安卓下多线程OpenGL共享Context
Posted 荆棘鸟
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之前的方案假定Java层更新纹理时使用的是RGB或RBGA格式的数据，但是在播放视频这种应用场景下，解码器解码出来的数据如果是YUV格式，渲染起来就比较麻烦了。一种方式是使用CPU进行YUV转RGB，然后再进行渲染，但是这种方式性能极差；另一种方式是使用GPU进行转换，利用GPU的并行计算能力加速转换。我们需要编写Shader来实现。如前文所述，Unity只需要Java层的纹理ID，当使用Shader进行YUV转RGB时，怎么实现更新该纹理的数据呢？答案是Render to Texture (参见[1])。具体做法是，创建一个FrameBuffer，调用glFramebufferTexture2D将纹理与FrameBuffer关联起来，这样在FrameBuffer上进行的绘制，就会被写入到该纹理中。Java代码如下：
public void setupGL(int width, int height) { // 创建纹理 int tempBuffer[] = new int[1]; GLES20.glGenTextures(1, tempBuffer, 0); mTextureId = tempBuffer[0]; if (mTextureId == 0) { glLogE("setupGL, glGenTextures for render texture failed"); return; } GLES20.glBindTexture(GLES20.GL_TEXTURE_2D, mTextureId); GLES20.glTexParameteri(GLES20.GL_TEXTURE_2D, GLES20.GL_TEXTURE_MIN_FILTER, GLES20.GL_LINEAR); GLES20.glTexParameteri(GLES20.GL_TEXTURE_2D, GLES20.GL_TEXTURE_MAG_FILTER, GLES20.GL_LINEAR); GLES20.glTexParameteri(GLES20.GL_TEXTURE_2D, GLES20.GL_TEXTURE_WRAP_S, GLES20.GL_CLAMP_TO_EDGE); GLES20.glTexParameteri(GLES20.GL_TEXTURE_2D, GLES20.GL_TEXTURE_WRAP_T, GLES20.GL_CLAMP_TO_EDGE); GLES20.glTexImage2D(GLES20.GL_TEXTURE_2D, 0, GLES20.GL_RGBA, width, height, 0, GLES20.GL_RGBA, GLES20.GL_UNSIGNED_BYTE, null); GLES20.glBindTexture(GLES20.GL_TEXTURE_2D, 0); // 创建YUV纹理 GLES20.glGenTextures(3, mYuvTextures, 0); if (mYuvTextures[0] == 0 || mYuvTextures[1] == 0 || mYuvTextures[2] == 0) { MyLog.e(TAG, "setupGL, glGenTextures for yuv texture failed"); return; } for (int yuvTexture : mYuvTextures) { GLES20.glBindTexture(GLES20.GL_TEXTURE_2D, yuvTexture); GLES20.glTexParameteri(GLES20.GL_TEXTURE_2D, GLES20.GL_TEXTURE_MIN_FILTER, GLES20.GL_LINEAR); GLES20.glTexParameteri(GLES20.GL_TEXTURE_2D, GLES20.GL_TEXTURE_MAG_FILTER, GLES20.GL_LINEAR); GLES20.glTexParameteri(GLES20.GL_TEXTURE_2D, GLES20.GL_TEXTURE_WRAP_S, GLES20.GL_CLAMP_TO_EDGE); GLES20.glTexParameteri(GLES20.GL_TEXTURE_2D, GLES20.GL_TEXTURE_WRAP_T, GLES20.GL_CLAMP_TO_EDGE); } GLES20.glBindTexture(GLES20.GL_TEXTURE_2D, 0); // 创建帧缓冲区 GLES20.glGenFramebuffers(1, tempBuffer, 0); mFramebuffer = tempBuffer[0]; if (mFramebuffer == 0) { glLogE("setupGL, glGenFramebuffers failed"); return; } GLES20.glBindFramebuffer(GLES20.GL_FRAMEBUFFER, mFramebuffer); GLES20.glFramebufferTexture2D(GLES20.GL_FRAMEBUFFER, GLES20.GL_COLOR_ATTACHMENT0, GLES20.GL_TEXTURE_2D, mTextureId, 0); int errCode = GLES20.glCheckFramebufferStatus(GLES20.GL_FRAMEBUFFER); if (errCode != GLES20.GL_FRAMEBUFFER_COMPLETE) { glLogE("setupGL, glCheckFramebufferStatus failed, errCode=0x" + Integer.toHexString(errCode)); return; } GLES20.glBindFramebuffer(GLES20.GL_FRAMEBUFFER, 0); }
