# OpenGL Mipmaps and FBOs

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I am having a hell of a time generating mipmaps for my FBO texture. My mipmaps just aren't being generated properly. I render a grid to the FBO on every frame, and try to generate a full set of mipmaps immediately. Then I draw my FBO texture to the screen: When I move the camera back from the render position, the grid disappears completely within my black quad. Obviously, the renderer is trying to use my mipmaps to render my quad to the screen, but those mipmap texture levels haven't been generated. My FBO texture is 1024x1024. I'm using OpenGL Version 3.2 on an nVidia Quadro 4800 card, Windows 7. Here is my code for creating my FBO:

texData.width = w;
texData.height = h;
texData.bFlipTexture = true;
texData.glType = GL_RGB;

this->autoClear = autoClear;

// create & setup texture
glGenTextures(1, (GLuint *)texData.textureName);   // could be more then one, but for now, just one

glEnable(GL_TEXTURE_2D);

const GLubyte* ver = glGetString(GL_VERSION);
glBindTexture(GL_TEXTURE_2D, (GLuint)texData.textureName[0]);

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_MAG_FILTER, GL_LINEAR);
glTexParameteri(GL_TEXTURE_2D, GL_TEXTURE_MIN_FILTER, GL_LINEAR_MIPMAP_LINEAR);		//GL_LINEAR

//glTexParameteri(GL_TEXTURE_2D, GL_GENERATE_MIPMAP, GL_TRUE); // automatic mipmap - when I include this line and draw my texture
//at a small scale, I see the imagery, but it does not appear to be mipmapped -- it looks like a typical (jagged) linear shrinkage. (See image below)

glTexImage2D(GL_TEXTURE_2D, 0, GL_RGB8 , texData.tex_w, texData.tex_h, 0, /*texData.glType*/ GL_RGB, GL_UNSIGNED_BYTE, 0);

// when I use glGenerateMipmapEXT(), I see proper transitions between mipmap levels as I scale down my texture, but every mipmap level
//is just black with no imagery. (See image above.) Level 0 crossfades to black as it reaches a 50% scale.
glGenerateMipmapEXT(GL_TEXTURE_2D);  //Allocate necessary space for mipmaps

glTexEnvf(GL_TEXTURE_ENV, GL_TEXTURE_ENV_MODE, GL_MODULATE);

// create & setup FBO
glGenFramebuffersEXT(1, &fbo);
glBindFramebufferEXT(GL_FRAMEBUFFER_EXT, fbo);

// attach it to the FBO so we can render to it
glFramebufferTexture2DEXT(GL_FRAMEBUFFER_EXT, GL_COLOR_ATTACHMENT0_EXT, GL_TEXTURE_2D, (GLuint)texData.textureName[0], 0);

glGenRenderbuffersEXT(1, &depthBuffer);
glBindRenderbufferEXT(GL_RENDERBUFFER_EXT, depthBuffer);
glRenderbufferStorageEXT(GL_RENDERBUFFER_EXT, GL_DEPTH_COMPONENT, texData.tex_w, texData.tex_h);

// Attach the depth render buffer to the FBO as it's depth attachment
glFramebufferRenderbufferEXT(GL_FRAMEBUFFER_EXT, GL_DEPTH_ATTACHMENT_EXT, GL_RENDERBUFFER_EXT, depthBuffer);

GLenum status = glCheckFramebufferStatusEXT(GL_FRAMEBUFFER_EXT);
if(status != GL_FRAMEBUFFER_COMPLETE_EXT) {
cout<<"glBufferTexture failed to initialize. Perhaps your graphics card doesnt support the framebuffer extension? If you are running osx prior to system 10.5, that could be the cause"<<endl;
std::exit(1);
}


