Mar 10
25

20. OpenGL MipMap Generation

Prev: 19. OpenGL Fullscreen Mode
Next: 21. OpenGL Display Lists

Mipmaps are a group of the same image, scaled to different sizes which are then selected according to how far away the viewer is.
Instead of resizing the texture to fit the object on the fly,
opengl will pick the mipmap that best fits the shape and textures
it with that. This fixes dodgy looking textures, and smooths
out the overall effect. Making a better looking image.

To do this all we need is this line in our LoadTexture function:
gluBuild2DMipmaps( GL_TEXTURE_2D, 3, width, height, GL_RGB, GL_UNSIGNED_BYTE, data );
Instead of the line:
glTexImage2D(GL_TEXTURE_2D, 0, GL_RGB, width, height, 0, GL_RGB, GL_UNSIGNED_BYTE, data);
The ‘data’ being the texture, the width and height being the width and height
of the texture file.

Then you just bind it like you would a normal texture.

But because we are now using mipmaps, we also want to change:
    glTexParameterf( GL_TEXTURE_2D, GL_TEXTURE_MIN_FILTER, GL_LINEAR );
    glTexParameterf( GL_TEXTURE_2D, GL_TEXTURE_MAG_FILTER, GL_LINEAR );
To
    glTexParameterf( GL_TEXTURE_2D, GL_TEXTURE_MIN_FILTER, GL_LINEAR_MIPMAP_LINEAR );
    glTexParameterf( GL_TEXTURE_2D, GL_TEXTURE_MAG_FILTER, GL_LINEAR_MIPMAP_LINEAR );

Simply so that we can get better looking textures.

And wasn’t that simple, we now have mipmaps
If you have any questions, please email me at swiftless@gmail.com

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#include <GL/gl.h>
#include <GL/glut.h>
#include <windows.h>
#include <stdio.h>

GLuint texture; //the array for our texture

GLfloat angle = 0.0;

GLuint LoadTexture( const char * filename, int width, int
height )
{
    GLuint texture;
    unsigned char * data;
    FILE * file;

    //The following code will read in our RAW file
    file = fopen( filename, “rb” );
    if ( file == NULL ) return 0;
    data = (unsigned char *)malloc( width * height * 3 );
    fread( data, width * height * 3, 1, file );
    fclose( file );

    glGenTextures( 1, &texture ); //generate the texture with
the loaded data
    glBindTexture( GL_TEXTURE_2D, texture ); //bind the texture
to it’s array
    glTexEnvf( GL_TEXTURE_ENV, GL_TEXTURE_ENV_MODE,
GL_MODULATE ); //set texture environment parameters

    //here we are setting what textures to use and when. The MIN
filter is which quality to show
    //when the texture is near the view, and the MAG filter is which
quality to show when the texture
    //is far from the view.

    //The qualities are (in order from worst to best)
    //GL_NEAREST
    //GL_LINEAR
    //GL_LINEAR_MIPMAP_NEAREST
    //GL_LINEAR_MIPMAP_LINEAR

    //And if you go and use extensions, you can use Anisotropic
filtering textures which are of an
    //even better quality, but this will do for now.
    glTexParameterf( GL_TEXTURE_2D, GL_TEXTURE_MIN_FILTER,
GL_LINEAR_MIPMAP_LINEAR );
    glTexParameterf( GL_TEXTURE_2D, GL_TEXTURE_MAG_FILTER,
GL_LINEAR_MIPMAP_LINEAR );

    //Here we are setting the parameter to repeat the texture
instead of clamping the texture
    //to the edge of our shape.
    glTexParameterf( GL_TEXTURE_2D, GL_TEXTURE_WRAP_S,
GL_REPEAT );
    glTexParameterf( GL_TEXTURE_2D, GL_TEXTURE_WRAP_T,
GL_REPEAT );