绘制时，先绑定FrameBuffer，再进行绘制操作，Java代码如下：
public void updateTexture() { GLES20.glBindFramebuffer(GLES20.GL_FRAMEBUFFER, mFramebuffer); GLES20.glViewport(0, 0, mWidth, mHeight); GLES20.glClearColor(0, 0, 0, 1); GLES20.glClear(GLES20.GL_COLOR_BUFFER_BIT); // 此处添加绘制操作 GLES20.glBindFramebuffer(GLES20.GL_FRAMEBUFFER, 0); }
为了实现YUV数据的渲染，需要编写Shader。此处，我们渲染的YUV数据格式为YUV420 (YUV420格式的具体介绍，请读者自行百度)，Y、U、V通道数据分别存放在三个缓冲区中。将YUV数据分别赋给三个纹理，然后指定Shader的顶点坐标和纹理坐标，绘制一个矩形即可 (参见[2])。
首先，在setupGL函数中为YUV生成三个纹理，Java代码如下：
// 创建YUV纹理 GLES20.glGenTextures(3, mYuvTextures, 0); if (mYuvTextures[0] == 0 || mYuvTextures[1] == 0 || mYuvTextures[2] == 0) { MyLog.e(TAG, "setupGL, glGenTextures for yuv texture failed"); return; } for (int yuvTexture : mYuvTextures) { GLES20.glBindTexture(GLES20.GL_TEXTURE_2D, yuvTexture); GLES20.glTexParameteri(GLES20.GL_TEXTURE_2D, GLES20.GL_TEXTURE_MIN_FILTER, GLES20.GL_LINEAR); GLES20.glTexParameteri(GLES20.GL_TEXTURE_2D, GLES20.GL_TEXTURE_MAG_FILTER, GLES20.GL_LINEAR); GLES20.glTexParameteri(GLES20.GL_TEXTURE_2D, GLES20.GL_TEXTURE_WRAP_S, GLES20.GL_CLAMP_TO_EDGE); GLES20.glTexParameteri(GLES20.GL_TEXTURE_2D, GLES20.GL_TEXTURE_WRAP_T, GLES20.GL_CLAMP_TO_EDGE); } GLES20.glBindTexture(GLES20.GL_TEXTURE_2D, 0);
绘制时，将YUV数据分别赋给三个纹理，并将三个纹理分别与GLES20.GL_TEXTURE0，GLES20.GL_TEXTURE1，GLES20.GL_TEXTURE2绑定，Java代码如下：
for (int i = 0; i < 3; ++i) { GLES20.glActiveTexture(GLES20.GL_TEXTURE0 + i); GLES20.glBindTexture(GLES20.GL_TEXTURE_2D, mYuvTextures[i]); int w = i == 0 ? yuvFrame.yuvStrides[0] : (yuvFrame.yuvStrides[0] / 2); int h = i == 0 ? yuvFrame.height : (yuvFrame.height / 2); GLES20.glTexImage2D(GLES20.GL_TEXTURE_2D, 0, GLES20.GL_LUMINANCE, w, h, 0, GLES20.GL_LUMINANCE, GLES20.GL_UNSIGNED_BYTE, yuvFrame.yuvPlanes[i]); }
然后，我们需要创建Program，为Program创建Vertex Shader和Fragment Shader (参见[2])。Java代码如下：
