Get depth stored in different FBOs that has different shaders - opengl

The goal of my program is to compare between the depth of the RGB-D camera and OpenGL depth.
The camera is facing a model (real one) and it has its own shaders (fragment and vertex).
To make comparison, a 3D Model is made, this one has also its own shaders (the depth is normally linearized as I followed what is explained in this tutorial: https://learnopengl.com/Advanced-OpenGL/Depth-testing).
So the comparison will be done on the depth received from the camera and the depth related to the 3D model.
So,the steps I have done are:
1.In the initialization function
Create two textures for each one (color and depth textures).
Generate two FBOs (one for the camera, the other one for the 3D Model).
Attach textures to their FBOs.
/* Other stuff needed for initialization */
...
// Generate color texture
glGenTextures(1, &colorTex);
glBindTexture(GL_TEXTURE_2D, colorTex);
glTexParameteri(GL_TEXTURE_2D, GL_TEXTURE_MAG_FILTER, GL_NEAREST);
glTexParameteri(GL_TEXTURE_2D, GL_TEXTURE_MIN_FILTER, GL_NEAREST);
glTexImage2D(GL_TEXTURE_2D, 0, GL_RGBA32F, _windowWidth, _windowHeight, 0, GL_RGBA, GL_UNSIGNED_BYTE, NULL);
glBindTexture(GL_TEXTURE_2D, 0);
// Generate depth texture
glGenTextures(1, &depthTex);
glBindTexture(GL_TEXTURE_2D, depthTex);
glTexParameteri(GL_TEXTURE_2D, GL_TEXTURE_MAG_FILTER, GL_NEAREST);
glTexParameteri(GL_TEXTURE_2D, GL_TEXTURE_MIN_FILTER, GL_NEAREST);
glTexImage2D(GL_TEXTURE_2D, 0, GL_DEPTH_COMPONENT, _windowWidth, _windowHeight, 0, GL_DEPTH_COMPONENT, GL_FLOAT, NULL);
glBindTexture(GL_TEXTURE_2D, 0);
// Attach the color and depth textures to current FrameBuffer
glGenFramebuffers(1, &_fboGL);
glBindFramebuffer(GL_FRAMEBUFFER, _fboGL);
glFramebufferTexture2D(GL_FRAMEBUFFER, GL_COLOR_ATTACHMENT0, GL_TEXTURE_2D, colorTex, 0);
glFramebufferTexture2D(GL_FRAMEBUFFER, GL_DEPTH_ATTACHMENT, GL_TEXTURE_2D, depthTex, 0);
In the main function for Drawing and rendering
Render the camera stream by binding its FBO.
Draw 3D Model by binding its own FBO.
Try to print openGL depth related to the 3D Model.
A fragment of the code looks like below:
glBindFramebuffer(GL_FRAMEBUFFER, fbo);
glFramebufferTexture2D(GL_FRAMEBUFFER, GL_COLOR_ATTACHMENT0, GL_TEXTURE_2D, camColorTex, 0);
glFramebufferTexture2D(GL_FRAMEBUFFER, GL_DEPTH_ATTACHMENT, GL_TEXTURE_2D, camDepthTex, 0);
glViewport(0, 0, windowWidth, windowHeight);
glEnable(GL_DEPTH_TEST);
glClearColor(0.25f, 0.25f, 0.25f, 1.f);
glClear(GL_COLOR_BUFFER_BIT | GL_DEPTH_BUFFER_BIT);
/*******************************************/
/* Some code for displaying camera stream */
/*******************************************/
.............
