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medevilenemy

OpenGL
FBO woes

21 posts in this topic

I've been trying to come up with a way to draw an opaque quad over my scene through which holes would be punched to allow one to see the scene below. Through some research and with some help I've determined that the best way to do this is probably by rendering to a texture through a framebuffer object, alphablending the holes into the quad and then drawing the texture over the scene. I'm having trouble getting it to work, though, and I can't find examples of what I'm trying to do.

So far I've succeeded in creating my framebuffer object and target texture based on the example block at the bottom of [url="http://www.songho.ca/opengl/gl_fbo.html."]http://www.songho.ca...gl/gl_fbo.html[/url]. After that, I've attempted various configurations of drawing what I want to the texture, but all I can seem to manage is to draw an opaque quad to it. I can't seem to get depth, alpha blending, or even texture mapping to work properly (no matter how I map the texture coords, the result is the same). glClear(GL_COLOR_BUFFER_BIT) on the texture causes my whole scene to get messed up, as well. The following code demonstrates how I'm trying to set up rendering to the texture:

[code]
glPushMatrix();
// set rendering destination to FBO
glBindFramebuffer(GL_FRAMEBUFFER, 92);

glViewport(0,0,screen_width, screen_height);
glMatrixMode(GL_PROJECTION);
glLoadIdentity();

// Clearing buffers
glClearDepth(1.0f);
glClearColor(0.0f, 0.0f, 0.0f, 1.0f);
glClear(GL_DEPTH_BUFFER_BIT); // Clearing the GL_COLOR_BUFFER_BIT causes things to break

// Set up internal rendering projection
glOrtho(-0.5 * width, 0.5*width, -1, 1, 1, 0);

glEnable(GL_BLEND);
glEnable(GL_DEPTH_TEST);

// Set blend func
//glBlendFunc(GL_SRC_ALPHA, GL_ONE_MINUS_SRC_ALPHA);

// Draw stuff here! At the moment just trying to draw a couple overlapping quads as a proof of concept

// Unbind fbo
glBindFramebuffer(GL_FRAMEBUFFER, 0);
glPopMatrix();

// Set scene-relative blending function.
glBlendFunc(GL_SRC_ALPHA, GL_ONE_MINUS_SRC_ALPHA); // function for blending

// Draw the rendered texture on top of the underlying scene
glEnable(GL_TEXTURE_2D);
glBindTexture(GL_TEXTURE_2D, 90);
glBegin(GL_QUADS);
glTexCoord2f(0.0, 0.0f); glVertex3f(-0.1,-0.1,0.01); // Bottom-Left Vertex
glTexCoord2f(1.0f, 0.0f); glVertex3f(0.1,-0.1,0.01); // Bottom-Right Vertex
glTexCoord2f(1.0f, 1.0f); glVertex3f(0.1,0.1,0.01); // Top-Right Vertex
glTexCoord2f(0.0f, 1.0f); glVertex3f(-0.1,0.1,0.01); // Top-Left Vertex
glEnd();
glDisable(GL_TEXTURE_2D);
[/code]

Any help is greatly appreciated!
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I've been doing some more tweaking, and have managed to be able to glClear(GL_COLOR_BUFFER_BIT) on the texture without messing other things up, but that's about as far as I've been able to get. I still can't seem to get something to take up only part of the texture, and can't seem to get multiple quads drawn to it (as a test) to actually blend together... I just see whichever was drawn later.

My best guess is that the texture itself isn't functioning properly (its as if it isn't mapping at all)... but that doesn't make any sense... I've done everything the couple examples I've seen does. Edited by medevilenemy
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The link seems to be down so I can't check your FBO setup. Can you post that code too? Did you actually bind at least one color buffer and a depth buffer to the FBO?

In the rendering code, after binding your FBO, you can check it with
[CODE]
GLenum status = glCheckFramebufferStatusEXT(GL_FRAMEBUFFER_EXT);
if (status != GL_FRAMEBUFFER_COMPLETE_EXT)
cry_out_in_pain();
[/CODE]
Just to see if s.th. broke it on the way.