public static final String VERTEX_SHADER_STRING = "attribute vec4 in_pos;\\n" + "attribute vec2 in_tc;\\n" + "varying vec2 out_tc;\\n" + "void main() {\\n" + " gl_Position = in_pos;\\n" + " out_tc = in_tc;\\n" + "}\\n"; public static final String FRAGMENT_SHADER_STRING = "precision mediump float;\\n" + "uniform sampler2D tex_y;\\n" + "uniform sampler2D tex_u;\\n" + "uniform sampler2D tex_v;\\n" + "varying vec2 out_tc;\\n" + "void main() {\\n" + " vec4 c = vec4((texture2D(tex_y, out_tc).r - 16./255.) * 1.164);\\n" + " vec4 U = vec4(texture2D(tex_u, out_tc).r - 128./255.);\\n" + " vec4 V = vec4(texture2D(tex_v, out_tc).r - 128./255.);\\n" + " c += V * vec4(1.596, -0.813, 0, 0);\\n" + " c += U * vec4(0, -0.392, 2.017, 0);\\n" + " c.a = 1.0;\\n" + " gl_FragColor = c;\\n" + "}\\n"; protected final void addShaderTo(int type, String source, int program) throws RuntimeException { int shader = GLES20.glCreateShader(type); if (shader == 0) { throw new RuntimeException("Create shader failed, err=" + GLES10.glGetError()); } GLES20.glShaderSource(shader, source); GLES20.glCompileShader(shader); int[] result = new int[]{GLES20.GL_FALSE}; GLES20.glGetShaderiv(shader, GLES20.GL_COMPILE_STATUS, result, 0); if (result[0] != GLES20.GL_TRUE) { GLES20.glDeleteShader(shader); throw new RuntimeException("Compile shader failed, err=" + GLES10.glGetError()); } GLES20.glAttachShader(program, shader); GLES20.glDeleteShader(shader); } public void setupGL(int width, int height) { ... // 创建Program mProgram = GLES20.glCreateProgram(); addShaderTo(GLES20.GL_VERTEX_SHADER, EglRender.VERTEX_SHADER_STRING, mProgram); addShaderTo(GLES20.GL_FRAGMENT_SHADER, EglRender.FRAGMENT_SHADER_STRING, mProgram); GLES20.glLinkProgram(mProgram); GLES20.glGetProgramiv(mProgram, GLES20.GL_LINK_STATUS, tempBuffer, 0); if (tempBuffer[0] != GLES20.GL_TRUE) { glLogE("setupGL, create program failed"); return; } GLES20.glUseProgram(mProgram); int y_tex = GLES20.glGetUniformLocation(mProgram, "tex_y"); GLES20.glUniform1i(y_tex, 0); int u_tex = GLES20.glGetUniformLocation(mProgram, "tex_u"); GLES20.glUniform1i(u_tex, 1); int v_tex = GLES20.glGetUniformLocation(mProgram, "tex_v"); GLES20.glUniform1i(v_tex, 2); mVertexLocation = GLES20.glGetAttribLocation(mProgram, "in_pos"); mTextureLocation = GLES20.glGetAttribLocation(mProgram, "in_tc"); GLES20.glUseProgram(0); }
完整绘制代码如下：