drawModelFunction();
printDepthFunction();
glBindFramebuffer(GL_FRAMEBUFFER, fbo);
glFramebufferTexture2D(GL_FRAMEBUFFER, GL_COLOR_ATTACHMENT0, GL_TEXTURE_2D, camColorTex, 0);
glFramebufferTexture2D(GL_FRAMEBUFFER, GL_DEPTH_ATTACHMENT, GL_TEXTURE_2D, camDepthTex, 0);
/***********************************************************/
/* Some stuff to render a GUI to interact with the program */
/***********************************************************/
glBindFramebuffer(GL_DRAW_FRAMEBUFFER, 0);
glBlitFramebuffer(0, 0, _windowWidth, _windowHeight, 0, 0, _windowWidth, _windowHeight, GL_COLOR_BUFFER_BIT, GL_LINEAR);
glBlitFramebuffer(0, 0, _windowWidth, _windowHeight, 0, 0, _windowWidth, _windowHeight, GL_DEPTH_BUFFER_BIT, GL_NEAREST);
A fragment code for drawModelFunction()
glBindFramebuffer(GL_FRAMEBUFFER, fboGL);
glFramebufferTexture2D(GL_FRAMEBUFFER, GL_COLOR_ATTACHMENT0, GL_TEXTURE_2D, colorTex, 0);
glFramebufferTexture2D(GL_FRAMEBUFFER, GL_DEPTH_ATTACHMENT, GL_TEXTURE_2D, depthTex, 0);
/*************************************/
/* Some stuff to render the 3D Model */
/*************************************/
A fragment code for printDepthFunction()
GLfloat *pixels = (GLfloat *)malloc(_windowWidth * _windowHeight * sizeof *pixels);
assert(pixels);
glBindTexture(GL_TEXTURE_2D, depthTex);
glGetTexImage(GL_TEXTURE_2D, 0, GL_DEPTH_COMPONENT, GL_FLOAT, pixels);
/* print the the values stored in **pixels** */
free(pixels);
glBindFramebuffer(GL_FRAMEBUFFER, 0);
When I try to display the values stored in the depth, I only obtain one value "1.0f" and the color returns me only the value "0.0f". In addition to that, the rendering is not here as I mixed everything with FBOs. I have a screen with mixture of camera stream, a red square inserted in the middle of the screen with the GUI in it (that took the red color of the square). And No 3D Model. (This is the result I have obtained after binding the framebuffers everywhere like shown before).

Related

OpenGL MSAA in 2 different ways. What are the differences?

I have been looking for OpenGL Multi Sample Anti Aliasing tutorials and I found many but I'll take 2.
They use a different way to do this. I have tested both ways and both work for my project so I can use any of them.
I use this to render my game engine scene to a texture.
This is the 1st way:
Create the FBO with MSAA
// create a texture object
glGenTextures(1, &textureId);
glBindTexture(GL_TEXTURE_2D, textureId);
glTexParameteri(GL_TEXTURE_2D, GL_TEXTURE_MAG_FILTER, GL_LINEAR);
glTexParameteri(GL_TEXTURE_2D, GL_TEXTURE_MIN_FILTER, GL_LINEAR);
glTexParameteri(GL_TEXTURE_2D, GL_TEXTURE_WRAP_S, GL_CLAMP_TO_EDGE);
glTexParameteri(GL_TEXTURE_2D, GL_TEXTURE_WRAP_T, GL_CLAMP_TO_EDGE);
glTexImage2D(GL_TEXTURE_2D, 0, GL_RGBA8, width, height, 0, GL_RGBA, GL_UNSIGNED_BYTE, 0);
glBindTexture(GL_TEXTURE_2D, 0);
// create a MSAA framebuffer object
// NOTE: All attachment images must have the same # of samples.
// Ohterwise, the framebuffer status will not be completed.
glGenFramebuffers(1, &fboMsaaId);
glBindFramebuffer(GL_FRAMEBUFFER, fboMsaaId);
// create a MSAA renderbuffer object to store color info
glGenRenderbuffers(1, &rboColorId);
glBindRenderbuffer(GL_RENDERBUFFER, rboColorId);
glRenderbufferStorageMultisample(GL_RENDERBUFFER, MSAA_level, GL_RGB8, width, height);
glBindRenderbuffer(GL_RENDERBUFFER, 0);
// create a MSAA renderbuffer object to store depth info
// NOTE: A depth renderable image should be attached the FBO for depth test.
// If we don't attach a depth renderable image to the FBO, then
// the rendering output will be corrupted because of missing depth test.
