egl离屏渲染中的pbuffer vs fbo
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【中文标题】egl离屏渲染中的pbuffer vs fbo【英文标题】：pbuffer vs fbo in egl offscreen rendering 【发布时间】：2015-03-02 19:20:18 【问题描述】：我对 egl pbuffer 表面感到非常困惑。在我看来，pbuffer 表面是一个独立于平台的表面，就像 windows 表面或像素图表面一样。绘制到该表面的东西虽然不可见，但应该能够被读回。
这个问题的答案似乎证实了我的理解：
Difference from eglCreatePbufferSurface and eglCreatePixmapSurface with OpenGL ES(EGL)
但是，我的实验表明，除了使用 pbuffer 表面之外，我还需要创建一个 fbo 缓冲区。
这段代码似乎对我有用，它创建了一个 pbuffer 表面，然后是一个 fbo。
#include <GLES2/gl2.h>
#include <EGL/egl.h>
int main(int argc, char *argv[])
EGLint ai32ContextAttribs[] = EGL_CONTEXT_CLIENT_VERSION, 2,
EGL_NONE ;
// Step 1 - Get the default display.
EGLDisplay eglDisplay = eglGetDisplay((EGLNativeDisplayType)0);
// Step 2 - Initialize EGL.
eglInitialize(eglDisplay, 0, 0);
// Step 3 - Make OpenGL ES the current API.
eglBindAPI(EGL_OPENGL_ES_API);
// Step 4 - Specify the required configuration attributes.
EGLint pi32ConfigAttribs[5];
pi32ConfigAttribs[0] = EGL_SURFACE_TYPE;
pi32ConfigAttribs[1] = EGL_WINDOW_BIT;
pi32ConfigAttribs[2] = EGL_RENDERABLE_TYPE;
pi32ConfigAttribs[3] = EGL_OPENGL_ES2_BIT;
pi32ConfigAttribs[4] = EGL_NONE;
// Step 5 - Find a config that matches all requirements.
int iConfigs;
EGLConfig eglConfig;
eglChooseConfig(eglDisplay, pi32ConfigAttribs, &eglConfig, 1, &iConfigs);
if (iConfigs != 1)
printf("Error: eglChooseConfig(): config not found.\n");
exit(-1);
// Step 6 - Create a surface to draw to.
EGLSurface eglSurface;
eglSurface = eglCreatePbufferSurface(eglDisplay, eglConfig, NULL);
// Step 7 - Create a context.
EGLContext eglContext;
eglContext = eglCreateContext(eglDisplay, eglConfig, NULL, ai32ContextAttribs);
// Step 8 - Bind the context to the current thread
eglMakeCurrent(eglDisplay, eglSurface, eglSurface, eglContext);
GLuint fboId = 0;
GLuint renderBufferWidth = 1920;
GLuint renderBufferHeight = 1080;
// Step 9 - create a framebuffer object
glGenFramebuffers(1, &fboId);
glBindFramebuffer(GL_FRAMEBUFFER, fboId);
GLuint renderBuffer;
glGenRenderbuffers(1, &renderBuffer);
glBindRenderbuffer(GL_RENDERBUFFER, renderBuffer);
glRenderbufferStorage(GL_RENDERBUFFER, GL_RGB565, renderBufferWidth, renderBufferHeight);
glFramebufferRenderbuffer(GL_FRAMEBUFFER, GL_COLOR_ATTACHMENT0, GL_RENDERBUFFER, renderBuffer);
GLuint depthRenderbuffer;
glGenRenderbuffers(1, &depthRenderbuffer);
glBindRenderbuffer(GL_RENDERBUFFER, depthRenderbuffer);
glRenderbufferStorage(GL_RENDERBUFFER, GL_DEPTH_COMPONENT16, renderBufferWidth, renderBufferHeight);
glFramebufferRenderbuffer(GL_FRAMEBUFFER, GL_DEPTH_ATTACHMENT, GL_RENDERBUFFER, depthRenderbuffer);
// Step 10 - check FBO status
GLenum status = glCheckFramebufferStatus(GL_FRAMEBUFFER);
if(status != GL_FRAMEBUFFER_COMPLETE)
printf("Problem with OpenGL framebuffer after specifying color render buffer: \n%x\n", status);
else
printf("FBO creation succedded\n");
int size = 4 * renderBufferHeight * renderBufferWidth;
unsigned char *data2 = new unsigned char[size];
// Step 11 - clear the screen in Red and read it back
glClearColor(1.0,0.0,0.0,1.0);
glClear(GL_COLOR_BUFFER_BIT);
eglSwapBuffers( eglDisplay, eglSurface);
glReadPixels(0,0,renderBufferWidth,renderBufferHeight,GL_RGBA, GL_UNSIGNED_BYTE, data2);
... save data2 to image ...
但是，如果我删除 fbo，并尝试直接绘制到 pbuffer，我会在调用 glClear() 函数后立即看到分段错误：
#include <GLES2/gl2.h>
#include <EGL/egl.h>
int main(int argc, char *argv[])
EGLint ai32ContextAttribs[] = EGL_CONTEXT_CLIENT_VERSION, 2,
EGL_NONE ;
// Step 1 - Get the default display.
