egl离屏渲染中的pbuffer vs fbo
pbuffer vs fbo in egl offscreen rendering
我对egl pbuffer 表面很困惑。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 ubuntu 14，intel graphics/mesa。
你知道我为什么看到分段错误吗？ （我查看了eglcontext，好像创建成功了。）你能确认pbuffer surface需要fbo吗？
编辑：正如 Reto 所指出的，我的问题是因为缺少属性。
设置这些属性后，我能够使用 opengl es 2 上下文进行操作。但是，我仍然对桌面 opengl 上下文有疑问。
在桌面 opengl 上下文中，我没有读回红色图像，而是只能得到透明图像。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();
}
这段代码中有几个问题：
配置属性中指定的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 表明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 次的表面时会发生什么好事0. 指定尺寸：
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。该代码使用 Java 绑定，但应该很容易适应。
为什么在 glReadPixels 之前执行 eglSwapBuffers？
大概 eglSwapBuffers 对 PBufferSurface 没有影响（因为它不是 double-buffer 表面）但如果它是你会尝试从未定义的缓冲区读取像素，结果未定义..
我对egl pbuffer 表面很困惑。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 ubuntu 14，intel graphics/mesa。
你知道我为什么看到分段错误吗？ （我查看了eglcontext，好像创建成功了。）你能确认pbuffer surface需要fbo吗？
编辑：正如 Reto 所指出的，我的问题是因为缺少属性。
设置这些属性后，我能够使用 opengl es 2 上下文进行操作。但是，我仍然对桌面 opengl 上下文有疑问。
在桌面 opengl 上下文中，我没有读回红色图像，而是只能得到透明图像。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();
}
这段代码中有几个问题：
配置属性中指定的
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 表明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 次的表面时会发生什么好事0. 指定尺寸：
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。该代码使用 Java 绑定，但应该很容易适应。
为什么在 glReadPixels 之前执行 eglSwapBuffers？ 大概 eglSwapBuffers 对 PBufferSurface 没有影响（因为它不是 double-buffer 表面）但如果它是你会尝试从未定义的缓冲区读取像素，结果未定义..