Rendering multiple depth information with FBOs

Question

I am trying to implement a shader computing the light refraction through two surfaces: the back and front of the object. To do so, I need to render the refractive geometry with the normal depth test (GL_LESS), and the reversed depth test (GL_GREATER). It would allow me to compute the distance from the back face to the front face. Unfortunately, I only manage to render one of those at a time, and I can't figure out how to pass both depth information as textures to the shader.

The shader itself shouldn't be a problem, but I'm struggling with setting up opengl so that it gives everything needed to the shader!

To be perfectly clear, I need to give two textures to my shader: - A texture with the depth information of the front faces of my object - A texture with the depth information of the back face of my object

Here is roughly what I did (simplified so that the code isn't too messy to read).

void FBO::init() {
    initDepthTexture();
    initFBO();
}

void FBO::initDepthTexture() {
    //32 bit depth texture, mWidth*mHeight
    glGenTextures(1, &mDepthTex);
    glBindTexture(GL_TEXTURE_2D, mDepthTex);
    glTexParameteri(GL_TEXTURE_2D, GL_TEXTURE_WRAP_S, GL_CLAMP_TO_EDGE);
    glTexParameteri(GL_TEXTURE_2D, GL_TEXTURE_WRAP_T, GL_CLAMP_TO_EDGE);
    glTexParameteri(GL_TEXTURE_2D, GL_TEXTURE_MIN_FILTER, GL_NEAREST);
    glTexParameteri(GL_TEXTURE_2D, GL_TEXTURE_MAG_FILTER, GL_NEAREST);
    glTexParameteri(GL_TEXTURE_2D, GL_DEPTH_TEXTURE_MODE, GL_INTENSITY);
    glTexParameteri(GL_TEXTURE_2D, GL_TEXTURE_COMPARE_MODE,
            GL_COMPARE_R_TO_TEXTURE);
    glTexParameteri(GL_TEXTURE_2D, GL_TEXTURE_COMPARE_FUNC, GL_LEQUAL);

    //NULL means reserve texture memory, but texels are undefined
    //You can also try GL_DEPTH_COMPONENT16, GL_DEPTH_COMPONENT24 for the internal format.
    //If GL_DEPTH24_STENCIL8_EXT, go ahead and use it (GL_EXT_packed_depth_stencil)
    glTexImage2D(GL_TEXTURE_2D, 0, GL_DEPTH_COMPONENT32, mWidth, mHeight, 0,
            GL_DEPTH_COMPONENT, GL_UNSIGNED_INT, NULL);
}

void FBO::initFBO() {
    glGenFramebuffersEXT(1, &mFrameBuffer);
    glBindFramebufferEXT(GL_FRAMEBUFFER_EXT, mFrameBuffer);
    //Attach
    glFramebufferTexture2DEXT(GL_FRAMEBUFFER_EXT, GL_DEPTH_ATTACHMENT_EXT,
            GL_TEXTURE_2D, mDepthTex, 0);
    //-------------------------
    //Does the GPU support current FBO configuration?
    //Before checking the configuration, you should call these 2 according to the spec.
    //At the very least, you need to call glDrawBuffer(GL_NONE)
    glDrawBuffer(GL_NONE);
    glReadBuffer(GL_NONE);

    checkFBO();

    renderToScreen();
}


void FBO::renderToFBO() {
    cout << "Render to FBO: " << mFrameBuffer << endl;
    glBindFramebufferEXT(GL_FRAMEBUFFER_EXT, mFrameBuffer); // Bind our frame buffer for rendering
    //-------------------------
    //----and to render to it, don't forget to call
    //At the very least, you need to call glDrawBuffer(GL_NONE)
    glDrawBuffer(GL_NONE);
    glReadBuffer(GL_NONE);
}

/**
 * Static
 */
void FBO::renderToScreen() {
    cout << "Render to screen " << endl;
    // Finish all operations
    //glFlush();
    //-------------------------
    //If you want to render to the back buffer again, you must bind 0 AND THEN CALL glDrawBuffer(GL_BACK)
    //else GL_INVALID_OPERATION will be raised
    glBindFramebufferEXT(GL_FRAMEBUFFER_EXT, 0); // Unbind our texture
    glDrawBuffer(GL_BACK);
    glReadBuffer(GL_BACK);
}