EGLDisplay eglDisplay = eglGetDisplay((EGLNativeDisplayType)0);
// Step 2 - Initialize EGL.
eglInitialize(eglDisplay, 0, 0);
// Step 3 - Make OpenGL ES the current API.
eglBindAPI(EGL_OPENGL_ES_API);
// Step 4 - Specify the required configuration attributes.
EGLint pi32ConfigAttribs[5];
pi32ConfigAttribs[0] = EGL_SURFACE_TYPE;
pi32ConfigAttribs[1] = EGL_WINDOW_BIT;
pi32ConfigAttribs[2] = EGL_RENDERABLE_TYPE;
pi32ConfigAttribs[3] = EGL_OPENGL_ES2_BIT;
pi32ConfigAttribs[4] = EGL_NONE;
// Step 5 - Find a config that matches all requirements.
int iConfigs;
EGLConfig eglConfig;
eglChooseConfig(eglDisplay, pi32ConfigAttribs, &eglConfig, 1, &iConfigs);
if (iConfigs != 1)
printf("Error: eglChooseConfig(): config not found.\n");
exit(-1);
// Step 6 - Create a surface to draw to.
EGLSurface eglSurface;
eglSurface = eglCreatePbufferSurface(eglDisplay, eglConfig, NULL);
// Step 7 - Create a context.
EGLContext eglContext;
eglContext = eglCreateContext(eglDisplay, eglConfig, NULL, ai32ContextAttribs);
// Step 8 - Bind the context to the current thread
eglMakeCurrent(eglDisplay, eglSurface, eglSurface, eglContext);
int size = 4 * renderBufferHeight * renderBufferWidth;
unsigned char *data2 = new unsigned char[size];
// Step 11 - clear the screen in Red and read it back
glClearColor(1.0,0.0,0.0,1.0);
glClear(GL_COLOR_BUFFER_BIT);
eglSwapBuffers( eglDisplay, eglSurface);
glReadPixels(0,0,renderBufferWidth,renderBufferHeight,GL_RGBA, GL_UNSIGNED_BYTE, data2);
... save data2 to image ...
我的环境是带有英特尔图形/台面的 ubuntu 14。
你知道我为什么看到分段错误吗？ （我检查了eglcontext，似乎创建成功了。）你能确认pbuffer表面需要fbo吗？
编辑：正如 Reto 所指出的，我的问题是因为缺少属性。
在设置了这些属性之后，我能够让事情在 opengl es 2 上下文中工作。但是，我的桌面 opengl 上下文仍然存在问题。
使用桌面 opengl 上下文，我只能读取透明图像，而不是读取红色图像。这是我当前的代码：
#include <QCoreApplication>
#include <QDebug>
#include <QImage>
#include <GL/gl.h>
#include <EGL/egl.h>
#include <QElapsedTimer>
int main(int argc, char *argv[])
// Step 1 - Get the default display.
EGLDisplay eglDisplay = eglGetDisplay((EGLNativeDisplayType)0);
// Step 2 - Initialize EGL.
eglInitialize(eglDisplay, 0, 0);
// Step 3 - Make OpenGL ES the current API.
eglBindAPI(EGL_OPENGL_API);
// Step 4 - Specify the required configuration attributes.
EGLint pi32ConfigAttribs[5];
pi32ConfigAttribs[0] = EGL_SURFACE_TYPE;
pi32ConfigAttribs[1] = EGL_PBUFFER_BIT;
pi32ConfigAttribs[2] = EGL_RENDERABLE_TYPE;
pi32ConfigAttribs[3] = EGL_OPENGL_BIT;
pi32ConfigAttribs[4] = EGL_CONFORMANT;
pi32ConfigAttribs[5] = EGL_OPENGL_BIT;
pi32ConfigAttribs[6] = EGL_COLOR_BUFFER_TYPE;
pi32ConfigAttribs[7] = EGL_RGB_BUFFER;
pi32ConfigAttribs[8] = EGL_LUMINANCE_SIZE;
pi32ConfigAttribs[9] = 0;
pi32ConfigAttribs[10] = EGL_RED_SIZE;
pi32ConfigAttribs[11] = 8;
pi32ConfigAttribs[12] = EGL_GREEN_SIZE;
pi32ConfigAttribs[13] = 8;
pi32ConfigAttribs[14] = EGL_BLUE_SIZE;
pi32ConfigAttribs[15] = 8;
pi32ConfigAttribs[16] = EGL_ALPHA_SIZE;
pi32ConfigAttribs[17] = 8;
pi32ConfigAttribs[18] = EGL_DEPTH_SIZE;
pi32ConfigAttribs[19] = 8;
pi32ConfigAttribs[20] = EGL_LEVEL;
pi32ConfigAttribs[21] = 0;
pi32ConfigAttribs[22] = EGL_BUFFER_SIZE;
pi32ConfigAttribs[23] = 24;
pi32ConfigAttribs[24] = EGL_NONE;
// Step 5 - Find a config that matches all requirements.
