NDK OpenGLES3.0 开发(五):FBO 离屏渲染
什么是 FBO
FBO(Frame Buffer Object)即帧缓冲区对象,实际上是一个可添加缓冲区的容器,可以为其添加纹理或渲染缓冲区对象(RBO)。
FBO 本身不能用于渲染,只有添加了纹理或者渲染缓冲区之后才能作为渲染目标,它仅且提供了 3 个附着(Attachment),分别是颜色附着、深度附着和模板附着。
RBO(Render Buffer Object)即渲染缓冲区对象,是一个由应用程序分配的 2D 图像缓冲区。渲染缓冲区可以用于分配和存储颜色、深度或者模板值,可以用作 FBO 中的颜色、深度或者模板附着。
使用 FBO 作为渲染目标时,首先需要为 FBO 的附着添加连接对象,如颜色附着需要连接纹理或者渲染缓冲区对象的颜色缓冲区。
为什么用 FBO
默认情况下,OpenGL ES 通过绘制到窗口系统提供的帧缓冲区,然后将帧缓冲区的对应区域复制到纹理来实现渲染到纹理,但是此方法只有在纹理尺寸小于或等于帧缓冲区尺寸才有效。
另一种方式是通过使用连接到纹理的 pbuffer 来实现渲染到纹理,但是与上下文和窗口系统提供的可绘制表面切换开销也很大。因此,引入了帧缓冲区对象 FBO 来解决这个问题。
NDK OpenGLES 开发中,一般使用 GLSurfaceView 将绘制结果显示到屏幕上,然而在实际应用中,也有许多场景不需要渲染到屏幕上,如利用 GPU 在后台完成一些图像转换、缩放等耗时操作,这个时候利用 FBO 可以方便实现类似需求。
使用 FBO 可以让渲染操作不用再渲染到屏幕上,而是渲染到离屏 Buffer 中,然后可以使用 glReadPixels 或者 HardwareBuffer 将渲染后的图像数据读出来,从而实现在后台利用 GPU 完成对图像的处理。
怎么用 FBO
创建并初始化 FBO 的步骤:
// 创建一个 2D 纹理用于连接 FBO 的颜色附着
glGenTextures(1, &m_FboTextureId);
glBindTexture(GL_TEXTURE_2D, m_FboTextureId);
glTexParameterf(GL_TEXTURE_2D, GL_TEXTURE_WRAP_S, GL_CLAMP_TO_EDGE);
glTexParameterf(GL_TEXTURE_2D, GL_TEXTURE_WRAP_T, GL_CLAMP_TO_EDGE);
glTexParameteri(GL_TEXTURE_2D, GL_TEXTURE_MIN_FILTER, GL_LINEAR);
glTexParameteri(GL_TEXTURE_2D, GL_TEXTURE_MAG_FILTER, GL_LINEAR);
glBindTexture(GL_TEXTURE_2D, GL_NONE);
// 创建 FBO
glGenFramebuffers(1, &m_FboId);
// 绑定 FBO
glBindFramebuffer(GL_FRAMEBUFFER, m_FboId);
// 绑定 FBO 纹理
glBindTexture(GL_TEXTURE_2D, m_FboTextureId);
// 将纹理连接到 FBO 附着
glFramebufferTexture2D(GL_FRAMEBUFFER, GL_COLOR_ATTACHMENT0, GL_TEXTURE_2D, m_FboTextureId, 0);
// 分配内存大小
glTexImage2D(GL_TEXTURE_2D, 0, GL_RGBA, m_RenderImage.width, m_RenderImage.height, 0, GL_RGBA, GL_UNSIGNED_BYTE, nullptr);
// 检查 FBO 的完整性状态
if (glCheckFramebufferStatus(GL_FRAMEBUFFER)!= GL_FRAMEBUFFER_COMPLETE) {
LOGCATE("FBOSample::CreateFrameBufferObj glCheckFramebufferStatus status != GL_FRAMEBUFFER_COMPLETE");
return false;
}
// 解绑纹理
glBindTexture(GL_TEXTURE_2D, GL_NONE);
// 解绑 FBO
glBindFramebuffer(GL_FRAMEBUFFER, GL_NONE);
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使用 FBO 的一般步骤:
// 绑定 FBO
glBindFramebuffer(GL_FRAMEBUFFER, m_FboId);
// 选定离屏渲染的 Program,绑定 VAO 和图像纹理,进行绘制(离屏渲染)
// m_ImageTextureId 为另外一个用于纹理映射的图片纹理
glUseProgram(m_FboProgramObj);
glBindVertexArray(m_VaoIds[1]);
glActiveTexture(GL_TEXTURE0);
// 绑定图像纹理
glBindTexture(GL_TEXTURE_2D, m_ImageTextureId);
glUniform1i(m_FboSamplerLoc, 0);
glDrawElements(GL_TRIANGLES, 6, GL_UNSIGNED_SHORT, (const void *)0);
glBindVertexArray(0);
glBindTexture(GL_TEXTURE_2D, 0);
// 解绑 FBO
glBindFramebuffer(GL_FRAMEBUFFER, 0);