Also (for debugging purposes) throw in this define somewhere
[CODE]
#define CHECKGLERROR(f, l) \
{ \
GLuint error = glGetError(); \
if (error) \
std::cout << "GL Error in " << f << " " << l << ": " << gluErrorString(error) << " (" << error << "). " << std::endl; \
}
[/CODE]
and do a CHECKGLERROR(__FILE__, __LINE__) after each OpenGL call.


Edit: fixed the preprocessor macro Edited by Ohforf sake
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somehow the period got added to the link... the page itself is still there: [url="http://www.songho.ca...gl/gl_fbo.html"]http://www.songho.ca...gl/gl_fbo.html[/url].

I tested for FBO completeness, and it seems to be ok. I'll give the macro you suggest a try, and get back to you. Based on my most recent tests, the best I can guess is that for whatever reason, the texture isn't functional (either not rendering or not complete or something) which results in the textured quad that's supposed to be drawing it just drawing as a straight-up colored quad.
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Uppon closer inspection this might also be a problem:
glOrtho(-0.5 * width, 0.5*width, -1, 1, 1, 0);

I think you switched the zNear and zFar plane at the end and that it should be:
glOrtho(-0.5 * width, 0.5*width, -1, 1, [b]0[/b], [b]1[/b]);
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That is the ortho setup I use in my main scene as well... it works just fine (I like to have bigger z = farther, I find it easier to remember). It seems like the problem has something to do with the rbo/texture setup or something, but I haven't a clue what. The fbo tests as complete.
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But don't you have to clear the depth to 0.0f then?
glClearDepth(0.0f);
Edit: Also glDepthFunc(GL_GEQUAL); but maybe thats hiding somewhere outside of that snippet. Edited by Ohforf sake
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No effect. All I see is a box of the size of the textured quad I'm drawing as a test in whatever the active color is. It does not seem to succesfully map a texture (which suggests the texture is somehow invalid).

I've done some testing for gl errors and I get the following when I try to bind the framebuffer object as I prepare to draw to it:
GL ERROR: invalid operation (1282)

This doesn't make any sense as the framebuffer object itself tests as complete.

[edit] by the way, thanks for your help so far. Edited by medevilenemy
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Could you post your updated source?

And, quick question, you use '92' and '90' as the GLuint reference, why not use a GLuint variable?
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After doing some sanity checking with a friend of mine, I determined that I was using glgenframebuffers wrong, so I have fixed that... so now instead of getting a quad with whatever the active color is, I now see... nothing at all. The updated version of the RTT code is as follows:

[CODE]
float bleh = 2 * ((double)screen_width/(double)screen_height);

// set rendering destination to FBO
glBindFramebuffer(GL_FRAMEBUFFER, fbonum);

glPushAttrib(GL_VIEWPORT_BIT | GL_ENABLE_BIT);

// clear buffers
//glClear(GL_COLOR_BUFFER_BIT | GL_DEPTH_BUFFER_BIT);

//glDepthFunc(GL_EQUAL);
glClearDepth(0.0f);
glClearColor(0.0f, 0.0f, 0.0f, 1.0f);
glClear(GL_DEPTH_BUFFER_BIT | GL_COLOR_BUFFER_BIT);
//glLoadIdentity();
glOrtho(-0.5 * bleh, 0.5*bleh, -1, 1, 1, 0);
//gluLookAt(Position.x,Position.y,Position.z,ViewPoint.x,ViewPoint.y,ViewPoint.z,UpVector.x,UpVector.y,UpVector.z);
glViewport(0,0,screen_width, screen_height);
glEnable(GL_BLEND);
glEnable(GL_DEPTH_TEST);