int iConfigs;
EGLConfig eglConfig;
eglChooseConfig(eglDisplay, pi32ConfigAttribs, &eglConfig, 1, &iConfigs);
qDebug() << "egl error" << eglGetError();
if (iConfigs != 1)
printf("Error: eglChooseConfig(): config not found.\n");
exit(-1);
EGLint pbufferAttribs[5];
pbufferAttribs[0] = EGL_WIDTH;
pbufferAttribs[1] = 1920;
pbufferAttribs[2] = EGL_HEIGHT;
pbufferAttribs[3] = 1080;
pbufferAttribs[4] = EGL_NONE;
// Step 6 - Create a surface to draw to.
EGLSurface eglSurface;
eglSurface = eglCreatePbufferSurface(eglDisplay, eglConfig, pbufferAttribs);
qDebug() << "egl error" << eglGetError();
if (eglSurface == EGL_NO_SURFACE)
qDebug() << "surface issue";
// Step 7 - Create a context.
EGLContext eglContext;
eglContext = eglCreateContext(eglDisplay, eglConfig, NULL, NULL);
qDebug() << "egl error" << eglGetError();
if (eglContext == EGL_NO_CONTEXT)
qDebug() << "context issue";
// Step 8 - Bind the context to the current thread
bool result = eglMakeCurrent(eglDisplay, eglSurface, eglSurface, eglContext);
if (!result)
qDebug() << "make current error" << eglGetError();
qDebug() << "egl error" << eglGetError();
GLuint renderBufferWidth = 1920;
GLuint renderBufferHeight = 1080;
QElapsedTimer benchmarkTimer;
int size = 4 * renderBufferHeight * renderBufferWidth;
unsigned char *data2 = new unsigned char[size];
int i = 0;
benchmarkTimer.start();
while(i<1000)
glClearColor(1.0,0.0,0.0,1.0);
glClear(GL_COLOR_BUFFER_BIT);
eglSwapBuffers( eglDisplay, eglSurface);
glReadPixels(0,0,renderBufferWidth,renderBufferHeight,GL_RGBA, GL_UNSIGNED_BYTE, data2);
++i;
qDebug() << "fps" << 1000.0*1000.0/benchmarkTimer.elapsed();
QImage saveImage(data2, renderBufferWidth, renderBufferHeight, QImage::Format_RGBA8888_Premultiplied);
saveImage.save("haha.png");
QCoreApplication a(argc, argv);
qDebug() << "done";
return a.exec();
【问题讨论】：
你为什么不检查eglMakeCurrent()的返回值？
我检查了，也检查了 eglGetError()，一切看起来都很好。为了简洁，我没有将它们粘贴在这里。
【参考方案1】：
这段代码有几个问题：
config属性中指定的EGL_SURFACE_TYPE错误：
pi32ConfigAttribs[0] = EGL_SURFACE_TYPE;
pi32ConfigAttribs[1] = EGL_WINDOW_BIT;
要渲染到 PBuffer，这需要使用匹配值：
pi32ConfigAttribs[0] = EGL_SURFACE_TYPE;
pi32ConfigAttribs[1] = EGL_PBUFFER_BIT;
没有为 PBuffer 指定大小。虽然man page 建议在不指定大小的情况下创建 PBuffer 是合法的，但宽度和高度的默认值为 0。我无法想象在尝试渲染到大小为 0 乘以 0 的表面时会发生什么好事。指定大小：
EGLint pbufferAttribs[5];
pbufferAttribs[0] = EGL_WIDTH;
pbufferAttribs[1] = DesiredWidthOfPBuffer;
pbufferAttribs[2] = EGL_HEIGHT;
pbufferAttribs[3] = DesiredHeightOfPBuffer;
pbufferAttribs[4] = EGL_NONE;
eglSurface = eglCreatePbufferSurface(eglDisplay, eglConfig, pbufferAttribs);
我对这个问题的回答包含为两个不同版本的 EGL 创建上下文和 PBuffer 表面的完整代码：GLES10.glGetIntegerv returns 0 in Lollipop only。代码使用 Java 绑定，但应该很容易适应。
【讨论】：
非常感谢，在根据您的建议编辑了我的代码后，我设法让它在 opengl es 2 上下文中工作。我读回的图像确实是红色的。但是，一旦我将 opengl es 更改为桌面 opengl，我仍然遇到问题。我读回的图像现在完全透明了。我尝试设置缓冲区大小、深度大小和我认为必要的所有内容，但我仍然无法解决问题。 :( 我的代码在问题中更新了。【参考方案2】：为什么你在 glReadPixels 之前做 eglSwapBuffers？ 大概 eglSwapBuffers 对 PBufferSurface 没有影响（因为它不是双缓冲表面），但如果是，你会尝试从未定义的缓冲区读取像素，结果未定义..
【讨论】：
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