Hello everyone, I want to render a scene larger that the screen size, if I got it correctly (source: http://www.songho.ca/opengl/gl_fbo.html), I should use FBO because the default framebuffer called window-system-provided will be controlled and I won't be able to render a scene larger than the screen resolution. I'm sort of confused, I can't get the code work, here is part of the code: myProjection.initOpenGL(inputImage->width , inputImage->height); // initOpenGL is for glutInitWindowSize(), glViewport(), and gluPerspective() //(If my image width and height are larger than my screen resolution, the program without //FBO crash) glGenFramebuffersEXT(1, &fboId); glBindFramebufferEXT(GL_FRAMEBUFFER_EXT, fboId); glGenRenderbuffersEXT(1, &rboId); glBindRenderbufferEXT(GL_RENDERBUFFER_EXT, rboId); glRenderbufferStorageEXT(GL_RENDERBUFFER_EXT, GL_DEPTH_COMPONENT, windowWidth, windowHeight); // glBindRenderbufferEXT(GL_RENDERBUFFER_EXT, 0); // attach a texture to FBO color attachement point // glFramebufferTexture2DEXT(GL_FRAMEBUFFER_EXT, GL_COLOR_ATTACHMENT0_EXT, GL_TEXTURE_2D, textureId, 0); // attach a renderbuffer to depth attachment point glFramebufferRenderbufferEXT(GL_FRAMEBUFFER_EXT, GL_DEPTH_ATTACHMENT_EXT, GL_RENDERBUFFER_EXT, rboId); // I the beginning of the drawScene() I add glBindFramebufferEXT(GL_FRAMEBUFFER_EXT, fboId); // then I draw the scene, which contains a loaded 3D model with texture // at the end of the drawing scene, I add // back to normal window-system-provided framebuffer glBindFramebufferEXT(GL_FRAMEBUFFER_EXT, 0); // unbind // and after that, I want to read the render scene to OpenCV void Projection::OpenGL2OpenCV(IplImage * imgO) {// source: http://webeng.cs.ait.ac.th/cvwiki/gltocv:main int width = imgO->width; int height = imgO->height; . . . IplImage *img = cvCreateImage(cvSize(width,height),IPL_DEPTH_8U,3); glReadPixels(0, 0, width, height, GL_RGB, GL_UNSIGNED_BYTE, imageData); . . . } I can't find where is the problem, it's not working. I don't want to use the render image as texture, I just want to read it after the rendering, so is it enough to use a renderbuffer even than my scene has a texture? I also check for errors in FBO generation, I render my scene twice, the first time I get an error "GL_FRAMEBUFFER_INCOMPLETE_DRAW_BUFFER_EXT", and the second rendering is fine, it pass the test GL_FRAMEBUFFER_COMPLETE_EXT. Many thanks for any help :) Bye

I think you need to tell it where to draw to. Try putting these two lines after your call to glFramebufferTexture2DEXT:

glDrawBuffer(GL_FRONT);
glReadBuffer(GL_FRONT);

I'm not too sure about the GL_FRONT param - I use FBO's just for a depth render and use GL_NONE...