// 完成离屏渲染后,结果图数据便保存在我们之前连接到 FBO 的纹理 m_FboTextureId 。
// 我们再拿 FBO 纹理 m_FboTextureId 做一次普通渲染便可将之前离屏渲染的结果绘制到屏幕上。
// 这里我们编译连接了 2 个 program ,一个用作离屏渲染的 m_FboProgramObj,一个用于普通渲染的 m_ProgramObj
//选定另外一个着色器程序,以 m_FboTextureId 纹理作为输入进行普通渲染
glUseProgram(m_ProgramObj);
glBindVertexArray(m_VaoIds[0]);
glActiveTexture(GL_TEXTURE0);
//绑定 FBO 纹理
glBindTexture(GL_TEXTURE_2D, m_FboTextureId);
glUniform1i(m_SamplerLoc, 0);
glDrawElements(GL_TRIANGLES, 6, GL_UNSIGNED_SHORT, (const void *)0);
glBindTexture(GL_TEXTURE_2D, GL_NONE);
glBindVertexArray(GL_NONE);
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示例:
创建并初始化 FBO
bool FBOSample::CreateFrameBufferObj()
{
// 创建并初始化 FBO 纹理
glGenTextures(1, &m_FboTextureId);
glBindTexture(GL_TEXTURE_2D, m_FboTextureId);
glTexParameterf(GL_TEXTURE_2D, GL_TEXTURE_WRAP_S, GL_CLAMP_TO_EDGE);
glTexParameterf(GL_TEXTURE_2D, GL_TEXTURE_WRAP_T, GL_CLAMP_TO_EDGE);
glTexParameteri(GL_TEXTURE_2D, GL_TEXTURE_MIN_FILTER, GL_LINEAR);
glTexParameteri(GL_TEXTURE_2D, GL_TEXTURE_MAG_FILTER, GL_LINEAR);
glBindTexture(GL_TEXTURE_2D, GL_NONE);
// 创建并初始化 FBO
glGenFramebuffers(1, &m_FboId);
glBindFramebuffer(GL_FRAMEBUFFER, m_FboId);
glBindTexture(GL_TEXTURE_2D, m_FboTextureId);
glFramebufferTexture2D(GL_FRAMEBUFFER, GL_COLOR_ATTACHMENT0, GL_TEXTURE_2D, m_FboTextureId, 0);
glTexImage2D(GL_TEXTURE_2D, 0, GL_RGBA, m_RenderImage.width, m_RenderImage.height, 0, GL_RGBA, GL_UNSIGNED_BYTE, nullptr);
if (glCheckFramebufferStatus(GL_FRAMEBUFFER)!= GL_FRAMEBUFFER_COMPLETE) {
LOGCATE("FBOSample::CreateFrameBufferObj glCheckFramebufferStatus status != GL_FRAMEBUFFER_COMPLETE");
return false;
}
glBindTexture(GL_TEXTURE_2D, GL_NONE);
glBindFramebuffer(GL_FRAMEBUFFER, GL_NONE);
return true;
}
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编译链接 2 个着色器程序,创建 VAO、VBO 和图像纹理
void FBOSample::Init()
{
//顶点坐标
GLfloat vVertices[] = {
-1.0f, -1.0f, 0.0f,
1.0f, -1.0f, 0.0f,
-1.0f, 1.0f, 0.0f,
1.0f, 1.0f, 0.0f,
};
//正常纹理坐标
GLfloat vTexCoors[] = {
0.0f, 1.0f,
1.0f, 1.0f,
0.0f, 0.0f,
1.0f, 0.0f,
};
//fbo 纹理坐标与正常纹理方向不同,原点位于左下角
GLfloat vFboTexCoors[] = {
0.0f, 0.0f,
1.0f, 0.0f,
0.0f, 1.0f,
1.0f, 1.0f,
};
GLushort indices[] = { 0, 1, 2, 1, 3, 2 };
char vShaderStr[] =
"#version 300 es \n"
"layout(location = 0) in vec4 a_position; \n"
"layout(location = 1) in vec2 a_texCoord; \n"
"out vec2 v_texCoord; \n"
"void main() \n"
"{ \n"
" gl_Position = a_position; \n"
" v_texCoord = a_texCoord; \n"
"} \n";
// 用于普通渲染的片段着色器脚本,简单纹理映射
char fShaderStr[] =