// If you also need stencil test for your rendering, then you must
// attach additional image to the stencil attachement point, too.
glGenRenderbuffers(1, &rboDepthId);
glBindRenderbuffer(GL_RENDERBUFFER, rboDepthId);
glRenderbufferStorageMultisample(GL_RENDERBUFFER, MSAA_level, GL_DEPTH_COMPONENT, width, height);
glBindRenderbuffer(GL_RENDERBUFFER, 0);
// attach msaa RBOs to FBO attachment points
glFramebufferRenderbuffer(GL_FRAMEBUFFER, GL_COLOR_ATTACHMENT0, GL_RENDERBUFFER, rboColorId);
glFramebufferRenderbuffer(GL_FRAMEBUFFER, GL_DEPTH_ATTACHMENT, GL_RENDERBUFFER, rboDepthId);
// create a normal (no MSAA) FBO to hold a render-to-texture
glGenFramebuffers(1, &fboId);
glBindFramebuffer(GL_FRAMEBUFFER, fboId);
glGenRenderbuffers(1, &rboId);
glBindRenderbuffer(GL_RENDERBUFFER, rboId);
glRenderbufferStorage(GL_RENDERBUFFER, GL_DEPTH_COMPONENT, width, height);
glBindRenderbuffer(GL_RENDERBUFFER, 0);
// attach a texture to FBO color attachement point
glFramebufferTexture2D(GL_FRAMEBUFFER, GL_COLOR_ATTACHMENT0, GL_TEXTURE_2D, textureId, 0);
// attach a rbo to FBO depth attachement point
glFramebufferRenderbuffer(GL_FRAMEBUFFER, GL_DEPTH_ATTACHMENT, GL_RENDERBUFFER, rboId);
//## disable color buffer if you don't attach any color buffer image,
//## for example, rendering the depth buffer only to a texture.
//## Otherwise, glCheckFramebufferStatus will not be complete.
//glDrawBuffer(GL_NONE);
//glReadBuffer(GL_NONE);
// check FBO status
if (glCheckFramebufferStatus(GL_FRAMEBUFFER) != GL_FRAMEBUFFER_COMPLETE)
return false;
glBindFramebuffer(GL_FRAMEBUFFER, 0);
And when I need to draw the scene
glBindFramebuffer(GL_FRAMEBUFFER, fboMsaaId);
glViewport(0, 0, width, height);
glClearColor(0.1f, 0.1f, 0.1f, 1.0f);
glClear(GL_COLOR_BUFFER_BIT | GL_DEPTH_BUFFER_BIT);
DrawScene();
glBindFramebuffer(GL_READ_FRAMEBUFFER, fboMsaaId);
glBindFramebuffer(GL_DRAW_FRAMEBUFFER, fboId);
glBlitFramebuffer(0, 0, width, height, // src rect
0, 0, width, height, // dst rect
GL_COLOR_BUFFER_BIT, // buffer mask
GL_LINEAR); // scale filter
glBindFramebuffer(GL_FRAMEBUFFER, 0);
glViewport(0, 0, App->window->GetWidth(), App->window->GetHeight());
The 2nd way:
Create the FBO with MSAA
unsigned int framebuffer;
glGenFramebuffers(1, &framebuffer);
glBindFramebuffer(GL_FRAMEBUFFER, framebuffer);
// create a multisampled color attachment texture
unsigned int textureColorBufferMultiSampled;
glGenTextures(1, &textureColorBufferMultiSampled);
glBindTexture(GL_TEXTURE_2D_MULTISAMPLE, textureColorBufferMultiSampled);
glTexImage2DMultisample(GL_TEXTURE_2D_MULTISAMPLE, 4, GL_RGB, SCR_WIDTH, SCR_HEIGHT, GL_TRUE);
glBindTexture(GL_TEXTURE_2D_MULTISAMPLE, 0);
glFramebufferTexture2D(GL_FRAMEBUFFER, GL_COLOR_ATTACHMENT0, GL_TEXTURE_2D_MULTISAMPLE, textureColorBufferMultiSampled, 0);
// create a (also multisampled) renderbuffer object for depth and stencil attachments
unsigned int rbo;
glGenRenderbuffers(1, &rbo);
glBindRenderbuffer(GL_RENDERBUFFER, rbo);