//glBlendFunc(GL_SRC_ALPHA, GL_ONE_MINUS_SRC_ALPHA); // function for blending
glBlendFunc(GL_ONE, GL_ONE); // function for blending
glColor4d(0.0, 1.0, 1.0, 0.0);
glBegin(GL_QUADS);
glVertex3d(0.0, 0.5, 0.0);
glVertex3d(0.5, 0.5, 0.0);
glVertex3d(0.5, -0.5, 0.5);
glVertex3d(0.0, -0.5, 0.5);
glEnd();

glColor4d(1.0, 0.0, 1.0, 1.0);
glBegin(GL_QUADS);
glVertex3d(0.1, 0.5, 0.0);
glVertex3d(0.6, 0.5, 0.0);
glVertex3d(0.6, -0.5, 0.0);
glVertex3d(0.1, -0.5, 0.0);
glEnd();

glPopAttrib();
glBindFramebuffer(GL_FRAMEBUFFER, 0);



glColor4d(1.0, 1.0, 0.0, 1.0);
//glBlendFunc(GL_SRC_ALPHA, GL_ONE_MINUS_SRC_ALPHA);
glBlendFunc(GL_ONE, GL_ONE);
glEnable(GL_TEXTURE_2D);
glBindTexture(GL_TEXTURE_2D, rttnum);

glBegin(GL_QUADS);
glTexCoord2f(0.0f, 0.0f); glVertex3f(-0.3,-0.3,0.0); // Bottom-Left Vertex
glTexCoord2f(1.0f, 0.0f); glVertex3f(0.3,-0.3,0.0); // Bottom-Right Vertex
glTexCoord2f(1.0f, 1.0f); glVertex3f(0.3,0.3,0.01); // Top-Right Vertex
glTexCoord2f(0.0f, 1.0f); glVertex3f(-0.3,0.3,0.01); // Top-Left Vertex
glEnd();
glDisable(GL_TEXTURE_2D);
[/CODE]

Apologies for the silly names and stuff... its just a proof of concept so I wasn't trying to use serious names in all places.
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Some random ideas:

I believe there is a restriction, that a rendertarget texture can not be bound while the FBO is bound and the FBO in turn can not be bound while the texture is bound. I could be wrong though ^^.
Anyways, try doing a

glBindTexture(GL_TEXTURE_2D, 0);

after you used the texture and before you loop back to binding the FBO.


Also, glClear(...) only affects the current viewport so you should set the viewport before doing glClear(...).


Did you use glDrawBuffers to specifiy, which RT to clear/render to?



It would really help to see the full code, including FBO setup and all the states you set for rendering!
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No effect. Moved glClear stuff around and unbound whatever texture might be active right before the FBO bind, but I still see nothing. I'm currently using glBlendFunc(GL_ONE,GL_ZERO) for both stages of blending (onto tex and tex onto scene) just for testing purposes, FYI. Not familiar with glDrawBuffers.

Here's the test code I'm using:

My init stuff...
[CODE]
GLenum err = glewInit();
if (GLEW_OK != err)
{
exit(0);
}
else
{
if (GLEW_ARB_framebuffer_object)
{
}
}

glEnable(GL_TEXTURE_2D);
glEnable(GL_COLOR_MATERIAL);
glGenTextures(1, &rttnum);

glBindTexture(GL_TEXTURE_2D, rttnum);
glTexParameterf(GL_TEXTURE_2D, GL_TEXTURE_MAG_FILTER, GL_LINEAR);
glTexParameterf(GL_TEXTURE_2D, GL_TEXTURE_MIN_FILTER, GL_LINEAR_MIPMAP_LINEAR);
glTexParameterf(GL_TEXTURE_2D, GL_TEXTURE_WRAP_S, GL_CLAMP_TO_EDGE);
glTexParameterf(GL_TEXTURE_2D, GL_TEXTURE_WRAP_T, GL_CLAMP_TO_EDGE);
glTexParameteri(GL_TEXTURE_2D, GL_GENERATE_MIPMAP, GL_TRUE); // automatic mipmap
glTexImage2D(GL_TEXTURE_2D, 0, GL_RGBA8, screen_width, screen_height, 0, GL_RGBA, GL_UNSIGNED_BYTE, 0);

glBindTexture(GL_TEXTURE_2D, 0);