In the code above, you may notice the commented line in which I set: glTexParameteri(GL_TEXTURE_2D, GL_GENERATE_MIPMAP, GL_TRUE); ...if I use this call instead of glGenerateMipmapEXT(), then I can see my entire FBO texture rendered in my quad, but it is obviously NOT mipmapped: Anyway, when it's time to actually render to my FBO texture, here is my pseudocode:

glBindTexture(GL_TEXTURE_2D, 0);
glBindFramebufferEXT(GL_FRAMEBUFFER_EXT, fbo);

//DRAW to FBO

glBindFramebufferEXT(GL_FRAMEBUFFER_EXT, 0); //unbind the FBO

glBindTexture(GL_TEXTURE_2D, (GLuint)texData.textureName);
//glEnable(GL_TEXTURE_2D); //I'm told I need this on an ATI card, but not necessary for nVidia?
glGenerateMipmapEXT(GL_TEXTURE_2D);


I have tried lots of combinations of tiny tweaks to the different calls, especially glTexImage2D, but haven't gotten anywhere. I've tried to mimic every example usage of glGenerateMipmapEXT() I can find online. I'd rather not deal with gluBuild2DMipmaps(), and I don't know why it should work where these other methods fail. Please help! I've been stuck on this for two days!

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Perhaps the 3.2 profile isn't working too well. Can you try it with GL 2 since you are calling glTexEnvf(GL_TEXTURE_ENV, GL_TEXTURE_ENV_MODE, GL_MODULATE);
and using the glGenerateMipmapEXT (the FBO_EXT version)

//glEnable(GL_TEXTURE_2D); //I'm told I need this on an ATI card, but not necessary for nVidia?
That's for regular texture on ATI. For FBO texture, you don't need it even on ATI. Also, it was for really old drivers. Perhaps it is already fixed.

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V-man,

Using the 2.0 or 2.1 profile is something I hadn't thought of. I'd love to try, but I have no idea how to do this. I can use glGetString(GL_VERSION) to see which version I'm using, but I can't find a reference online for how to specify an older version/profile. I don't know if it means using a different nVidia driver, gl_.dll, setting preprocessor constants... Where do I start?

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Making an old context is done with the old fashion way. Look at any code such as at Nehe. Of course, the version will always say 3.0 even if you have old profile.

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You suggested that I try 2.0 because I was using glGenerateMipmapEXT and a lot of other EXT function pointers. I have since updated glee.h and am now using glGenerateMipmap(). Should that make a difference? My results on screen are the same.

I'm using a fairly large code base that does not easily lend itself to being pushed backwards. I don't think that it is worth trying to revert everything to an older version of OpenGL. Is there anything in more recent versions that might work better?

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But are you creating a GL 3 context?
Some tutorial here http://www.opengl.org/wiki/Main_Page

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• By Lewa
So, i stumbled upon the topic of gamma correction.
So from what i've been able to gather: (Please correct me if i'm wrong)
Old CRT monitors couldn't display color linearly, that's why gamma correction was nessecary. Modern LCD/LED monitors don't have this issue anymore but apply gamma correction anyway. (For compatibility reasons? Can this be disabled?) All games have to apply gamma correction? (unsure about that) All textures stored in file formats (.png for example) are essentially stored in SRGB color space (as what we see on the monitor is skewed due to gamma correction. So the pixel information is the same, the percieved colors are just wrong.) This makes textures loaded into the GL_RGB format non linear, thus all lighting calculations are wrong You have to always use the GL_SRGB format to gamma correct/linearise textures which are in SRGB format
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What i tried to do is to apply a gamma correction shader shown in the tutorial above which looks essentially like this: (it's a postprocess shader which is applied at the end of the renderpipeline)
vec3 gammaCorrection(vec3 color){ // gamma correction color = pow(color, vec3(1.0/2.2)); return color; } void main() { vec3 color; vec3 tex = texture2D(texture_diffuse, vTexcoord).rgb; color = gammaCorrection(tex); outputF = vec4(color,1.0f); } The results look like this:
No gamma correction:
With gamma correction:

The colors in the gamma corrected image look really wased out. (To the point that it's damn ugly. As if someone overlayed a white half transparent texture. I want the colors to pop.)
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