glRenderbufferStorageMultisample(GL_RENDERBUFFER, 4, GL_DEPTH24_STENCIL8, SCR_WIDTH, SCR_HEIGHT);
glBindRenderbuffer(GL_RENDERBUFFER, 0);
glFramebufferRenderbuffer(GL_FRAMEBUFFER, GL_DEPTH_STENCIL_ATTACHMENT, GL_RENDERBUFFER, rbo);
if (glCheckFramebufferStatus(GL_FRAMEBUFFER) != GL_FRAMEBUFFER_COMPLETE)
cout << "ERROR::FRAMEBUFFER:: Framebuffer is not complete!" << endl;
glBindFramebuffer(GL_FRAMEBUFFER, 0);
// configure second post-processing framebuffer
unsigned int intermediateFBO;
glGenFramebuffers(1, &intermediateFBO);
glBindFramebuffer(GL_FRAMEBUFFER, intermediateFBO);
// create a color attachment texture
unsigned int screenTexture;
glGenTextures(1, &screenTexture);
glBindTexture(GL_TEXTURE_2D, screenTexture);
glTexImage2D(GL_TEXTURE_2D, 0, GL_RGB, SCR_WIDTH, SCR_HEIGHT, 0, GL_RGB, GL_UNSIGNED_BYTE, NULL);
glTexParameteri(GL_TEXTURE_2D, GL_TEXTURE_MIN_FILTER, GL_LINEAR);
glTexParameteri(GL_TEXTURE_2D, GL_TEXTURE_MAG_FILTER, GL_LINEAR);
glFramebufferTexture2D(GL_FRAMEBUFFER, GL_COLOR_ATTACHMENT0, GL_TEXTURE_2D, screenTexture, 0); // we only need a color buffer
if (glCheckFramebufferStatus(GL_FRAMEBUFFER) != GL_FRAMEBUFFER_COMPLETE)
cout << "ERROR::FRAMEBUFFER:: Intermediate framebuffer is not complete!" << endl;
glBindFramebuffer(GL_FRAMEBUFFER, 0);
And when I need to draw the scene:
glBindFramebuffer(GL_FRAMEBUFFER, framebuffer);
glViewport(0, 0, width, height);
glClearColor(0.1f, 0.1f, 0.1f, 1.0f);
glClear(GL_COLOR_BUFFER_BIT | GL_DEPTH_BUFFER_BIT);
DrawScene();
glBindFramebuffer(GL_READ_FRAMEBUFFER, framebuffer);
glBindFramebuffer(GL_DRAW_FRAMEBUFFER, intermediateFBO);
glBlitFramebuffer(0, 0, SCR_WIDTH, SCR_HEIGHT, 0, 0, SCR_WIDTH, SCR_HEIGHT, GL_COLOR_BUFFER_BIT, GL_LINEAR);
glBindFramebuffer(GL_FRAMEBUFFER, 0);
glViewport(0, 0, App->window->GetWidth(), App->window->GetHeight());
Summarizing:
1st way: Create FBO, create RBO for color and RBO for depth and use glRenderbufferStorageMultisample(...) to specify the MSAA level. Then create other FBO with texture for color and RBO for depth.
2nd way: Create FBO, create Texture for color and RBO for depth and using glTexImage2DMultisample(...) for MSAA level in the texture. Then create other FBO and a texture.
What are the differences on using one way or other? Is one better than the other?
MSAA setup in your example is actually the same in both cases. The only difference between the two methods you depicted is - different FBO attachment type. In general, you will want to attach a texture and not render buffer when you later need to use the information from that FBO for further render passes. In such a case you would plug previous render pass FBO's texture attachment into texture unit, and sample from it in the next pass shader program. Shadow mapping is one of such cases.

What's wrong with my Depth Texture?

At the moment I'm playing with FBO, which I don't really master. I have no problems with color atachments, but I can't figure out how to use depth textures in my render.