And here is how I use the FBOs: I first create two FBO outside of the render function, see the init() function to see how it is initialized. On the first FBO, I render the geometry depth from the front On the second FBO, I render the geometry depth from the back Then I render both depth texture to a full screen quad.

void Viewer::onRender() {
        FBO::renderToScreen();

            // XXX: Need of Z-Depth sorting to get alpha blending right!!
            glEnable(GL_DEPTH_TEST);

            glClearColor(0., 0., 0.2, 1.);
            glClear(GL_DEPTH_BUFFER_BIT | GL_COLOR_BUFFER_BIT);

            glClearDepth(1.);
            glDepthFunc(GL_LESS);

            // set the projection transformation
            glMatrixMode(GL_PROJECTION);
            glLoadIdentity();
            gluPerspective(45.0f, (GLdouble) m_width / (GLdouble) m_height,
                    m_scale * 5.0, m_scale * 10000.0);


            // set the model transformation
            glMatrixMode(GL_MODELVIEW);
            glLoadIdentity();
            glm::vec3 pos = mCamera->getPosition();
            glm::vec3 view = mCamera->getView();
            glm::vec3 up = mCamera->getUp();
            gluLookAt(pos.x, pos.y, pos.z, view.x, view.y, view.z, up.x, up.y,
                    up.z);


            static float rotationAngle = 0;
            rotationAngle+=5;

            static int i = 0;
            if(i++ < 200) {
            /**
             * Render geometry twice to FBOs
             */
            mFBO->renderToFBO();
            glClear(GL_DEPTH_BUFFER_BIT);
            glClearDepth(0.);
            glDepthFunc(GL_LESS);
            glPushMatrix();
            glRotatef(1, 1, 0, 120);
            glColor3f(0., 1., 0.);
            // Draw teapot
            glutSolidTeapot(1.8);
            glPopMatrix();

            mFBO2->renderToFBO();
            glClear(GL_DEPTH_BUFFER_BIT);
            glClearDepth(0.);
            glDepthFunc(GL_GREATER);
            glPushMatrix();
            glColor3f(0., 1., 0.);
            // Draw teapot
            glutSolidTeapot(3.5);
            glPopMatrix();


            /**
             * Render the same geometry to the screen
             */
            FBO::renderToScreen();
            } else {
            mShader->enable();
            mShader->setTextureFromId("frontDepth", mFBO->getDepthTextureId());
            mShader->setTextureFromId("backDepth", mFBO2->getDepthTextureId());
            glBegin(GL_QUADS); // Draw A Quad
            glTexCoord2f(0, 1);
            glVertex3f(-1.0f, 1.0f, 0.0f); // Top Left
            glTexCoord2f(1, 1);
            glVertex3f(1.0f, 1.0f, 0.0f); // Top Right
            glTexCoord2f(1, 0);
            glVertex3f(1.0f, -1.0f, 0.0f); // Bottom Right
            glTexCoord2f(0, 0);
            glVertex3f(-1.0f, -1.0f, 0.0f); // Bottom Left
            glEnd(); // Done Drawing The Quad
            mShader->disable();
        }
    }

This is working perfectly if render to the FBO and then render on the quad. In the example above, I render 200 times to the FBO, then stop rendering to the FBO and display the texture on my fullscreen quad. Here is the result, as expected (for display purposes, I rendered the second geometry smaller than the first one):

Here is the code (nearly the same as for the working image, but rendering the quad for every frame)

void Viewer::onRender() {
            FBO::renderToScreen();

        // XXX: Need of Z-Depth sorting to get alpha blending right!!
        glEnable(GL_DEPTH_TEST);

        glClearColor(0., 0., 0.2, 1.);
        glClear(GL_DEPTH_BUFFER_BIT | GL_COLOR_BUFFER_BIT);

        glClearDepth(1.);
        glDepthFunc(GL_LESS);

        // set the projection transformation
        glMatrixMode(GL_PROJECTION);
        glLoadIdentity();
        gluPerspective(45.0f, (GLdouble) m_width / (GLdouble) m_height,
                m_scale * 5.0, m_scale * 10000.0);


        // set the model transformation
        glMatrixMode(GL_MODELVIEW);
        glLoadIdentity();
        glm::vec3 pos = mCamera->getPosition();
        glm::vec3 view = mCamera->getView();
        glm::vec3 up = mCamera->getUp();
        gluLookAt(pos.x, pos.y, pos.z, view.x, view.y, view.z, up.x, up.y,
                up.z);


        static float rotationAngle = 0;
        rotationAngle+=5;


