How to egl offscreen render to an image on linux? - c++

Question

I'm trying to use egl to do offscreen rendering to an image.
my code doesn't generate any error. the egl part seems to be correct, the fbo is also complete. but when I read pixels using glReadPixels, I always get a black image (I cleared the entire scene with red, so the image should be red too).
I can't figure out what's wrong.
Also, I noticed that glRenderbufferStorage can only support 16bit color depth. GL_RGBA8 is consider an invalid parameter for this function. Isn't 16bit a bit low for serious opengl application?
My environment is Ubuntu 14.10 with mesa and intel graphics.
#include <QCoreApplication>
#include <QDebug>
#include <QImage>
#include <GLES2/gl2.h>
#include <EGL/egl.h>
int main(int argc, char *argv[])
{
#define CONTEXT_ES20
#ifdef CONTEXT_ES20
EGLint ai32ContextAttribs[] = { EGL_CONTEXT_CLIENT_VERSION, 2,
EGL_NONE };
#endif
// Step 1 - Get the default display.
EGLDisplay eglDisplay = eglGetDisplay((EGLNativeDisplayType)0);
// Step 2 - Initialize EGL.
eglInitialize(eglDisplay, 0, 0);
#ifdef CONTEXT_ES20
// 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;
#else
EGLint pi32ConfigAttribs[3];
pi32ConfigAttribs[0] = EGL_SURFACE_TYPE;
pi32ConfigAttribs[1] = EGL_WINDOW_BIT;
pi32ConfigAttribs[2] = EGL_NONE;
#endif
// 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 = eglCreateWindowSurface(eglDisplay, eglConfig,
(EGLNativeWindowType)NULL, NULL);
// Step 7 - Create a context.
EGLContext eglContext;
#ifdef CONTEXT_ES20
eglContext = eglCreateContext(eglDisplay, eglConfig, NULL,
ai32ContextAttribs);
#else
eglContext = eglCreateContext(eglDisplay, eglConfig, NULL, NULL);
#endif
// Step 8 - Bind the context to the current thread
eglMakeCurrent(eglDisplay, eglSurface, eglSurface, eglContext);
GLuint fboId = 0;
GLuint renderBufferWidth = 1280;
GLuint renderBufferHeight = 720;
// create a framebuffer object
glGenFramebuffers(1, &fboId);
glBindFramebuffer(GL_FRAMEBUFFER, fboId);
// create a texture object
/* GLuint textureId;
glGenTextures(1, &textureId);
glBindTexture(GL_TEXTURE_2D, textureId);
glTexParameterf(GL_TEXTURE_2D, GL_TEXTURE_MAG_FILTER, GL_LINEAR);
glTexParameterf(GL_TEXTURE_2D, GL_TEXTURE_MIN_FILTER, GL_LINEAR);
//GL_LINEAR_MIPMAP_LINEAR
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_GENERATE_MIPMAP_HINT, GL_TRUE); // automatic mipmap
glTexImage2D(GL_TEXTURE_2D, 0, GL_RGB, renderBufferWidth, renderBufferHeight, 0,
GL_RGB, GL_UNSIGNED_BYTE, 0);
glBindTexture(GL_TEXTURE_2D, 0);
// attach the texture to FBO color attachment point
glFramebufferTexture2D(GL_FRAMEBUFFER, GL_COLOR_ATTACHMENT0,
GL_TEXTURE_2D, textureId, 0);
*/
qDebug() << glGetError();
GLuint renderBuffer;
glGenRenderbuffers(1, &renderBuffer);
glBindRenderbuffer(GL_RENDERBUFFER, renderBuffer);
qDebug() << glGetError();
glRenderbufferStorage(GL_RENDERBUFFER,
GL_RGB565,
renderBufferWidth,
renderBufferHeight);
qDebug() << glGetError();
glFramebufferRenderbuffer(GL_FRAMEBUFFER,
GL_COLOR_ATTACHMENT0,
GL_RENDERBUFFER,
renderBuffer);
qDebug() << glGetError();
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);
// 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");
}
glClearColor(1.0,0.0,0.0,1.0);
glClear(GL_COLOR_BUFFER_BIT);
qDebug() << eglSwapBuffers( eglDisplay, eglSurface);
int size = 4 * renderBufferHeight * renderBufferWidth;
printf("print size");
printf("size %d", size);
qDebug() << size;
unsigned char *data2 = new unsigned char[size];
glReadPixels(0,0,renderBufferWidth,renderBufferHeight,GL_RGB, GL_RGB565, data2);
QImage image(data2, renderBufferWidth, renderBufferHeight,renderBufferWidth*2, QImage::Format_RGB16);
image.save("result.png");
qDebug() << "done";
QCoreApplication a(argc, argv);
return a.exec();
}

Answer 1

OpenGL ES 2.0 has a very limited number of formats/types that are supported for glReadPixels(). The ones you are trying to use are not guaranteed to be supported:
glReadPixels(0 ,0, renderBufferWidth, renderBufferHeight,
GL_RGB, GL_RGB565, data2);
Only two formats/types are supported:
GL_RGBA/GL_UNSIGNED_BYTE.
An implementation dependent combination.
The format and type of the implementation dependent combination can be queried with:
GLint format = 0, type = 0;
glGetIntegerv(GL_IMPLEMENTATION_COLOR_READ_FORMAT, &format);
glGetIntegerv(GL_IMPLEMENTATION_COLOR_READ_TYPE, &type);
This can give you one of the following combinations:
GL_RGB/GL_UNSIGNED_BYTE.
GL_RGB/GL_UNSIGNED_SHORT_5_6_5.
GL_RGBA/GL_UNSIGNED_SHORT_4_4_4_4.
GL_RGBA/GL_UNSIGNED_SHORT_5_5_5_1.
GL_ALPHA/GL_UNSIGNED_BYTE.
So the combination you tried to use could be supported by an implementation, if it returns the corresponding values from the glGetIntegerv() calls above. However, there was a subtle but important error in the arguments of your glReadPixels() call even if it is supported: GL_RGB565 is a value for a format, while the 6th argument is a type. The call would have to be:
glReadPixels(0 ,0, renderBufferWidth, renderBufferHeight,
GL_RGB, GL_UNSIGNED_SHORT_5_6_5, data2);