At the moment, I render my scene on a FBO (color+depth), then I draw a textured quad using the depth texture... But it doesn't work, my screen stays black (everyhting is 0 : even if i multiply by 1000 it stays at 0). (Depth test works : if I render the color texture instead of the depth, depth test works correctly)
Rendering loop :
///Render to FBO
glEnable(GL_DEPTH_TEST);
FBO.Bind(); //Do I really need to explain what it does ?
glUseProgram(ShaderProgram->getProgramID());
glClear(GL_COLOR_BUFFER_BIT | GL_DEPTH_BUFFER_BIT);
///Setting uniforms etc
mainScene.drawAll();
//draw the quad
glBindFramebuffer(GL_FRAMEBUFFER, 0);
glClear(GL_COLOR_BUFFER_BIT);
glDisable(GL_DEPTH_TEST);
glViewport(0, 0, m_window.getSize().x, m_window.getSize().y);
glUseProgram(2DShader.getProgramID());
quadModel.bindVAO();
glBindTexture(GL_TEXTURE_2D, FBO.DepthTexureID);
glDrawArrays(GL_TRIANGLES, 0, 6);
And FBO creation :
glGenFramebuffers(1, &FBO_ID);
glBindFramebuffer(GL_FRAMEBUFFER, FBO_ID);
glGenTextures(1, &ColorBufferID);
glBindTexture(GL_TEXTURE_2D, ColorBufferID);
glTexImage2D(GL_TEXTURE_2D, 0, GL_RGBA, width, height, 0, GL_RGBA, GL_UNSIGNED_BYTE, 0);
glTexParameteri(GL_TEXTURE_2D, GL_TEXTURE_MIN_FILTER, GL_LINEAR);
glTexParameteri(GL_TEXTURE_2D, GL_TEXTURE_MAG_FILTER, GL_LINEAR);
glFramebufferTexture2D(GL_FRAMEBUFFER, GL_COLOR_ATTACHMENT0, GL_TEXTURE_2D, ColorBufferID, 0);
glBindTexture(GL_TEXTURE_2D, 0);
glGenTextures(1, &DepthTexture);
glBindTexture(GL_TEXTURE_2D, DepthTexture);
glTexImage2D(GL_TEXTURE_2D, 0, GL_DEPTH_COMPONENT, width, height, 0, GL_DEPTH_COMPONENT, GL_UNSIGNED_BYTE, 0);
glFramebufferTexture2D(GL_FRAMEBUFFER, GL_DEPTH_ATTACHMENT, GL_TEXTURE_2D, DepthTexture, 0);
glBindTexture(GL_TEXTURE_2D, 0);
Any idea ?
Your depth texture is not mipmap-complete, but you are using the default mipmap filtering, so sampling will fail. Just specify some non-mipmapping filters like GL_NEAREST or LINEAR for sampling that texture.

Deferred shading skybox, rendered after full screen quad

So as a final step in my deferred shader I've been trying to get a skybox to work. Essentially I go through all the regular deferred shader steps and then use some post processing stuff like HDR and bloom, this final step is rendered to a full screen quad using a forward shader. Now if I just render the skybox separately or the deferred shading results everything looks fine and normal. It's when I try to render the skybox last that it doesn't end up on the screen.
So basically after I render the quad with the results I want to render the skybox using the depth values from the geometry pass. I've tried blitting the depth buffer as Deferred Rendering Skybox OpenGL suggests but it doesn't have any effect and I think the issue is with how I set up my depth buffer for the FBO since it shares a stencil attachment.