// create a renderbuffer object to store depth info
GLuint rboId = 1;
glGenRenderbuffers(1, &rboId);

glBindRenderbuffer(GL_RENDERBUFFER, rboId);

glRenderbufferStorage(GL_RENDERBUFFER, GL_DEPTH_COMPONENT24, screen_width, screen_height);

// create a framebuffer object
glGenFramebuffers(1, &fbonum);

glBindFramebuffer(GL_FRAMEBUFFER, fbonum);

// attach the texture to FBO color attachment point
glFramebufferTexture2D(GL_FRAMEBUFFER, GL_COLOR_ATTACHMENT0, GL_TEXTURE_2D, rttnum, 0);

// attach the renderbuffer to depth attachment point
glFramebufferRenderbuffer(GL_FRAMEBUFFER, GL_DEPTH_ATTACHMENT, GL_RENDERBUFFER, rboId);

// switch back to window-system-provided framebuffer
glBindRenderbuffer(GL_RENDERBUFFER, 0);
glBindFramebuffer(GL_FRAMEBUFFER, 0);
[/CODE]

And the rendering related stuff...
[CODE]
float bleh = 2 * ((double)screen_width/(double)screen_height);

// Unbind any texture that might currently be bound
glBindTexture(GL_TEXTURE_2D, 0);
// set rendering destination to FBO
glBindFramebuffer(GL_FRAMEBUFFER, fbonum);

glPushAttrib(GL_VIEWPORT_BIT | GL_ENABLE_BIT);

// clear buffers
//glClear(GL_COLOR_BUFFER_BIT | GL_DEPTH_BUFFER_BIT);

//glDepthFunc(GL_EQUAL);
glOrtho(-0.5 * bleh, 0.5*bleh, -1, 1, 1, 0);
glViewport(0,0,screen_width, screen_height);
glClearDepth(0.0f);
glClearColor(0.0f, 0.0f, 0.0f, 1.0f);
glClear(GL_DEPTH_BUFFER_BIT | GL_COLOR_BUFFER_BIT);
//glLoadIdentity();
glEnable(GL_BLEND);
glEnable(GL_DEPTH_TEST);

//glBlendFunc(GL_SRC_ALPHA, GL_ONE_MINUS_SRC_ALPHA); // function for blending
//glBlendFunc(GL_ONE, GL_ONE); // function for blending
glBlendFunc(GL_ONE, GL_ZERO);
glColor4d(0.0, 1.0, 1.0, 1.0);
glBegin(GL_QUADS);
glVertex3d(0.0, 0.5, 0.0);
glVertex3d(0.5, 0.5, 0.0);
glVertex3d(0.5, -0.5, 0.5);
glVertex3d(0.0, -0.5, 0.5);
glEnd();

glColor4d(1.0, 0.0, 1.0, 1.0);
glBegin(GL_QUADS);
glVertex3d(0.1, 0.5, 0.0);
glVertex3d(0.6, 0.5, 0.0);
glVertex3d(0.6, -0.5, 0.0);
glVertex3d(0.1, -0.5, 0.0);
glEnd();

glPopAttrib();
glBindFramebuffer(GL_FRAMEBUFFER, 0);



glColor4d(1.0, 1.0, 0.0, 1.0);
//glBlendFunc(GL_SRC_ALPHA, GL_ONE_MINUS_SRC_ALPHA);
glBlendFunc(GL_ONE, GL_ZERO);
glEnable(GL_TEXTURE_2D);
glBindTexture(GL_TEXTURE_2D, rttnum);

glBegin(GL_QUADS);
glTexCoord2f(0.0f, 0.0f); glVertex3f(-0.3,-0.3,0.0); // Bottom-Left Vertex
glTexCoord2f(1.0f, 0.0f); glVertex3f(0.3,-0.3,0.0); // Bottom-Right Vertex
glTexCoord2f(1.0f, 1.0f); glVertex3f(0.3,0.3,0.01); // Top-Right Vertex
glTexCoord2f(0.0f, 1.0f); glVertex3f(-0.3,0.3,0.01); // Top-Left Vertex
glEnd();
glDisable(GL_TEXTURE_2D);


glBindTexture(GL_TEXTURE_2D, 0);
[/CODE]

