OpenGL: Trouble with Render to Texture & Framebuffer Object

15
votes

I'd like to render a scene to an initially empty texture. To do so, I use a Framebuffer Object to which I attach an empty 2d texture and a depth buffer. After the set up, as for testing, I draw a simple quad into the scene. Every vertex has a different color, so I eventually expect a color-interpolated quad in the texture. I then use that texture containing the quad and map it on another quad. So evetually, I have a quad in my default Framebuffer which has a texture containing a colored quad. I hope this is not too confusing...

Anyway, I must be missing something here, since what I get is nothing but a grey texture. I basically followed this instructions which are pretty straight forward. However, I cannot quite figure out what I am missing here. I would be grateful if somebody could give me a clue.

Thanks Walter

This is the code I have so far: // create frame buffer object glGenFramebuffers(1, &frameBufferObject);

// create depth buffer
glGenRenderbuffers(1, &depthAttachment);

// create empty texture
int width = 512;
int height = 512;
int numberOfChannels = 3;
GLuint internalFormat = GL_RGB;
GLuint format = GL_RGB;

unsigned char* texels = new unsigned char[width * height * numberOfChannels];

glGenTextures(1, &texture);
glBindTexture(GL_TEXTURE_2D, texture);

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_LINEAR);
glTexParameteri(GL_TEXTURE_2D, GL_TEXTURE_MAG_FILTER, GL_LINEAR);

glTexImage2D(GL_TEXTURE_2D, 0, internalFormat, width, height, 0, format, GL_UNSIGNED_BYTE, texels);

glGenerateMipmap(GL_TEXTURE_2D);

delete[] texels;
texels = NULL;

// activate & bind empty texture
glActiveTexture(GL_TEXTURE0);
glBindTexture(GL_TEXTURE_2D, texture);

// attach empty texture to framebuffer object
glBindFramebuffer(GL_FRAMEBUFFER, frameBufferObject);
glFramebufferTexture2D(GL_FRAMEBUFFER, GL_COLOR_ATTACHMENT0, GL_TEXTURE_2D, texture, 0);

// define render targets (empty texture is at GL_COLOR_ATTACHMENT0)
glDrawBuffers(1, GL_COLOR_ATTACHMENT0);

// attach depth buffer
glBindRenderbuffer(GL_RENDERBUFFER, depthAttachment);
glRenderbufferStorage(GL_RENDERBUFFER, GL_DEPTH_COMPONENT, width, height);
glFramebufferRenderbuffer(GL_FRAMEBUFFER, GL_DEPTH_ATTACHMENT, GL_RENDERBUFFER, depthAttachment);

// use framebuffer object
glBindFramebuffer(GL_FRAMEBUFFER, frameBufferObject);

// draw the colored quad into the initially empty texture
glDisable(GL_CULL_FACE);
glDisable(GL_DEPTH_TEST);

// store attibutes
glPushAttrib(GL_VIEWPORT_BIT | GL_COLOR_BUFFER_BIT | GL_DEPTH_BUFFER_BIT);

// reset viewport
glViewport(0, 0, width, height);

// make background yellow
glClearColor(1.0f, 1.0f, 0.0f, 1.0f);

// draw quad into texture attached to frame buffer object
glColor4f(1.0f, 1.0f, 1.0f, 1.0f);
glBegin(GL_QUADS);
    glColor4f(1.0f, 1.0f, 1.0f, 1.0f); glVertex2f(0.0f, 100.0f); // top left
    glColor4f(1.0f, 0.0f, 0.0f, 1.0f); glVertex2f(0.0f, 0.0f); // bottom left
    glColor4f(0.0f, 1.0f, 0.0f, 1.0f); glVertex2f(100.0f, 0.0f); // bottom right
    glColor4f(0.0f, 0.0f, 1.0f, 1.0f); glVertex2f(100.0f, 100.0f); // top right
glEnd();

// restore attributes
glPopAttrib();

glEnable(GL_DEPTH_TEST);
glEnable(GL_CULL_FACE);

// use default framebuffer
glBindFramebuffer(GL_FRAMEBUFFER, 0);

// clear default framebuffer
glClear(GL_COLOR_BUFFER_BIT | GL_DEPTH_BUFFER_BIT);

// draw the scene
glMatrixMode(GL_MODELVIEW);
glPushMatrix();
glLoadIdentity();

glColor4f(1.0, 1.0, 1.0, 1.0);