My render call for the skybox:
//draw skybox
glEnable(GL_DEPTH_TEST);
glBindFramebuffer(GL_READ_FRAMEBUFFER, mFBO);
glBindFramebuffer(GL_DRAW_FRAMEBUFFER, 0);
glBlitFramebuffer(0, 0, mWindowWidth, mWindowHeight, 0, 0, mWindowWidth, mWindowHeight, GL_DEPTH_BUFFER_BIT, GL_NEAREST);
glBindFramebuffer(GL_FRAMEBUFFER, 0);
// Draw skybox as last
glDepthFunc(GL_LEQUAL); // Change depth function so depth test passes when values are equal to depth buffer's content
ShaderMan.bindShader(SKY_BOX_SHADER);
glm::mat4 view = glm::mat4(glm::mat3(CameraMan.getActiveCamera()->getViewMatrix())); // Remove any translation component of the view matrix
glm::mat4 projection = CameraMan.getActiveCamera()->getProjectionMatrix();
glUniformMatrix4fv(ShaderMan.getActiveShader().getUniformLocation("viewMatrix"), 1, GL_FALSE, glm::value_ptr(view));
glUniformMatrix4fv(ShaderMan.getActiveShader().getUniformLocation("projectionMatrix"), 1, GL_FALSE, glm::value_ptr(projection));
// skybox cube
glUniform1i(ShaderMan.getActiveShader().getUniformLocation("skybox"), 0);
mSkybox.render();
glDepthFunc(GL_LESS); // Set depth function back to default
ShaderMan.unbindShader();
and how I set up the deferred FBO
//POSIITON
glGenTextures(1, &mPositionTexture);
glBindTexture(GL_TEXTURE_2D, mPositionTexture);
glTexImage2D(GL_TEXTURE_2D, 0, GL_RGB32F, mWindowWidth, mWindowHeight, 0, GL_RGB, GL_FLOAT, NULL);
//Nearest min mag filters since we dont want interpolated values
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_TEXTURE_WRAP_S, GL_CLAMP_TO_EDGE);
glTexParameteri(GL_TEXTURE_2D, GL_TEXTURE_WRAP_T, GL_CLAMP_TO_EDGE);
//glFrameBuffer attaches a texture as a color attachment, depth attachment or stencil etc
glFramebufferTexture2D(GL_FRAMEBUFFER, GL_COLOR_ATTACHMENT0, GL_TEXTURE_2D, mPositionTexture, 0);
glGenTextures(1, &mNormalTexture);
glBindTexture(GL_TEXTURE_2D, mNormalTexture);
glTexImage2D(GL_TEXTURE_2D, 0, GL_RGB32F, mWindowWidth, mWindowHeight, 0, GL_RGB, GL_FLOAT, NULL);
glTexParameteri(GL_TEXTURE_2D, GL_TEXTURE_MIN_FILTER, GL_NEAREST);
glTexParameteri(GL_TEXTURE_2D, GL_TEXTURE_MAG_FILTER, GL_NEAREST);
glFramebufferTexture2D(GL_FRAMEBUFFER, GL_COLOR_ATTACHMENT1, GL_TEXTURE_2D, mNormalTexture, 0);
//Combined color and specular buffer
glGenTextures(1, &mSpecularAlbedoTexture);
glBindTexture(GL_TEXTURE_2D, mSpecularAlbedoTexture);
glTexImage2D(GL_TEXTURE_2D, 0, GL_RGB32F, mWindowWidth, mWindowHeight, 0, GL_RGBA, GL_FLOAT, NULL);
glTexParameteri(GL_TEXTURE_2D, GL_TEXTURE_MIN_FILTER, GL_NEAREST);
glTexParameteri(GL_TEXTURE_2D, GL_TEXTURE_MAG_FILTER, GL_NEAREST);
glFramebufferTexture2D(GL_FRAMEBUFFER, GL_COLOR_ATTACHMENT2, GL_TEXTURE_2D, mSpecularAlbedoTexture, 0);
//Depth & stencil texture for the FBO
glGenTextures(1, &mDepthTexture);
glBindTexture(GL_TEXTURE_2D, mDepthTexture);
glTexImage2D(GL_TEXTURE_2D, 0, GL_DEPTH32F_STENCIL8, mWindowWidth, mWindowHeight, 0, GL_DEPTH_COMPONENT, GL_FLOAT, NULL);
glFramebufferTexture2D(GL_DRAW_FRAMEBUFFER, GL_DEPTH_STENCIL_ATTACHMENT, GL_TEXTURE_2D, mDepthTexture, 0);
// final
glGenTextures(1, &mFinalTexture);
glBindTexture(GL_TEXTURE_2D, mFinalTexture);
///Used to be GL_RGBA only
glTexImage2D(GL_TEXTURE_2D, 0, GL_RGBA32F, mWindowWidth, mWindowHeight, 0, GL_RGB, GL_FLOAT, NULL);
glTexParameteri(GL_TEXTURE_2D, GL_TEXTURE_MIN_FILTER, GL_NEAREST);
glTexParameteri(GL_TEXTURE_2D, GL_TEXTURE_MAG_FILTER, GL_NEAREST);
glFramebufferTexture2D(GL_DRAW_FRAMEBUFFER, GL_COLOR_ATTACHMENT4, GL_TEXTURE_2D, mFinalTexture, 0);
I'm thinking the GL_DEPTH_STENCIL_ATTACHMENT is what messes something up, but I also can't see why you shouldnt be able to blit the depth attachment if you specify it in the blit function
An image of the result, no skybox: result
EDIT
I tried searching around a bit and changed the format to GL_DEPTH_24_STENCIL8 in case the main fbo had a different float format, but that did not help either.