[edit] Just attempted glDrawBuffer(GL_COLOR_ATTACHMENT0); right after binding the framebuffer and before drawing to it... no effect. Edited by medevilenemy
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ok, so I got it working, but without automatic mipmap generation. There might be a bug in NVidia's driver implementation (see [url="http://www.opengl.org/discussion_boards/showthread.php/175732-GL_GENERATE_MIPMAP-bug-%28nVidia%29"]http://www.opengl.or...AP-bug-(nVidia)[/url]) but I don't have the time to look into that right now. Do you actually need that?

Here is my code. Note, that I switched the depth interval (it confused the hell out of me ^^) and removed some stuff.
I think the two important changes are proper setup of the projection matrix (you were accumulating glOrthos) and removal of automatic mipmap generation.
[CODE]
GLuint rttnum;
glEnable(GL_TEXTURE_2D);
glEnable(GL_COLOR_MATERIAL);
glGenTextures(1, &rttnum);
glBindTexture(GL_TEXTURE_2D, rttnum);
glTexParameterf(GL_TEXTURE_2D, GL_TEXTURE_MAG_FILTER, GL_LINEAR);
glTexParameterf(GL_TEXTURE_2D, GL_TEXTURE_MIN_FILTER, GL_LINEAR);//GL_LINEAR_MIPMAP_LINEAR);
glTexParameterf(GL_TEXTURE_2D, GL_TEXTURE_WRAP_S, GL_CLAMP_TO_EDGE);
glTexParameterf(GL_TEXTURE_2D, GL_TEXTURE_WRAP_T, GL_CLAMP_TO_EDGE);
//glTexParameteri(GL_TEXTURE_2D, GL_GENERATE_MIPMAP, GL_TRUE); // automatic mipmap
glTexImage2D(GL_TEXTURE_2D, 0, GL_RGBA8, 800, 600, 0, GL_RGBA, GL_UNSIGNED_BYTE, 0);
CHECKGLERROR(__FILE__, __LINE__)
glBindTexture(GL_TEXTURE_2D, 0);
// create a renderbuffer object to store depth info
GLuint rboId = 1;
glGenRenderbuffers(1, &rboId);
glBindRenderbuffer(GL_RENDERBUFFER, rboId);
glRenderbufferStorage(GL_RENDERBUFFER, GL_DEPTH_COMPONENT24, 800, 600);
GLuint fbonum;
// create a framebuffer object
glGenFramebuffers(1, &fbonum);
glBindFramebuffer(GL_FRAMEBUFFER, fbonum);
// attach the texture to FBO color attachment point
glFramebufferTexture2D(GL_FRAMEBUFFER, GL_COLOR_ATTACHMENT0, GL_TEXTURE_2D, rttnum, 0);
// attach the renderbuffer to depth attachment point
glFramebufferRenderbuffer(GL_FRAMEBUFFER, GL_DEPTH_ATTACHMENT, GL_RENDERBUFFER, rboId);
GLenum status = glCheckFramebufferStatusEXT(GL_FRAMEBUFFER_EXT);
if (status != GL_FRAMEBUFFER_COMPLETE_EXT)
std::cout << "incomplete: " << std::hex << status << std::endl;
// switch back to window-system-provided framebuffer
glBindRenderbuffer(GL_RENDERBUFFER, 0);
glBindFramebuffer(GL_FRAMEBUFFER, 0);
CHECKGLERROR(__FILE__, __LINE__)
bool shutdown = false;
while (!shutdown) {
// Unbind any texture that might currently be bound
glBindTexture(GL_TEXTURE_2D, 0);
// set rendering destination to FBO
glBindFramebuffer(GL_FRAMEBUFFER, fbonum);
GLenum status = glCheckFramebufferStatusEXT(GL_FRAMEBUFFER_EXT);
if (status != GL_FRAMEBUFFER_COMPLETE_EXT)
std::cout << "incomplete: " << std::hex << status << std::endl;
glViewport(0,0,800, 600);
glClearColor(0.0f, 0.0f, 0.0f, 1.0f);
glClear(GL_DEPTH_BUFFER_BIT | GL_COLOR_BUFFER_BIT);
glMatrixMode(GL_PROJECTION);
glLoadIdentity();
glOrtho(-1, 1, -1, 1, -1, 1);
glMatrixMode(GL_MODELVIEW);
glLoadIdentity();
glEnable(GL_DEPTH_TEST);