        /**
         * Render geometry twice to FBOs
         */
        mFBO->renderToFBO();
        glClear(GL_DEPTH_BUFFER_BIT);
        glClearDepth(0.);
        glDepthFunc(GL_LESS);
        glPushMatrix();
        glRotatef(1, 1, 0, 120);
        glColor3f(0., 1., 0.);
        // Draw teapot
        glutSolidTeapot(1.8);
        glPopMatrix();

        mFBO2->renderToFBO();
        glClear(GL_DEPTH_BUFFER_BIT);
        glClearDepth(0.);
        glDepthFunc(GL_GREATER);
        glPushMatrix();
        glColor3f(0., 1., 0.);
        // Draw teapot
        glutSolidTeapot(3.5);
        glPopMatrix();


            /**
             * Render both depth texture on a fullscreen quad
             **/
        FBO::renderToScreen();
        mShader->enable();
        mShader->setTextureFromId("frontDepth", mFBO->getDepthTextureId());
        mShader->setTextureFromId("backDepth", mFBO2->getDepthTextureId());
        glBegin(GL_QUADS); // Draw A Quad
        glTexCoord2f(0, 1);
        glVertex3f(-1.0f, 1.0f, 0.0f); // Top Left
        glTexCoord2f(1, 1);
        glVertex3f(1.0f, 1.0f, 0.0f); // Top Right
        glTexCoord2f(1, 0);
        glVertex3f(1.0f, -1.0f, 0.0f); // Bottom Right
        glTexCoord2f(0, 0);
        glVertex3f(-1.0f, -1.0f, 0.0f); // Bottom Left
        glEnd(); // Done Drawing The Quad
        mShader->disable();
    }
}

But now, my problem occurs when I render to the FBOs, and then try to display the quad at every frame. I get a weird result, which seems to take into account only a small part of the geometry:

I can't figure out why this is happening. It's definitely rendering to the depth texture, but it seems that for some reason rendering a full screen quad alters the rendering of the FBO geometry.

[Edit] I just tried saving opengl state, and restoring it after the quad...

            FBO::renderToScreen();
    glPushAttrib(GL_ALL_ATTRIB_BITS);

        mShader->enable();
        mShader->setTextureFromId("frontDepth", mFBO->getDepthTextureId());
        mShader->setTextureFromId("backDepth", mFBO2->getDepthTextureId());
        glBegin(GL_QUADS); // Draw A Quad
        glTexCoord2f(0, 1);
        glVertex3f(-1.0f, 1.0f, 0.0f); // Top Left
        glTexCoord2f(1, 1);
        glVertex3f(1.0f, 1.0f, 0.0f); // Top Right
        glTexCoord2f(1, 0);
        glVertex3f(1.0f, -1.0f, 0.0f); // Bottom Right
        glTexCoord2f(0, 0);
        glVertex3f(-1.0f, -1.0f, 0.0f); // Bottom Left
        glEnd(); // Done Drawing The Quad
        mShader->disable();
            glPopAttrib();

Well, that works, I can move around the scene add objects and everthing without any kind of trouble. However I'm still curious as to which state change could have cause the rendering process to fail so much, any idea?

Answer 1

Since the problem is fixed (even if I don't understand which state was putting everything off), here is the full code of my FBO class and render function, in case someone runs into the same kind of problem:

    /*
 * FBO.cpp
 *
 *  Created on: 28 Mar 2013
 *      Author: arnaud
 */

// Include GLEW
#include <GL/glew.h>
#include "FBO.h"
#include <GL/glext.h>
#include <iostream>
#include "FBOException.h"

using namespace std;

FBO::FBO(int width, int height) {
    mWidth = width;
    mHeight = height;
    // TODO Auto-generated constructor stubc
    init();