You shouldn't attempt to blit an FBO depth buffer to the default depth buffer. To blit from one depth buffer to another the internal formats of both must be exactly the same. Whilst you can define the format of an FBO, the default depth buffer is implementation defined. You might be able to deduce its format by trial and error but the solution will not be portable.
One solution would be to write out your depths to an R32F texture colour attachment during the geometry pass. Later when you're drawing your skybox to the default framebuffer you can bind the R32F texture to a sampler and read the depth for that pixel in the fragment shader. Only output the skybox to fragments where the depth is 0.0f.

Use FBO's position buffer and color buffer for rendering (post-rendering 3D warp)

I want to implement post-rendering 3D warp using position data and color data stored in a FBO. How to do it efficiently in modern OpenGL? By position data, they are camera-relative XYZ coordinates.
I have two FBOs, say fbo1 and fbo2. fbo1 is for data (position+color) input, and fbo2 is for data output (the results after post-rendering 3D warp). FBO setup code for both FBOs is as following:
void setupFBO( int width, int height )
{
// Create and bind the FBO
glGenFramebuffers(1, &FBO);
glBindFramebuffer(GL_FRAMEBUFFER, FBO);
// The depth buffer
glGenRenderbuffers(1, &fboDepthBuf);
glBindRenderbuffer(GL_RENDERBUFFER, fboDepthBuf);
glRenderbufferStorage(GL_RENDERBUFFER, GL_DEPTH_COMPONENT, width, height);
// The color buffer
glActiveTexture(GL_TEXTURE0);
glGenTextures(1, &fboColorTex);
glBindTexture(GL_TEXTURE_2D, fboColorTex);
glTexImage2D(GL_TEXTURE_2D, 0, GL_RGBA16F, width, height, 0, GL_RGBA, GL_UNSIGNED_BYTE, NULL);
glTexParameteri(GL_TEXTURE_2D, GL_TEXTURE_MIN_FILTER, GL_NEAREST);
glTexParameteri(GL_TEXTURE_2D, GL_TEXTURE_MAG_FILTER, GL_NEAREST);
// The position buffer
glActiveTexture(GL_TEXTURE1);
glGenTextures(1, &fboPosTex);
glBindTexture(GL_TEXTURE_2D, fboPosTex);
glTexImage2D(GL_TEXTURE_2D, 0, GL_RGBA32F, width, height, 0, GL_RGBA, GL_UNSIGNED_BYTE, NULL);
glTexParameteri(GL_TEXTURE_2D, GL_TEXTURE_MIN_FILTER, GL_NEAREST);
glTexParameteri(GL_TEXTURE_2D, GL_TEXTURE_MAG_FILTER, GL_NEAREST);
// Attach the images to the framebuffer
glFramebufferRenderbuffer(GL_FRAMEBUFFER, GL_DEPTH_ATTACHMENT, GL_RENDERBUFFER, fboDepthBuf);
glFramebufferTexture2D(GL_FRAMEBUFFER, GL_COLOR_ATTACHMENT0, GL_TEXTURE_2D, fboColorTex, 0);
glFramebufferTexture2D(GL_FRAMEBUFFER, GL_COLOR_ATTACHMENT1, GL_TEXTURE_2D, fboPosTex, 0);
// Set the targets for the fragment output variables
GLenum drawBuffers[] = {GL_COLOR_ATTACHMENT0, GL_COLOR_ATTACHMENT1};
glDrawBuffers(2, drawBuffers);
glBindFramebuffer(GL_FRAMEBUFFER, 0);
}
Assume we've data stored in fbo1, i.e. color data in fbo1.fboColorTex and position data in fbo1.fboPosTex, and we know the transformation (modelview) matrix needed for transforming the position data and also the projection matrix. How to obtain the transformed position and color data? One way to implement it could be like the following:
glBindFramebuffer(GL_FRAMEBUFFER, fbo2);
glActiveTexture(GL_TEXTURE0);
glBindTexture(GL_TEXTURE_2D, fbo1.fboColorTex);
glActiveTexture(GL_TEXTURE1);
glBindTexture(GL_TEXTURE_2D, fbo1.fboPosTex);
// set shader parameters for textures
// set modelview, projection matrices
// what to render here? A full-screen quad?
glBindFramebuffer(GL_FRAMEBUFFER, 0);
What do render? And what shaders should look like?
