OpenGL进阶之构建句柄

先说明一下Opengl中的资源一般都是用一个句柄（handle）来引用，句柄一般由gl***接口返回，代表一个特定的资源。一个完整的Opengl程序需要创建定点着色器和片段着色器并将他们Link起来组成一个完整的OpenGL程序。

和创建Shader相关的：glCreateShader -> glShaderSource -> glCompileShader , 通过这几个接口的调用最终返回一个Shader的句柄。

/**
 * Helper function to compile a shader.
 *
 * @param shaderType   The shader type.
 * @param shaderSource The shader source code.
 * @return An OpenGL handle to the shader.
 */
public static int compileShader(final int shaderType, final String shaderSource) {
  int shaderHandle = GLES20.glCreateShader(shaderType);

  if (shaderHandle != 0) {
    // Pass in the shader source.
    GLES20.glShaderSource(shaderHandle, shaderSource);

    // Compile the shader.
    GLES20.glCompileShader(shaderHandle);

    // Get the compilation status.
    final int[] compileStatus = new int[1];
    GLES20.glGetShaderiv(shaderHandle, GLES20.GL_COMPILE_STATUS, compileStatus, 0);

    // If the compilation failed, delete the shader.
    if (compileStatus[0] == 0) {
      Log.e(TAG, "Error compiling shader: " + GLES20.glGetShaderInfoLog(shaderHandle));
      GLES20.glDeleteShader(shaderHandle);
      shaderHandle = 0;
    }
  }

  if (shaderHandle == 0) {
    throw new RuntimeException("Error creating shader.");
  }

  return shaderHandle;
}

和创建Shader程序相关的（每个shader程序都必须包含Vertex Shader 和 Fragment Shader两部分）：glCreateProgram -> glAttachShader -> glLinkProgram，通过这些接口的调用最终返回个Program的句柄。

/**
 * Helper function to compile and link a program.
 *
 * @param vertexShaderHandle   An OpenGL handle to an already-compiled vertex shader.
 * @param fragmentShaderHandle An OpenGL handle to an already-compiled fragment shader.
 * @param attributes           Attributes that need to be bound to the program.
 * @return An OpenGL handle to the program.
 */
public static int createAndLinkProgram(
    final int vertexShaderHandle,
    final int fragmentShaderHandle,
    final String[] attributes) {

  int programHandle = GLES20.glCreateProgram();

  if (programHandle != 0) {
    // Bind the vertex shader to the program.
    GLES20.glAttachShader(programHandle, vertexShaderHandle);

    // Bind the fragment shader to the program.
    GLES20.glAttachShader(programHandle, fragmentShaderHandle);

    // Bind attributes
    if (attributes != null) {
      final int size = attributes.length;
      for (int i = 0; i < size; i++) {
        GLES20.glBindAttribLocation(programHandle, i, attributes[i]);
      }
    }

    // Link the two shaders together into a program.
    GLES20.glLinkProgram(programHandle);

    // Get the link status.
    final int[] linkStatus = new int[1];
    GLES20.glGetProgramiv(programHandle, GLES20.GL_LINK_STATUS, linkStatus, 0);

    // If the link failed, delete the program.
    if (linkStatus[0] == 0) {
      Log.e(TAG, "Error compiling program: " + GLES20.glGetProgramInfoLog(programHandle));
      GLES20.glDeleteProgram(programHandle);
      programHandle = 0;
    }
  }

  if (programHandle == 0) {
    throw new RuntimeException("Error creating program.");
  }

  return programHandle;
}

获取Shader内部变量赋值和传递数据到gl

• mProgram，代表glCreateProgram返回的 Shader程序句柄

• “a_texcoord”和"tex_sampler"是 Vertex Shader和 Fragment Shader中定义的变量

• 对于不同类型的数据使用不同的接口获取句柄，便于向这些变量中传入值：

  GLES20.glGetUniformLocation(mProgram, "tex_sampler")

  GLES20.glGetAttribLocation(mProgram, "a_texcoord")

• glVertexAttribPointer() 获得变量的句柄之后就要向其中传值。

• 传值之后需要使glEnableVertexAttribArray才能将数据传入到gl中，传入数据使用glDrawArrays()。

aPositionLoc = GLES20.glGetAttribLocation(mProgramHandle, "aPosition");

GLES20.glEnableVertexAttribArray(aPositionLoc);

GLES20.glVertexAttribPointer(aPositionLoc, coordsPerVertex, GLES20.GL_FLOAT, false, vertexStride, vertexBuffer);

GLES20.glDrawArrays(GLES20.GL_TRIANGLE_STRIP, firstVertex, vertexCount);

获取GL错误

public static void checkGlError(String op) {
    int error;
    while ((error = GLES20.glGetError()) != GLES20.GL_NO_ERROR) {
          Log.e("ES20_ERROR", op + ": glError " + error);
          if (DEBUG) {
              throw new RuntimeException(op + ": glError " + error);
          }
    }
}