glColor4d(0.0, 1.0, 1.0, 1.0);
glBegin(GL_QUADS);
glVertex3d(0.0, 0.5, 0.5);
glVertex3d(0.5, 0.5, 0.5);
glVertex3d(0.5, -0.5, 0.5);
glVertex3d(0.0, -0.5, 0.5);
glEnd();
glColor4d(1.0, 0.0, 1.0, 1.0);
glBegin(GL_QUADS);
glVertex3d(0.1, 0.5, 0.0);
glVertex3d(0.6, 0.5, 0.0);
glVertex3d(0.6, -0.5, 0.0);
glVertex3d(0.1, -0.5, 0.0);
glEnd();
glBindFramebuffer(GL_FRAMEBUFFER, 0);
CHECKGLERROR(__FILE__, __LINE__)

glViewport(0,0,800, 600);
glClearColor(1.0f, 0.0f, 0.0f, 1.0f);
glClear(GL_DEPTH_BUFFER_BIT | GL_COLOR_BUFFER_BIT);
glMatrixMode(GL_PROJECTION);
glLoadIdentity();
glOrtho(-1, 1, -1, 1, -1, 1);
glMatrixMode(GL_MODELVIEW);
glLoadIdentity();

glColor4d(1.0, 1.0, 1.0, 1.0);
glEnable(GL_TEXTURE_2D);
glBindTexture(GL_TEXTURE_2D, rttnum);
glBegin(GL_QUADS);
glTexCoord2f(0.0f, 0.0f); glVertex3f(-0.3,-0.3,0.0); // Bottom-Left Vertex
glTexCoord2f(1.0f, 0.0f); glVertex3f(0.3,-0.3,0.0); // Bottom-Right Vertex
glTexCoord2f(1.0f, 1.0f); glVertex3f(0.3,0.3,0.01); // Top-Right Vertex
glTexCoord2f(0.0f, 1.0f); glVertex3f(-0.3,0.3,0.01); // Top-Left Vertex
glEnd();
glDisable(GL_TEXTURE_2D);
CHECKGLERROR(__FILE__, __LINE__)
glBindTexture(GL_TEXTURE_2D, 0);
SDL_Event event;
while (SDL_PollEvent(&event)) {
switch( event.type ) {
case SDL_QUIT:
shutdown = true;
break;
}
}
SDL_GL_SwapBuffers();
}
[/CODE]
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Thanks for the suggestion! I've tried adapting your approach to my context, with no effect. I think what is preventing it from working is my drawing of the underlying scene... Here's the code for the gameplay screen as I have it after trying to adapt your approach... This should let you see the context of the scene rendering happening so we can get them to mesh together. The setup stuff actually happens in a different source file outside the main loop. On the upside... the matrix mode changes have prevented bleedover and weirdness back into my menu screen... never really understood what modelview did before.

[CODE]
float bleh = 2 * ((double)screen_width/(double)screen_height);

// Unbind any texture that might currently be bound
glBindTexture(GL_TEXTURE_2D, 0);

// set rendering destination to FBO
glBindFramebuffer(GL_FRAMEBUFFER, fbonum);

// Set stuff up
glViewport(0,0,screen_width, screen_height);
glClearDepth(0.0f);
glClearColor(0.0f, 0.0f, 0.0f, 0.0f);
glClear(GL_DEPTH_BUFFER_BIT | GL_COLOR_BUFFER_BIT);
glMatrixMode(GL_PROJECTION);
glLoadIdentity();
glOrtho(-0.5 * bleh, 0.5*bleh, -1, 1, 1, 0);
glMatrixMode(GL_MODELVIEW);
glLoadIdentity();