1 public void updateTexture() { 2 GLES20.glBindFramebuffer(GLES20.GL_FRAMEBUFFER, mFramebuffer); 3 4 GLES20.glViewport(0, 0, mWidth, mHeight); 5 GLES20.glClearColor(0, 0, 0, 1); 6 GLES20.glClear(GLES20.GL_COLOR_BUFFER_BIT); 7 GLES20.glDisable(GLES20.GL_CULL_FACE); 8 GLES20.glDisable(GLES20.GL_DEPTH_TEST); 9 GLES20.glBindBuffer(GLES20.GL_ARRAY_BUFFER, 0); 10 GLES20.glBindBuffer(GLES20.GL_ELEMENT_ARRAY_BUFFER, 0); 11 12 GLES20.glUseProgram(mProgram); 13 // 更新YUV数据 14 for (int i = 0; i < 3; ++i) { 15 GLES20.glActiveTexture(GLES20.GL_TEXTURE0 + i); 16 GLES20.glBindTexture(GLES20.GL_TEXTURE_2D, mYuvTextures[i]); 17 int w = i == 0 ? yuvFrame.yuvStrides[0] : (yuvFrame.yuvStrides[0] / 2); 18 int h = i == 0 ? yuvFrame.height : (yuvFrame.height / 2); 19 GLES20.glTexImage2D(GLES20.GL_TEXTURE_2D, 0, GLES20.GL_LUMINANCE, w, h, 0, 20 GLES20.GL_LUMINANCE, GLES20.GL_UNSIGNED_BYTE, yuvFrame.yuvPlanes[i]); 21 } 22 // 设置顶点坐标和纹理坐标 23 GLES20.glEnableVertexAttribArray(mVertexLocation); 24 mVertexCoord.position(0); 25 GLES20.glVertexAttribPointer(mVertexLocation, 2, GLES20.GL_FLOAT, false, 0, mVertexCoord); 26 GLES20.glEnableVertexAttribArray(mTextureLocation); 27 mTextureCoord.position(0); 28 GLES20.glVertexAttribPointer(mTextureLocation, 2, GLES20.GL_FLOAT, false, 0, mTextureCoord); 29 // 绘制矩形 30 GLES20.glDrawArrays(GLES20.GL_TRIANGLE_STRIP, 0, 4); 31 32 GLES20.glDisableVertexAttribArray(mVertexLocation); 33 GLES20.glDisableVertexAttribArray(mTextureLocation); 34 for (int i = 0; i < 3; ++i) { 35 GLES20.glActiveTexture(GLES20.GL_TEXTURE0 + i); 36 GLES20.glBindTexture(GLES20.GL_TEXTURE_2D, 0); 37 } 38 39 GLES20.glUseProgram(0); 40 GLES20.glBindFramebuffer(GLES20.GL_FRAMEBUFFER, 0); 41 GLES20.glClear(0); 42 }
其中，9～10行是必需的，否则，glDrawArrays调用会失败，报GL_INVALID_OPERATION错误 (参见[3])。当绘制完成之后，需要通知Unity3D，在C#中调用GL.InvalidateState，否则会影响Unity的绘制 (参见[4])。C#代码如下：
void Update () { mPluginTexture.Call ("updateTexture"); GL.InvalidateState (); }
其中，Update函数是Unity在每次刷新帧时回调，我们在该回调中调用Java层的updateTexture函数更新纹理数据，然后调用GL.InvalidateState，通知Unity重置OpenGL状态。笔者一开始采用当Java解码出一帧时，便更新纹理数据进行绘制，然后通知C#调用GL.InvalidateState。但是这种方式存在两个问题，一是当App退到后台，绘制操作仍会进行，只是绘制会失败；二是画面更新一段时间之后，便会卡住，过很久才会恢复。具体原因并未查出。后来，改用从C#的Update回调更新纹理，这两个问题得以解决。
总结：
Unity官方给出的Plugin绘制方式是通过在C#层调用GL.IssuePluginEvent，C++层接收从Unity Render线程过来的回调，在该回调中更新纹理 (参见[5])。该方案需要编写JNI和C++，实现起来比较麻烦。本文给出的方案全部在Java层即可实现。供感兴趣的读者参考。
[参考文献]
[1] Tutorial 14 : Render To Texture
[2] 最简单的视音频播放示例6：OpenGL播放YUV420P（通过Texture，使用Shader）
[5] Low-level Native Plugin Interface
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