}

FBO::~FBO() {
    // TODO Auto-generated destructor stub
    glDeleteFramebuffersEXT(1, &mFrameBuffer);
}

void FBO::init() {
    initDepthTexture();
    initFBO();
}

void FBO::initDepthTexture() {
    //32 bit depth texture, mWidth*mHeight
    glGenTextures(1, &mDepthTex);
    glBindTexture(GL_TEXTURE_2D, mDepthTex);
    glTexParameteri(GL_TEXTURE_2D, GL_TEXTURE_WRAP_S, GL_CLAMP_TO_EDGE);
    glTexParameteri(GL_TEXTURE_2D, GL_TEXTURE_WRAP_T, GL_CLAMP_TO_EDGE);
    glTexParameteri(GL_TEXTURE_2D, GL_TEXTURE_MIN_FILTER, GL_NEAREST);
    glTexParameteri(GL_TEXTURE_2D, GL_TEXTURE_MAG_FILTER, GL_NEAREST);
    glTexParameteri(GL_TEXTURE_2D, GL_DEPTH_TEXTURE_MODE, GL_INTENSITY);
    glTexParameteri(GL_TEXTURE_2D, GL_TEXTURE_COMPARE_MODE,
            GL_COMPARE_R_TO_TEXTURE);
    glTexParameteri(GL_TEXTURE_2D, GL_TEXTURE_COMPARE_FUNC, GL_LEQUAL);

    //NULL means reserve texture memory, but texels are undefined
    //You can also try GL_DEPTH_COMPONENT16, GL_DEPTH_COMPONENT24 for the internal format.
    //If GL_DEPTH24_STENCIL8_EXT, go ahead and use it (GL_EXT_packed_depth_stencil)
    glTexImage2D(GL_TEXTURE_2D, 0, GL_DEPTH_COMPONENT32, mWidth, mHeight, 0,
            GL_DEPTH_COMPONENT, GL_UNSIGNED_INT, NULL);
}

void FBO::initFBO() {
    glGenFramebuffersEXT(1, &mFrameBuffer);
    glBindFramebufferEXT(GL_FRAMEBUFFER_EXT, mFrameBuffer);
    //Attach
    glFramebufferTexture2DEXT(GL_FRAMEBUFFER_EXT, GL_DEPTH_ATTACHMENT_EXT,
            GL_TEXTURE_2D, mDepthTex, 0);
    //-------------------------
    //Does the GPU support current FBO configuration?
    //Before checking the configuration, you should call these 2 according to the spec.
    //At the very least, you need to call glDrawBuffer(GL_NONE)
    glDrawBuffer(GL_NONE);
    glReadBuffer(GL_NONE);

    checkFBO();

    renderToScreen();
}

void FBO::checkFBO() throw () {
    GLenum status;
    status = glCheckFramebufferStatusEXT(GL_FRAMEBUFFER_EXT);
    switch (status) {
    case GL_FRAMEBUFFER_COMPLETE_EXT:
        cout << "Good Framebuffer" << endl;
        break;
    case GL_FRAMEBUFFER_UNDEFINED:
        throw new FBOException(
                "Framebuffer undefined. Usually returned if  returned if target is the default framebuffer, but the default framebuffer does not exist.");
        break;
    case GL_FRAMEBUFFER_INCOMPLETE_ATTACHMENT:
        throw new FBOException(
                "Incomplete Attachement: is returned if any of the framebuffer attachment points are framebuffer incomplete.");
        break;
    case GL_FRAMEBUFFER_INCOMPLETE_MISSING_ATTACHMENT:
        throw new FBOException(
                "Incomplete Missing Attachment: is returned if the framebuffer does not have at least one image attached to it.");
        break;
    case GL_FRAMEBUFFER_INCOMPLETE_DRAW_BUFFER:
        throw new FBOException(
                "Incomplete Draw Buffer: is returned if the value of GL_FRAMEBUFFER_ATTACHMENT_OBJECT_TYPE is GL_NONE for any color attachment point(s) named by GL_DRAWBUFFERi");
        break;
    case GL_FRAMEBUFFER_INCOMPLETE_READ_BUFFER:
        throw new FBOException(
                "Incomplete Read Buffer: is returned if GL_READ_BUFFER is not GL_NONE and the value of\\"
                        " GL_FRAMEBUFFER_ATTACHMENT_OBJECT_TYPE is GL_NONE for the color attachment point named by GL_READ_BUFFER");
        break;
    case GL_FRAMEBUFFER_UNSUPPORTED:
        throw new FBOException(
                "Framebuffer Unsupported: is returned if the combination of internal formats of the attached images violates an implementation-dependent set of restrictions.");
        break;
    case GL_FRAMEBUFFER_INCOMPLETE_MULTISAMPLE:
        throw new FBOException("Incomplete Multisample");
        break;
    case GL_FRAMEBUFFER_INCOMPLETE_LAYER_TARGETS:
        throw new FBOException("Incomplete Layer Targets");
        break;
    default:
        throw new FBOException("Bad Framebuffer");
    }
}