//glTranslatef(0.0, 0.0, 0.0);

//glLoadIdentity();
glEnable(GL_ALPHA_TEST);
glEnable(GL_BLEND);
glEnable(GL_DEPTH_TEST);

//glBlendFunc(GL_SRC_ALPHA, GL_ONE_MINUS_SRC_ALPHA); // function for blending
//glBlendFunc(GL_ONE, GL_ONE); // function for blending
//glBlendFunc(GL_ONE, GL_ZERO);
glColor4d(0.0, 1.0, 1.0, 1.0);
glBegin(GL_QUADS);
glVertex3d(0.0, 0.5, 0.0);
glVertex3d(0.5, 0.5, 0.0);
glVertex3d(0.5, -0.5, 0.0);
glVertex3d(0.0, -0.5, 0.0);
glEnd();

glColor4d(1.0, 0.0, 1.0, 1.0);
glBegin(GL_QUADS);
glVertex3d(0.1, 0.5, 0.0);
glVertex3d(0.6, 0.5, 0.0);
glVertex3d(0.6, -0.5, 0.0);
glVertex3d(0.1, -0.5, 0.0);
glEnd();

glBindFramebuffer(GL_FRAMEBUFFER, 0);


// DRAW MAIN SCENE STARTING HERE
glViewport(0,0,screen_width,screen_height);
glClearColor(0.0f, 0.0f, 0.0f, 0.0f);
glClearDepth(1.0f);
glClear(GL_COLOR_BUFFER_BIT | GL_DEPTH_BUFFER_BIT);
glMatrixMode(GL_PROJECTION);
glLoadIdentity();
float width = 2 * ((double)screen_width/(double)screen_height);
glOrtho(-0.5 * width, 0.5*width, -1, 1, 1, 0);
glMatrixMode(GL_MODELVIEW);
glLoadIdentity();

glEnable(GL_DEPTH_TEST);
glEnable(GL_BLEND); // allows blending (transparency)
glBlendFunc(GL_SRC_ALPHA, GL_ONE_MINUS_SRC_ALPHA); // function for blending
glEnable(GL_DEPTH_TEST);
glShadeModel(GL_SMOOTH); // allows smooth shading (gradients)
glEnable(GL_ALPHA_TEST);
glAlphaFunc(GL_GREATER, 0.0);

//glLoadIdentity();
Camera.Render();

// FOR REFERENCE: Less Z = Closer to camera, Bigger Z = farther away
glColor4d(0.0, 1.0, 0.0, 1.0);
if(player != NULL)
{
glRasterPos3d(Camera.GetX() - 1.0, Camera.GetY() - 0.9, 0.01);
DrawString(listBase[5], "FPS: %g, PLAYER POSITION: %g, %g MESSAGES: %i", fps, player->Get_X(), player->Get_Y(), msthissec);
}
if(interactable_nearby)
{
glColor4d(0.2, 0.2, 1.0, 0.9);
glRasterPos3d(Camera.GetX()-0.05, Camera.GetY() - 0.85, 0.01);
DrawString(listBase[2], "Press [%c] key to interact", interaction_key);
}

AudioHandler();
SceneryHandler();
SpriteHandler();
PlayerHandler();
triggers.Update();
ScriptHandler();


glColor4d(1.0, 1.0, 0.0, 1.0);
//glBlendFunc(GL_SRC_ALPHA, GL_ONE_MINUS_SRC_ALPHA);
glBlendFunc(GL_ONE, GL_ZERO);
glEnable(GL_TEXTURE_2D);
glBindTexture(GL_TEXTURE_2D, rttnum);

glBegin(GL_QUADS);
glTexCoord2f(0.0f, 0.0f); glVertex3f(-0.3,-0.3,0.01); // Bottom-Left Vertex
glTexCoord2f(1.0f, 0.0f); glVertex3f(0.3,-0.3,0.01); // Bottom-Right Vertex
glTexCoord2f(1.0f, 1.0f); glVertex3f(0.3,0.3,0.01); // Top-Right Vertex
glTexCoord2f(0.0f, 1.0f); glVertex3f(-0.3,0.3,0.01); // Top-Left Vertex
glEnd();
glDisable(GL_TEXTURE_2D);


glBindTexture(GL_TEXTURE_2D, 0);
[/CODE]

