
        GLES20.glDrawArrays(type, 0, count);
        checkError();
        GLES20.glDisableVertexAttribArray(positionHandle);
        checkError();
    }

四、总结

以上，说明了OpenGL绘制的三个流程。由于OpenGL是独立于语言的，这些流程不仅适用于Android，使用其它语言编写也可借鉴。从上面冗长的流程可以看到，每次要进行绘制都写那么多代码实在不经济，那么接下来的文章就会讨论如何封装以上的流程了。其实开头的github仓库里面的代码已经有封装好的类了，有需要的话可以先去看源代码。感谢阅读！