// begin texture mapping
glEnable(GL_TEXTURE_2D);
glTexEnvf(GL_TEXTURE_ENV, GL_TEXTURE_ENV_MODE, GL_MODULATE);

glBegin(GL_QUADS);
    glNormal3d(0.0f, 0.0f, 1.0f);
    glTexCoord2f(0.0f, 0.0f); glVertex3f(0.0f, 50.0f, -100.0f);    // top left
    glTexCoord2f(0.0f, 1.0f); glVertex3f(0.0f, 0.0f, -100.0f);     // bottom left
    glTexCoord2f(1.0f, 1.0f); glVertex3f(50.0f, 0.0f, -100.0f);    // bottom right
    glTexCoord2f(1.0f, 0.0f); glVertex3f(50.0f, 50.0f, -100.0f);   // top right
glEnd();

glDisable(GL_TEXTURE_2D);

glPopMatrix();

// swap buffers (I forgot to mention that I use SDL)
SDL_GL_SwapBuffers();

Expected result:

  • Texture with a colored quad on a yellow background
  • That texture is mapped on another quad

Actual result:

  • A grey texture
  • Texture can successfully be mapped on the other quad

EDIT: I missed to mention the error handling. This is how I check for errors. It includes the additional cases Paul S suggested. Thanks for that.

GLuint status = glCheckFramebufferStatus(GL_FRAMEBUFFER);
switch(status) {
    case GL_FRAMEBUFFER_COMPLETE:
        return;
        break;

case GL_FRAMEBUFFER_INCOMPLETE_ATTACHMENT:
    throw FramebufferIncompleteException("An attachment could not be bound to frame buffer object!");
    break;

case GL_FRAMEBUFFER_INCOMPLETE_MISSING_ATTACHMENT:
    throw FramebufferIncompleteException("Attachments are missing! At least one image (texture) must be bound to the frame buffer object!");
    break;

case GL_FRAMEBUFFER_INCOMPLETE_DIMENSIONS_EXT:
    throw FramebufferIncompleteException("The dimensions of the buffers attached to the currently used frame buffer object do not match!");
    break;

case GL_FRAMEBUFFER_INCOMPLETE_FORMATS_EXT:
    throw FramebufferIncompleteException("The formats of the currently used frame buffer object are not supported or do not fit together!");
    break;

case GL_FRAMEBUFFER_INCOMPLETE_DRAW_BUFFER:
    throw FramebufferIncompleteException("A Draw buffer is incomplete or undefinied. All draw buffers must specify attachment points that have images attached.");
    break;

case GL_FRAMEBUFFER_INCOMPLETE_READ_BUFFER:
    throw FramebufferIncompleteException("A Read buffer is incomplete or undefinied. All read buffers must specify attachment points that have images attached.");
    break;

case GL_FRAMEBUFFER_INCOMPLETE_MULTISAMPLE:
    throw FramebufferIncompleteException("All images must have the same number of multisample samples.");
    break;

case GL_FRAMEBUFFER_INCOMPLETE_LAYER_TARGETS :
    throw FramebufferIncompleteException("If a layered image is attached to one attachment, then all attachments must be layered attachments. The attached layers do not have to have the same number of layers, nor do the layers have to come from the same kind of texture.");
    break;

case GL_FRAMEBUFFER_UNSUPPORTED:
    throw FramebufferIncompleteException("Attempt to use an unsupported format combinaton!");
    break;

default:
    throw FramebufferIncompleteException("Unknown error while attempting to create frame buffer object!");
    break;
}

EDIT 2: This is the method I use to check for GL-errors

checkForGLErrors(string sourceFile, int line)
    GLenum error = glGetError();
    ostringstream o;

    switch(error) {
        case GL_NO_ERROR:
            return;
            break;

        case GL_INVALID_ENUM:
            o<<"OpenGL Error in "<<sourceFile<<" at line "<<line<<": Invalid enum!"<<endl;
            throw GLErrorException(o.str());
            break;

        case GL_INVALID_VALUE:
            o<<"OpenGL Error in "<<sourceFile<<" at line "<<line<<": Invalid value!"<<endl;
            throw GLErrorException(o.str());
            break;

        case GL_INVALID_OPERATION:
            o<<"OpenGL Error in "<<sourceFile<<" at line "<<line<<": Invalid operation!"<<endl;
            throw GLErrorException(o.str());
            break;

        case GL_STACK_OVERFLOW:
            o<<"OpenGL Error in "<<sourceFile<<" at line "<<line<<": Stack overflow!"<<endl;
            throw GLErrorException(o.str());
            break;

        case GL_STACK_UNDERFLOW:
            o<<"OpenGL Error in "<<sourceFile<<" at line "<<line<<": Stack underflow!"<<endl;
            throw GLErrorException(o.str());
            break;

        case GL_OUT_OF_MEMORY:
            o<<"OpenGL Error in "<<sourceFile<<" at line "<<line<<": Out Of memory!"<<endl;
            throw GLErrorException(o.str());
            break;

        case GL_TABLE_TOO_LARGE:
            o<<"OpenGL Error in "<<sourceFile<<" at line "<<line<<": Table too large!"<<endl;
            throw GLErrorException(o.str());
            break;