The other way I can think of is to use dynamic VBOs, but I'm not sure how to dynamically assign VBO's vertex and color data from textures (in fbo1)... The reason that I mention dynamic VBO is because fbo1's data can be changed but not frequently.

Unable to render depth texture

I'm having trouble with rendering depth texture using frame buffer in opengl and I can not find the problem by myself.
Here are the setup:
//initialize color texture
glGenTextures(1, &color_buffer);
glBindTexture(GL_TEXTURE_2D, color_buffer);
glTexImage2D(GL_TEXTURE_2D, 0, GL_RGBA8, width, height, 0, GL_RGBA, GL_FLOAT, NULL);
glTexParameteri(GL_TEXTURE_2D,GL_TEXTURE_MAG_FILTER,GL_NEAREST);
glTexParameteri(GL_TEXTURE_2D,GL_TEXTURE_MIN_FILTER,GL_NEAREST);
glTexParameteri(GL_TEXTURE_2D,GL_TEXTURE_WRAP_S,GL_CLAMP_TO_EDGE);
glTexParameteri(GL_TEXTURE_2D,GL_TEXTURE_WRAP_T,GL_CLAMP_TO_EDGE);
glBindTexture(GL_TEXTURE_2D, 0);
//initialize depth texture
glGenTextures(1, &depth_buffer);
glBindTexture(GL_TEXTURE_2D, depth_buffer);
glTexImage2D(GL_TEXTURE_2D, 0, GL_DEPTH_COMPONENT, width, height, 0, GL_DEPTH_COMPONENT, GL_FLOAT, NULL);
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_TEXTURE_WRAP_S, GL_CLAMP_TO_EDGE);
glTexParameteri(GL_TEXTURE_2D, GL_TEXTURE_WRAP_T, GL_CLAMP_TO_EDGE);
//bind both textures to a frame buffer
glGenFramebuffers(1, &frame_buffer);
glBindFramebuffer(GL_FRAMEBUFFER, frame_buffer);
glFramebufferTexture2D(GL_FRAMEBUFFER, GL_COLOR_ATTACHMENT0, GL_TEXTURE_2D, color_buffer, 0);
glFramebufferTexture2D(GL_FRAMEBUFFER, GL_DEPTH_ATTACHMENT, GL_TEXTURE_2D, depth_buffer, 0);
glBindFramebuffer(GL_FRAMEBUFFER, 0);
After rendering the scene to the framebuffer, I used the following codes to render the texture. When the color_buffer is used, the scene is correctly drawn. But when I use depth_buffer, the screen is all white. I'm not sure what is wrong in here. My fragment shader just use gl_FragColor = texture2D(texture_ID,texture_coord); to render. What is wrong with my codes? How can I render a depth texture?
glActiveTexture(GL_TEXTURE0);
glBindTexture(GL_TEXTURE_2D, ***color_buffer***);
glUseProgramObjectARB( program );
glUniform1i(glGetUniformLocation(program,"img"),0);
//codes that attach the texture to a quad
But when I use depth_buffer, the screen is all white.
My fragment shader just use gl_FragColor = texture2D(texture_ID,texture_coord); to render.
That is your depth buffer. It's not possible to be sure without knowing anything about what you have actually rendered (or about the projection matrix you use). But generally speaking, perspective projections tend to be a very skewed transform. It skews the Z to farther values, on the [0, 1] range, so most numbers will be closer to 1 than to 0.
If you want depth values in some kind of linear space, then you'll need to linearize them. That's going to be somewhat difficult without the clip-space W to multiply with. It's doable, but you'll need values from your perspective matrix to do it.