Once we get this working, I can take the method and adapt it to practical use for the effect I want, but for now I have it all in one function for ease of fiddling.
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Your z interval goes from 1.0f to 0.0f and you draw your quads at 0.0f. This may lead to them being culled by the far plane. Maybe you can draw them at 0.5f?
You will also need glDepthFunc(GEQUAL); because of the inverted depth interval.

BTW: Your comment
// FOR REFERENCE: Less Z = Closer to camera, Bigger Z = farther away

is not correct. The way you set up your projection matrix a z value of 1.0f is as close to the camera as possible, while a z value of 0.0f is as far away as possible (Ignoring for now, that for orthogonal projections this rather depends on the depth test, then on some actual "positioning").

I already asked this in a previous post, but are you sure that you want an inverted depth interval? This is so uncommon, that I have never seen this before.
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For the Gl ortho call for the fbo, I swapped the Z planes around and then set the depth func to LEQUAL. it now seems to draw to the texture such that I can see it. However, anything drawn at a z position greater than 0 is invisible, and when I draw the boxes such that they overlap even in the slightest, only the box drawn later is visible (with the one being drawn first becoming completely invisible). We're making progress! Though we're not quite there yet.

[edit] For now, I'm going to leave the main scene ortho the way it is, as it has always worked before. I wonder if the reason it works the way I expect is because of the camera view angle. Anyway, that's unimportant at the moment. Edited by medevilenemy
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That sounds good! With the normal z buffer setup you need to set glClearDepth(1.0f); before glClear-ing the depth buffer. Then it should work.

So for glOrtho(..., 0.0f, 1.0f): (I prefer this ^^)
glClearDepth(1.0f); // before glClear(... | GL_DEPTH_BUFFER_BIT)
glDepthFunc(GL_LEQUAL);

and for glOrtho(... 1.0f, 0.0f);
glClearDepth(0.0f); // before glClear(... | GL_DEPTH_BUFFER_BIT)
glDepthFunc(GL_GEQUAL);


[quote]I wonder if the reason it works the way I expect is because of the camera view angle.[/quote]

Yes, that is what I meant to hint in the previous post. If you look at a picture with perspective projection, you automatically assume the camera to be located at the focal point and if the depth test is the wrong way around, the image just feels wrong. For orthogonal projections however, where you don't have a view angle and thus no focal point, the camera could be located at either end of the view frustrum and the only thing that decides it for the human eye is the depth test.

You can of course stick with the inverted depth interval, but keep in mind that some aspects of the fixed function pipeline (for example fog) might not work as intended.
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Switching the depth clear had no effect, I still only see the box drawn second. FYI I have blend func within the context set to GL_ONE, GL_ZERO, but changing that has no apparant effect. I'll probably stick with the inverted depth interval for now, because I'd rather not have to flip around everything I'm rendering, but I'll consider doing so if it ever causes a problem.
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I've done some more tweaking, and I've managed to draw an opaque quad with a transparent window in its center. The reason one of the quads was clipping before seems to have been depth (I was drawing a couple of its vertices with 0.5 depth and for some reason it wasn't playing nice). The "window" is created by drawing a 0.0 alpha quad in the appropriate location with alpha func set to GL_ONE, GL_ZERO. The next thing I need is to be able to achieve the desired effect by subtracting the alpha of the "window" from the opaque quad (this way I could do overlapping lights and gradients). alpha func GL_SRC_ALPHA, GL_ONE_MINUS_SRC_ALPHA doesn't seem to work for this. Any suggestions?
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After lots more tweaking and digging around, the blending proof of concept is now functional (after some fiddling and experimentation with the blend subract and minmax extensions). Thanks for your help @Ohforf sake[size=3][b].[/b][/size]
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