    //Generate the texture with mipmaps
    gluBuild2DMipmaps( GL_TEXTURE_2D, 3, width, height,
GL_RGB, GL_UNSIGNED_BYTE, data );
    free( data ); //free the texture
    return texture; //return whether it was successfull
}

void FreeTexture( GLuint texture )
{
glDeleteTextures( 1, &texture );
}

void square (void) {
    glBindTexture( GL_TEXTURE_2D, texture ); //bind our texture

    glPushMatrix();
    glRotatef( angle, 1.0f, 1.0f, 1.0f );
    glBegin (GL_QUADS);
    glTexCoord2d(0.0,0.0); glVertex2d(-1.0,-1.0);
    glTexCoord2d(1.0,0.0); glVertex2d(+1.0,-1.0);
    glTexCoord2d(1.0,1.0); glVertex2d(+1.0,+1.0);
    glTexCoord2d(0.0,1.0); glVertex2d(-1.0,+1.0);
    glEnd();
    glPopMatrix();
    glPushMatrix();
    glTranslatef( 0,0,-5 );
    glBegin (GL_QUADS);
    glTexCoord2d(0.0,0.0); glVertex2d(-1.0,-1.0);
    glTexCoord2d(1.0,0.0); glVertex2d(+1.0,-1.0);
    glTexCoord2d(1.0,1.0); glVertex2d(+1.0,+1.0);
    glTexCoord2d(0.0,1.0); glVertex2d(-1.0,+1.0);
    glEnd();
    glPopMatrix();
}

void display (void) {
    glClearColor (0.0,0.0,0.0,1.0);
    glClear (GL_COLOR_BUFFER_BIT);
    glLoadIdentity();
    glEnable( GL_TEXTURE_2D ); //enable texturing
    gluLookAt (0.0, 0.0, 5.0, 0.0, 0.0, 0.0, 0.0, 1.0, 0.0);
    square();
    glutSwapBuffers();
    angle ++;
}
void reshape (int w, int h) {
    glViewport (0, 0, (GLsizei)w, (GLsizei)h);
    glMatrixMode (GL_PROJECTION);
    glLoadIdentity ();
    gluPerspective (60, (GLfloat)w / (GLfloat)h, 1.0, 100.0);
    glMatrixMode (GL_MODELVIEW);
}

int main (int argc, char **argv) {
    glutInit (&argc, argv);
    glutInitDisplayMode (GLUT_DOUBLE | GLUT_RGBA);
    glutInitWindowSize (500, 500);
    glutInitWindowPosition (100, 100);
    glutCreateWindow (“A basic OpenGL Window“);
    glutDisplayFunc (display);
    glutIdleFunc (display);
    glutReshapeFunc (reshape);

texture = LoadTexture( “texture.raw”, 256, 256 ); //load our texture

glutMainLoop ();

FreeTexture( texture );

return 0;
}

Brent says:

March 27, 2012 at 5:18 am

http://www.opengl.org/wiki/Common_Mistakes#gluBuild2DMipmaps

Mitchell says:

February 2, 2012 at 2:07 am

You should be using glGenerateMipmap(GL_TEXTURE_2D).

Never use gluBuild2DMipmaps

Geometrian says:

February 9, 2011 at 10:41 am

Right, so this line:
glTexParameterf( GL_TEXTURE_2D, GL_TEXTURE_MAG_FILTER, GL_LINEAR_MIPMAP_LINEAR );
. . . will give undefined behavior.

February 8, 2011 at 2:58 am

Setting the mipmap flag in the mag filter will have undefined results. Using mipmapping for magnification doesn’t make sense.

Swiftless says:

February 8, 2011 at 6:43 pm
Hi Geometrian,

You’re right, you can leave the mag filter as linear as this doesn’t make sense to set.

However it half makes sense in the case of magnification for effects such as bloom, where you take the smaller mipmap and apply it to a shape too large on purpose to get a blurry effect. I see this as magnification, albeit a skewed interpretation

Cheers,
Swiftless
- Geometrian says:
  
  February 9, 2011 at 1:03 am
  Magnification doesn’t work that way. Mipmapping is designed to work only in minification. This is why, according to the spec, no mipmapping is defined for mag filters. See (http://www.opengl.org/sdk/docs/man/xhtml/glTexParameter.xml).
  
  While mipmap levels can be blown up for the purposes you describe, it can’t be done simply by setting the mag filter to a mipmap. You’d need to select the actual level when you draw it in the fullscreen pass.
  - Swiftless says:
    
    February 9, 2011 at 10:17 am
    Hi Geometrian,
    
    I think we got our wires crossed, when you first mentioned magnification I was thinking as I described in my response (as an effect), I now understand where you are coming from (magnification as in the magnification level of the texture, not magnification as in the effect), and agree totally.
    
    Cheers,
    Swiftless

Swiftless Tutorials
Game Programming and Computer Graphics Tutorials

20. OpenGL MipMap Generation

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