Sign in to follow this  

OpenGL FBO and reading with glReadPixels

Recommended Posts

Hello everyone, I want to render a scene larger that the screen size, if I got it correctly (source:, 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: 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

Share this post

Link to post
Share on other sites
I think you need to tell it where to draw to. Try putting these two lines after your call to glFramebufferTexture2DEXT:


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

Share this post

Link to post
Share on other sites
You have to set the corresponding attachment you want to read when calling glReadPixels to extract the color data from an FBO such as:


Also make sure to call glWindowPos AFTER binding the FBO. Obviously the FBO needs to be bound while reading its color attachment pixels.

Note that you can also read pixels from the texture itself using glGetTexImage instead of glReadPixels. You already have the FBO texture and can just keep the FBO unbound and then bind its texture as GL texture and read its content.

Finally, find out why you get an FBO setup error and fix this before proceeding any further.

Share this post

Link to post
Share on other sites

Create an account or sign in to comment

You need to be a member in order to leave a comment

Create an account

Sign up for a new account in our community. It's easy!

Register a new account

Sign in

Already have an account? Sign in here.

Sign In Now

Sign in to follow this  

  • Forum Statistics

    • Total Topics
    • Total Posts
  • Similar Content

    • By DelicateTreeFrog
      Hello! As an exercise for delving into modern OpenGL, I'm creating a simple .obj renderer. I want to support things like varying degrees of specularity, geometry opacity, things like that, on a per-material basis. Different materials can also have different textures. Basic .obj necessities. I've done this in old school OpenGL, but modern OpenGL has its own thing going on, and I'd like to conform as closely to the standards as possible so as to keep the program running correctly, and I'm hoping to avoid picking up bad habits this early on.
      Reading around on the OpenGL Wiki, one tip in particular really stands out to me on this page:
      For something like a renderer for .obj files, this sort of thing seems almost ideal, but according to the wiki, it's a bad idea. Interesting to note!
      So, here's what the plan is so far as far as loading goes:
      Set up a type for materials so that materials can be created and destroyed. They will contain things like diffuse color, diffuse texture, geometry opacity, and so on, for each material in the .mtl file. Since .obj files are conveniently split up by material, I can load different groups of vertices/normals/UVs and triangles into different blocks of data for different models. When it comes to the rendering, I get a bit lost. I can either:
      Between drawing triangle groups, call glUseProgram to use a different shader for that particular geometry (so a unique shader just for the material that is shared by this triangle group). or
      Between drawing triangle groups, call glUniform a few times to adjust different parameters within the "master shader", such as specularity, diffuse color, and geometry opacity. In both cases, I still have to call glBindTexture between drawing triangle groups in order to bind the diffuse texture used by the material, so there doesn't seem to be a way around having the CPU do *something* during the rendering process instead of letting the GPU do everything all at once.
      The second option here seems less cluttered, however. There are less shaders to keep up with while one "master shader" handles it all. I don't have to duplicate any code or compile multiple shaders. Arguably, I could always have the shader program for each material be embedded in the material itself, and be auto-generated upon loading the material from the .mtl file. But this still leads to constantly calling glUseProgram, much more than is probably necessary in order to properly render the .obj. There seem to be a number of differing opinions on if it's okay to use hundreds of shaders or if it's best to just use tens of shaders.
      So, ultimately, what is the "right" way to do this? Does using a "master shader" (or a few variants of one) bog down the system compared to using hundreds of shader programs each dedicated to their own corresponding materials? Keeping in mind that the "master shaders" would have to track these additional uniforms and potentially have numerous branches of ifs, it may be possible that the ifs will lead to additional and unnecessary processing. But would that more expensive than constantly calling glUseProgram to switch shaders, or storing the shaders to begin with?
      With all these angles to consider, it's difficult to come to a conclusion. Both possible methods work, and both seem rather convenient for their own reasons, but which is the most performant? Please help this beginner/dummy understand. Thank you!
    • By JJCDeveloper
      I want to make professional java 3d game with server program and database,packet handling for multiplayer and client-server communicating,maps rendering,models,and stuffs Which aspect of java can I learn and where can I learn java Lwjgl OpenGL rendering Like minecraft and world of tanks
    • By AyeRonTarpas
      A friend of mine and I are making a 2D game engine as a learning experience and to hopefully build upon the experience in the long run.

      -What I'm using:
          C++;. Since im learning this language while in college and its one of the popular language to make games with why not.     Visual Studios; Im using a windows so yea.     SDL or GLFW; was thinking about SDL since i do some research on it where it is catching my interest but i hear SDL is a huge package compared to GLFW, so i may do GLFW to start with as learning since i may get overwhelmed with SDL.  
      Knowing what we want in the engine what should our main focus be in terms of learning. File managements, with headers, functions ect. How can i properly manage files with out confusing myself and my friend when sharing code. Alternative to Visual studios: My friend has a mac and cant properly use Vis studios, is there another alternative to it?  
    • By ferreiradaselva
      Both functions are available since 3.0, and I'm currently using `glMapBuffer()`, which works fine.
      But, I was wondering if anyone has experienced advantage in using `glMapBufferRange()`, which allows to specify the range of the mapped buffer. Could this be only a safety measure or does it improve performance?
      Note: I'm not asking about glBufferSubData()/glBufferData. Those two are irrelevant in this case.
    • By xhcao
      Before using void glBindImageTexture(    GLuint unit, GLuint texture, GLint level, GLboolean layered, GLint layer, GLenum access, GLenum format), does need to make sure that texture is completeness. 
  • Popular Now