Question

I knew it is possible to render offline without displaying it on screen.

How to do it, maybe

create an invisible window then draw.

Can I use specific fbo when render?

Answer 1

You can create a framebuffer object using something like:

GLint fbo, tex; // "handles" for framebuffer and it's texture
glGenFramebuffersEXT(1, &fbo);
glBindFramebufferEXT(GL_FRAMEBUFFER_EXT, fbo);

// setup texture for colour attachment
glGenTextures(1, &tex);
glEnable(GL_TEXTURE_2D);
glDisable(GL_COLOR_MATERIAL);
glBindTexture(GL_TEXTURE_2D, tex);
glTexParameteri(GL_TEXTURE_2D, GL_TEXTURE_WRAP_S, GL_REPEAT);
glTexParameteri(GL_TEXTURE_2D, GL_TEXTURE_WRAP_T, GL_REPEAT);
glTexParameteri(GL_TEXTURE_2D, GL_TEXTURE_MIN_FILTER, GL_NEAREST);
glTexParameteri(GL_TEXTURE_2D, GL_TEXTURE_MAG_FILTER, GL_NEAREST);
// set a size fotr the texture, but not any initial data
glTexImage2D(GL_TEXTURE_2D, 0, GL_RGBA8, fbo_resX, fbo_resY, 0, GL_BGRA, GL_UNSIGNED_BYTE, NULL);
// You might want a depth attachment here too perhaps?
glFramebufferTexture2DEXT(GL_FRAMEBUFFER_EXT, GL_COLOR_ATTACHMENT0_EXT, GL_TEXTURE_2D, tex, 0);

const GLenum status = glCheckFramebufferStatusEXT(GL_FRAMEBUFFER_EXT);
assert(status == GL_FRAMEBUFFER_COMPLETE_EXT);

glBindFramebufferEXT(GL_FRAMEBUFFER_EXT, 0);

You then use it like:

// specify which FBO to use
glBindFramebufferEXT(GL_FRAMEBUFFER_EXT, fbo);
// glDraw...

// return to the default
glBindFramebufferEXT(GL_FRAMEBUFFER_EXT, 0);

You can create multiple framebuffers as you like and bind to them when you please. (Give or take). You’ll need a valid OpenGL context to use this, which usually approximates to creating a window on most platforms, but you don’t ever have to draw anything into that window.