        default:
            o<<"OpenGL Error in "<<sourceFile<<" at line "<<line<<": Unknown error!"<<endl;
            throw GLErrorException(o.str());
            break;
    }
}
3
openglrenderingrendertexture-mappingframebuffer
Which version of OpenGL do you use? The framebuffer object functions are without any vendor suffix, so I'm inclined to believe 3.0 - 3.3, but I'm suspicious. Remember that there are major differences between OpenGL 3 core functionality, GL_ARB_framebuffer_object and GL_EXT_framebuffer_object. They are not equal! Also, calling just glCheckFramebufferStatus() is not sufficient. Make sure you handle normal OpenGL errors as well with glGetError() – Mads Elvheim
Your assumption is correct. I'm using OpenGL 3.1. I should have mentioned that earlier. Thanks for the glGetError() tip. I included that into my code. I added the method to the code listings above. I think I check for all the errors possibly fired by OpenGL 3.1, don't I? I have checked the whole code for errors, but there seems to be nothing. – Walter
I increasingly harbour a suspiscion that it is not the application which contains errors, but there might be some library or extension missing or wrongly set up, respectively. I'm not sure if that is of any help, but this is my system: Windows 7, OpenGL 3.1, NVidia Quadro FX 3700. I'm using GLEW 1.5.7, SDL 1.2, GL_EXT_framebuffer_object, GL_EXT_texture3D, GL_ARB_draw_buffers... Any ideas are appreciated. – Walter

3 Answers

5
votes

I resolved this issue.

So here is what I do. Maybe there is some better / smarter way to do it, but it works perfectly fine now. Feel free to suggest adaptions...

I don't list the error handling beneath. You can find that further up in this question.

// create frame buffer object
glGenFramebuffers(1, frameBufferObject);

// create empty texture
int width = 512;
int height = 512;
int numberOfChannels = 3;
GLuint internalFormat = GL_RGB8;
GLuint format = GL_RGB;

unsigned char* texels = new unsigned char[width * height * numberOfChannels];

glGenTextures(1, &texture);
glBindTexture(GL_TEXTURE_2D, texture);

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_LINEAR);
glTexParameteri(GL_TEXTURE_2D, GL_TEXTURE_MAG_FILTER, GL_LINEAR);

glTexImage2D(GL_TEXTURE_2D, 0, internalFormat, width, height, 0, format, GL_UNSIGNED_BYTE, texels);

delete[] texels;
texels = NULL;

// draw the colored quad into the initially empty texture
glDisable(GL_CULL_FACE);
glDisable(GL_DEPTH_TEST);

// store attibutes
glPushAttrib(GL_VIEWPORT_BIT | GL_COLOR_BUFFER_BIT | GL_DEPTH_BUFFER_BIT);

// reset viewport
glViewport(0, 0, width, height);

// setup modelview matrix
glMatrixMode(GL_MODELVIEW);
glPushMatrix();
glLoadIdentity();

// setup projection matrix
glMatrixMode(GL_PROJECTION);
glPushMatrix();
glLoadIdentity();

// setup orthogonal projection
glOrtho(-width / 2, width / 2, -height / 2, height / 2, 0, 1000);

// bind framebuffer object
glBindFramebuffer(GL_FRAMEBUFFER, frameBufferObject);

// attach empty texture to framebuffer object
glFramebufferTexture2D(GL_FRAMEBUFFER, GL_COLOR_ATTACHMENT0, GL_TEXTURE_2D, texture, 0);

// check framebuffer status (see above)

// bind framebuffer object (IMPORTANT! bind before adding color attachments!)
glBindFramebuffer(GL_FRAMEBUFFER, frameBufferObject);

// define render targets (empty texture is at GL_COLOR_ATTACHMENT0)
glDrawBuffers(1, GL_COLOR_ATTACHMENT0); // you can of course have more than only 1 attachment

// activate & bind empty texture
// I figured activating and binding must take place AFTER attaching texture as color attachment
glActiveTexture(GL_TEXTURE0);
glBindTexture(GL_TEXTURE_2D, texture);

// clear color attachments
glClear(GL_COLOR_BUFFER_BIT);

// make background yellow
glClearColor(1.0f, 1.0f, 0.0f, 1.0f);

// draw quad into texture attached to frame buffer object
glColor4f(1.0f, 1.0f, 1.0f, 1.0f);
glBegin(GL_QUADS);
    glColor4f(1.0f, 1.0f, 1.0f, 1.0f); glVertex2f(0.0f, 100.0f); // top left
    glColor4f(1.0f, 0.0f, 0.0f, 1.0f); glVertex2f(0.0f, 0.0f); // bottom left
    glColor4f(0.0f, 1.0f, 0.0f, 1.0f); glVertex2f(100.0f, 0.0f); // bottom right
    glColor4f(0.0f, 0.0f, 1.0f, 1.0f); glVertex2f(100.0f, 100.0f); // top right
glEnd();