/****
 * PUBLIC Functions
 */

void FBO::renderToFBO() {
    cout << "Render to FBO: " << mFrameBuffer << endl;
    glBindFramebufferEXT(GL_FRAMEBUFFER_EXT, mFrameBuffer); // Bind our frame buffer for rendering
    //-------------------------
    //----and to render to it, don't forget to call
    //At the very least, you need to call glDrawBuffer(GL_NONE)
    glDrawBuffer(GL_NONE);
    glReadBuffer(GL_NONE);
}

/**
 * Static
 */
void FBO::renderToScreen() {
    cout << "Render to screen " << endl;
    // Finish all operations
    //glFlush();
    //-------------------------
    //If you want to render to the back buffer again, you must bind 0 AND THEN CALL glDrawBuffer(GL_BACK)
    //else GL_INVALID_OPERATION will be raised
    glBindFramebufferEXT(GL_FRAMEBUFFER_EXT, 0); // Unbind our texture
    glDrawBuffer(GL_BACK);
    glReadBuffer(GL_BACK);
}

And here is the render function

void Viewer::onRender() {
    // Get elapsed time since last loop
    sf::Time time = mClock.getElapsedTime();
    float ellapsedTime = time.asMilliseconds();
    if (time.asMilliseconds() > 1000 / 60) {

        // XXX: Need of Z-Depth sorting to get alpha blending right!!
        glEnable(GL_DEPTH_TEST);

        glClearColor(0., 0., 0.2, 1.);
        glClear(GL_DEPTH_BUFFER_BIT | GL_COLOR_BUFFER_BIT);

        glClearDepth(1.);
        glDepthFunc(GL_LESS);

        // set the projection transformation
        glMatrixMode(GL_PROJECTION);
        glLoadIdentity();
        gluPerspective(45.0f, (GLdouble) m_width / (GLdouble) m_height,
                m_scale * 5.0, m_scale * 10000.0);


        // set the model transformation
        glMatrixMode(GL_MODELVIEW);
        glLoadIdentity();
        glm::vec3 pos = mCamera->getPosition();
        glm::vec3 view = mCamera->getView();
        glm::vec3 up = mCamera->getUp();
        gluLookAt(pos.x, pos.y, pos.z, view.x, view.y, view.z, up.x, up.y,
                up.z);


        static float rotationAngle = 0;
        rotationAngle+=5;

        /**
         * Render geometry twice to FBOs
         */
        mFBO->renderToFBO();
        glClear(GL_DEPTH_BUFFER_BIT);
        glClearDepth(0.);
        glDepthFunc(GL_LESS);
        glPushMatrix();
        glColor3f(0., 1., 0.);
        // Draw teapot
        glutSolidTeapot(3.5);
        glPopMatrix();

        mFBO2->renderToFBO();
        glClear(GL_DEPTH_BUFFER_BIT);
        glClearDepth(0.);
        glDepthFunc(GL_GREATER);
        glPushMatrix();
        glColor3f(0., 1., 0.);
        // Draw teapot
        glutSolidTeapot(3.5);
        glPopMatrix();


        /**
         * Render the same geometry to the screen
         */
        FBO::renderToScreen();
        // XXX: Save attribs bits to fix FBO problem... (why is this needed!?)
        glPushAttrib(GL_ALL_ATTRIB_BITS);
        mShader->enable();
        mShader->setTextureFromId("frontDepth", mFBO->getDepthTextureId());
        mShader->setTextureFromId("backDepth", mFBO2->getDepthTextureId());
        glBegin(GL_QUADS); // Draw A Quad
        glTexCoord2f(0, 1);
        glVertex3f(-1.0f, 1.0f, 0.0f); // Top Left
        glTexCoord2f(1, 1);
        glVertex3f(1.0f, 1.0f, 0.0f); // Top Right
        glTexCoord2f(1, 0);
        glVertex3f(1.0f, -1.0f, 0.0f); // Bottom Right
        glTexCoord2f(0, 0);
        glVertex3f(-1.0f, -1.0f, 0.0f); // Bottom Left
        glEnd(); // Done Drawing The Quad
        mShader->disable();
        glPopAttrib();
    }

}

Answer 2

What you're trying to do is a technique called "Depth Peeling" which can essentially be described as a form of insertion sort into a number of depth buffer layers. There are a number of presentations and papers available online.