我对egl pbuffer感到非常困惑。在我看来，pbuffer表面是独立于平台的表面，就像windows表面或pixmap表面一样。吸引到该表面的事物虽然不可见，但应该能够被读取。
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表面外，还需要创建fbo缓冲区。
该代码似乎对我有用，它创建了一个pbuffer表面，然后创建了一个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，如果删除fbo，并尝试直接绘制到pbuffer，则在调用glClear（）函数后，我会立即看到分段错误：
#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 ...
}
我的环境是具有Intel图形/介面的ubuntu 14。
您知道我为什么看到细分错误吗？ （我检查了eglcontext，它似乎已成功创建。）您可以确认pbuffer表面是否需要fbo吗？
编辑：正如雷托指出，我的问题是由于缺少属性。
设置完这些属性后，我就可以在opengl es 2上下文中工作。但是，我仍然对桌面opengl上下文有疑问。
我不能通过桌面opengl上下文读回红色图像，而只能得到透明图像。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();
}
最佳答案
这段代码有两个问题：
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，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表示创建不指定大小的PBuffer是合法的，但是width和height的默认值为0。我无法想象当尝试渲染大小为0乘以0的表面时会发生什么好事。要指定大小：
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有完整的代码，可以为两种不同版本的EGL：GLES10.glGetIntegerv returns 0 in Lollipop only创建上下文和PBuffer表面。该代码使用Java绑定，但应易于调整。
关于opengl-es - EGL离屏渲染中的pbuffer vs fbo，我们在Stack Overflow上找到一个类似的问题：https://stackoverflow.com/questions/28817777/