"#version 300 es\n"
"precision mediump float;\n"
"in vec2 v_texCoord;\n"
"layout(location = 0) out vec4 outColor;\n"
"uniform sampler2D s_TextureMap;\n"
"void main()\n"
"{\n"
" outColor = texture(s_TextureMap, v_texCoord);\n"
"}";
// 用于离屏渲染的片段着色器脚本,取每个像素的灰度值
char fFboShaderStr[] =
"#version 300 es\n"
"precision mediump float;\n"
"in vec2 v_texCoord;\n"
"layout(location = 0) out vec4 outColor;\n"
"uniform sampler2D s_TextureMap;\n"
"void main()\n"
"{\n"
" vec4 tempColor = texture(s_TextureMap, v_texCoord);\n"
" float luminance = tempColor.r * 0.299 + tempColor.g * 0.587 + tempColor.b * 0.114;\n"
" outColor = vec4(vec3(luminance), tempColor.a);\n"
"}"; // 输出灰度图
// 编译链接用于普通渲染的着色器程序
m_ProgramObj = GLUtils::CreateProgram(vShaderStr, fShaderStr, m_VertexShader, m_FragmentShader);
// 编译链接用于离屏渲染的着色器程序
m_FboProgramObj = GLUtils::CreateProgram(vShaderStr, fFboShaderStr, m_FboVertexShader, m_FboFragmentShader);
if (m_ProgramObj == GL_NONE || m_FboProgramObj == GL_NONE)
{
LOGCATE("FBOSample::Init m_ProgramObj == GL_NONE");
return;
}
m_SamplerLoc = glGetUniformLocation(m_ProgramObj, "s_TextureMap");
m_FboSamplerLoc = glGetUniformLocation(m_FboProgramObj, "s_TextureMap");
// 生成 VBO ,加载顶点数据和索引数据
// Generate VBO Ids and load the VBOs with data
glGenBuffers(4, m_VboIds);
glBindBuffer(GL_ARRAY_BUFFER, m_VboIds[0]);
glBufferData(GL_ARRAY_BUFFER, sizeof(vVertices), vVertices, GL_STATIC_DRAW);
glBindBuffer(GL_ARRAY_BUFFER, m_VboIds[1]);
glBufferData(GL_ARRAY_BUFFER, sizeof(vTexCoors), vTexCoors, GL_STATIC_DRAW);
glBindBuffer(GL_ARRAY_BUFFER, m_VboIds[2]);
glBufferData(GL_ARRAY_BUFFER, sizeof(vFboTexCoors), vFboTexCoors, GL_STATIC_DRAW);
glBindBuffer(GL_ELEMENT_ARRAY_BUFFER, m_VboIds[3]);
glBufferData(GL_ELEMENT_ARRAY_BUFFER, sizeof(indices), indices, GL_STATIC_DRAW);
GO_CHECK_GL_ERROR();
// 生成 2 个 VAO,一个用于普通渲染,另一个用于离屏渲染
// Generate VAO Ids
glGenVertexArrays(2, m_VaoIds);
// 初始化用于普通渲染的 VAO
// Normal rendering VAO
glBindVertexArray(m_VaoIds[0]);
glBindBuffer(GL_ARRAY_BUFFER, m_VboIds[0]);
glEnableVertexAttribArray(VERTEX_POS_INDX);
glVertexAttribPointer(VERTEX_POS_INDX, 3, GL_FLOAT, GL_FALSE, 3 * sizeof(GLfloat), (const void *)0);
glBindBuffer(GL_ARRAY_BUFFER, GL_NONE);
glBindBuffer(GL_ARRAY_BUFFER, m_VboIds[1]);
glEnableVertexAttribArray(TEXTURE_POS_INDX);
glVertexAttribPointer(TEXTURE_POS_INDX, 2, GL_FLOAT, GL_FALSE, 2 * sizeof(GLfloat), (const void *)0);
glBindBuffer(GL_ARRAY_BUFFER, GL_NONE);
glBindBuffer(GL_ELEMENT_ARRAY_BUFFER, m_VboIds[3]);