// reset projection matrix
glMatrixMode(GL_PROJECTION);
glPopMatrix();

// reset modelview matrix
glMatrixMode(GL_MODELVIEW);
glPopMatrix();

// restore attributes
glPopAttrib();

glEnable(GL_DEPTH_TEST);
glEnable(GL_CULL_FACE);

// I guess, now it's OK to create MipMaps

// draw the scene
glMatrixMode(GL_MODELVIEW);
glPushMatrix();
glLoadIdentity();

glColor4f(1.0, 1.0, 1.0, 1.0);

// begin texture mapping
glEnable(GL_TEXTURE_2D);
    glTexEnvf(GL_TEXTURE_ENV, GL_TEXTURE_ENV_MODE, GL_MODULATE);

    // normal faces "camera"
    glNormal3d(0.0f, 0.0f, 1.0f);

    glBegin(GL_QUADS);
        glNormal3d(0.0f, 0.0f, 1.0f);
        glTexCoord2f(0.0f, 0.0f); glVertex3f(0.0f, 50.0f, -100.0f);    // top left
        glTexCoord2f(0.0f, 1.0f); glVertex3f(0.0f, 0.0f, -100.0f);     // bottom left
        glTexCoord2f(1.0f, 1.0f); glVertex3f(50.0f, 0.0f, -100.0f);    // bottom right
        glTexCoord2f(1.0f, 0.0f); glVertex3f(50.0f, 50.0f, -100.0f);   // top right
    glEnd();

glDisable(GL_TEXTURE_2D);

glPopMatrix();

// finish rendering
glFlush();
glFinish();

// swap buffers (I forgot to mention that I use SDL)
SDL_GL_SwapBuffers();

// do the clean up!

As you see, I got rid of the depth buffer attachment. I figured that I don't really need it for my task. As soon as I actually could draw things, I added the orthogonal projection. Without that, drawing to a texture results in pretty awkward images...

Hope this is useful for somebody out there Walter

4
votes

Weird, both glDrawBuffers and glFramebufferTexture2D don't look like the specs! The first one takes an array of GLenum, and not a GLenum (check warnings: it can be silently casted!). And the second one only take 4 arguments on the specs!

EDIT:

void glDrawBuffers(GLsizei  n,  const GLenum * bufs);
1
votes

I expect what is happening either:

  • You're rendering from an unpopulated mip-map level. You're calling glGenerateMipmap well before you've rendered the coloured quad. Just try GL_NEAREST for now.
  • You're getting an error when you create your framebuffer object. If that happens your rendering will just fall down a black hole.

Here's a snippet of code I use to check the frame buffer status. It uses the EXT version of FBOs and is in Objective-C (hence the @"" strings), but it should be obvious. It comes after I've called glBindFramebuffer, glFramebufferTexture2D & glDrawBuffers.

    GLenum framebufferStatus = glCheckFramebufferStatusEXT(GL_FRAMEBUFFER_EXT);

    switch (framebufferStatus) {
        case GL_FRAMEBUFFER_COMPLETE_EXT: break;
        case GL_FRAMEBUFFER_INCOMPLETE_ATTACHMENT_EXT:
            NSLog(@"Framebuffer Object %d Error: Attachment Point Unconnected", i);
            break;
        case GL_FRAMEBUFFER_INCOMPLETE_MISSING_ATTACHMENT_EXT:
            NSLog(@"Framebuffer Object %d Error: Missing Attachment", i);
            break;
        case GL_FRAMEBUFFER_INCOMPLETE_DIMENSIONS_EXT:
            NSLog(@"Framebuffer Object %d Error: Dimensions do not match", i);
            break;
        case GL_FRAMEBUFFER_INCOMPLETE_FORMATS_EXT:
            NSLog(@"Framebuffer Object %d Error: Formats", i);
            break;
        case GL_FRAMEBUFFER_INCOMPLETE_DRAW_BUFFER_EXT:
            NSLog(@"Framebuffer Object %d Error: Draw Buffer", i);
            break;
        case GL_FRAMEBUFFER_INCOMPLETE_READ_BUFFER_EXT:
            NSLog(@"Framebuffer Object %d Error: Read Buffer", i);
            break;
        case GL_FRAMEBUFFER_UNSUPPORTED_EXT:
            NSLog(@"Framebuffer Object %d Error: Unsupported Framebuffer Configuration", i);
            break;
        default:
            NSLog(@"Framebuffer Object %d Error: Unkown Framebuffer Object Failure", i);
            break;
    }

The problem is likely to be the depth buffer, as drivers tend to be quite picky about this. For this test you don't need depth so you could remove the render buffer stuff if it's causing issues.

Another note, the texture doesn't need to be bound and active for rendering to it. You could postpone that code until you want to use it to render your final image.