draw in FrameBuffer but get only black - opengl

Question

windows
using glew
I'm trying to render offscreen and save the img opengl rendered to a png file.
I followed a highly rated answer on stackoverflow:
How to render offscreen on OpenGL?
But the png file I get is only a black screen.
Here's my code relating to it:
glutCreateWindow(argv[0]);
if(GLEW_OK!=glewInit())
{
return -1;
}
initScene();
GLuint fbo, render_buf;
glGenFramebuffers(1,&fbo);
glGenRenderbuffers(1,&render_buf);
glBindRenderbuffer(GL_RENDERBUFFER,render_buf);
glRenderbufferStorage(GL_RENDERBUFFER, GL_RGB8, viewport.w, viewport.h);
glBindFramebuffer(GL_DRAW_FRAMEBUFFER,fbo);
glFramebufferRenderbuffer(GL_DRAW_FRAMEBUFFER, GL_COLOR_ATTACHMENT0, GL_RENDERBUFFER, render_buf);
//Before drawing
glBindFramebuffer(GL_DRAW_FRAMEBUFFER,fbo);
glClear(GL_COLOR_BUFFER_BIT); // clear the color buffer
glMatrixMode(GL_MODELVIEW); // indicate we are specifying camera transformations
glLoadIdentity(); // make sure transformation is "zero'd"
//draw...
//glBegin(GL_POINTS) glColor3f, glVertex2f
//glFlush();
glFinish();
/*glutDisplayFunc(myDisplay);
glutPostRedisplay();*/
glBindFramebuffer(GL_READ_FRAMEBUFFER, fbo);
savePNG(outputPNGName,0,0,viewport.w,viewport.h);
//At deinit:
glDeleteFramebuffers(1,&fbo);
glDeleteRenderbuffers(1,&render_buf);
glBindFramebuffer(GL_DRAW_FRAMEBUFFER,0);
How to solve the problem?
Thank you
savePNG (related code):
glReadBuffer(GL_COLOR_ATTACHMENT0);
glReadPixels(x, y, width, height, GL_RGB, GL_UNSIGNED_BYTE, (GLvoid *)image);

Answer 1

There are at least two problems in this code:
GL_RGB8 is not a valid format for a renderbuffer. From the glRenderbufferStorage() man page:
internalformat specifies the internal format to be used for the renderbuffer object's storage and must be a color-renderable, depth-renderable, or stencil-renderable format.
Table 8.13 in the latest spec document (4.5, downloadable from https://www.opengl.org/registry) lists all formats, with a column showing which of them are color-renderable. RGB8 does not have a checkmark in that column. You can use GL_RGBA8 instead, which is color-renderable.
You may also want to check out the glCheckFramebufferStatus() function, which allows you to check if your framebuffer setup is valid.
While we don't see the code for savePNG(), there is no way it can know that you want to read the pixel data from your FBO. It will most likely use glReadPixels(), which reads data from the current read framebuffer, while your code only sets the draw framebuffer. Before calling savePNG(), add this call to set the read framebuffer to your FBO:
glBindFramebuffer(GL_READ_FRAMEBUFFER, fbo);