GO_CHECK_GL_ERROR();
glBindVertexArray(GL_NONE);
// 初始化用于离屏渲染的 VAO
// FBO off screen rendering VAO
glBindVertexArray(m_VaoIds[1]);
glBindBuffer(GL_ARRAY_BUFFER, m_VboIds[0]);
glEnableVertexAttribArray(VERTEX_POS_INDX);
glVertexAttribPointer(VERTEX_POS_INDX, 3, GL_FLOAT, GL_FALSE, 3 * sizeof(GLfloat), (const void *)0);
glBindBuffer(GL_ARRAY_BUFFER, GL_NONE);
glBindBuffer(GL_ARRAY_BUFFER, m_VboIds[2]);
glEnableVertexAttribArray(TEXTURE_POS_INDX);
glVertexAttribPointer(TEXTURE_POS_INDX, 2, GL_FLOAT, GL_FALSE, 2 * sizeof(GLfloat), (const void *)0);
glBindBuffer(GL_ARRAY_BUFFER, GL_NONE);
glBindBuffer(GL_ELEMENT_ARRAY_BUFFER, m_VboIds[3]);
GO_CHECK_GL_ERROR();
glBindVertexArray(GL_NONE);
// 创建并初始化图像纹理
glGenTextures(1, &m_ImageTextureId);
glBindTexture(GL_TEXTURE_2D, m_ImageTextureId);
glTexParameterf(GL_TEXTURE_2D, GL_TEXTURE_WRAP_S, GL_CLAMP_TO_EDGE);
glTexParameterf(GL_TEXTURE_2D, GL_TEXTURE_WRAP_T, GL_CLAMP_TO_EDGE);
glTexParameteri(GL_TEXTURE_2D, GL_TEXTURE_MIN_FILTER, GL_LINEAR);
glTexParameteri(GL_TEXTURE_2D, GL_TEXTURE_MAG_FILTER, GL_LINEAR);
glTexImage2D(GL_TEXTURE_2D, 0, GL_RGBA, m_RenderImage.width, m_RenderImage.height, 0, GL_RGBA, GL_UNSIGNED_BYTE, m_RenderImage.ppPlane[0]);
glBindTexture(GL_TEXTURE_2D, GL_NONE);
GO_CHECK_GL_ERROR();
if (!CreateFrameBufferObj())
{
LOGCATE("FBOSample::Init CreateFrameBufferObj fail");
return;
}
}
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离屏渲染和普通渲染
void FBOSample::Draw(int screenW, int screenH)
{
// 离屏渲染
glPixelStorei(GL_UNPACK_ALIGNMENT,1);
glViewport(0, 0, m_RenderImage.width, m_RenderImage.height);
// Do FBO off screen rendering
glBindFramebuffer(GL_FRAMEBUFFER, m_FboId);
glUseProgram(m_FboProgramObj);
glBindVertexArray(m_VaoIds[1]);
glActiveTexture(GL_TEXTURE0);
glBindTexture(GL_TEXTURE_2D, m_ImageTextureId);
glUniform1i(m_FboSamplerLoc, 0);
GO_CHECK_GL_ERROR();
glDrawElements(GL_TRIANGLES, 6, GL_UNSIGNED_SHORT, (const void *)0);
GO_CHECK_GL_ERROR();
glBindVertexArray(0);
glBindTexture(GL_TEXTURE_2D, 0);
glBindFramebuffer(GL_FRAMEBUFFER, 0);
// 普通渲染
// Do normal rendering
glViewport(0, 0, screenW, screenH);
glUseProgram(m_ProgramObj);
GO_CHECK_GL_ERROR();
glBindVertexArray(m_VaoIds[0]);(http://www.amjmh.com)
glActiveTexture(GL_TEXTURE0);
glBindTexture(GL_TEXTURE_2D, m_FboTextureId);
glUniform1i(m_SamplerLoc, 0);
GO_CHECK_GL_ERROR();
glDrawElements(GL_TRIANGLES, 6, GL_UNSIGNED_SHORT, (const void *)0);
GO_CHECK_GL_ERROR();
glBindTexture(GL_TEXTURE_2D, GL_NONE);
glBindVertexArray(GL_NONE);
}