/* Simple demo of computing textures as arrays of values and
   texture-mapping them onto a 2D quad.  There are two textures:  the
   classic checkerboard (which is simple but boring because it is
   invariant almost no matter what you do to it) and the US flag.  Toggle
   between them using a 'u'.

   Scott D. Anderson
   Fall 2000 original
   Fall 2003 adapted to use TW
*/

#include <math.h>
#include <tw.h>

bool USflag = false;                // which flag to show

void toggleFlag(unsigned char key, int x, int y) {
    USflag = !USflag;
    glutPostRedisplay();
}

/* ================================================================
   Checkerboard Texture variables.  The checkerboard texture is an array
   of unsigned 8-bit values (GLubyte) which are interpreted as a grayscale
   value.  We only use 0xFF and 0, namely white and black, respectively.
*/

const int CheckFlagWidth  = 8;
const int CheckFlagHeight = 8;

GLubyte CheckFlagArray[CheckFlagHeight][CheckFlagWidth];

void init_check_flag() {
    for(int i=0; i<CheckFlagWidth; i++) 
        for(int j=0; j<CheckFlagWidth; j++) 
            // The following cleverness just alternates between black and
            // white in the appropriate way
            CheckFlagArray[i][j] = ((i+j)&1)==0 ? 0xFF : 0;
}

/* ================================================================ */

void display() {
    twDisplayInit();
    twCamera();

    glPushAttrib(GL_ALL_ATTRIB_BITS);

    glTexEnvf(GL_TEXTURE_ENV, GL_TEXTURE_ENV_MODE, GL_DECAL);
    glTexParameterf(GL_TEXTURE_2D, GL_TEXTURE_WRAP_S, GL_REPEAT);
    glTexParameterf(GL_TEXTURE_2D, GL_TEXTURE_WRAP_T, GL_REPEAT);
    glTexParameterf(GL_TEXTURE_2D, GL_TEXTURE_MIN_FILTER, GL_LINEAR);
    glTexParameterf(GL_TEXTURE_2D, GL_TEXTURE_MAG_FILTER, GL_NEAREST);

    if(USflag) {
        // This is *not* an efficient use of this function.  It recomputes
        // the flag texture every time.  You should use texture binding.
        // You can find the code for twUSflag in tw/src/tw-textures.cc
        twUSflag();
    } else {
        glTexImage2D(GL_TEXTURE_2D, 0, 3, CheckFlagWidth, CheckFlagHeight, 0,
                     GL_LUMINANCE, GL_UNSIGNED_BYTE, CheckFlagArray);
    }

    glEnable(GL_TEXTURE_2D);

    // There seems to be a roundoff error of some sort in our graphics
    // card, because the following code ends up with a little bit (1
    // pixel) of the left edge of the texture on the right edge.  If you
    // say one=1, you get this effect, but if you change it to 0.99, you
    // don't get it.
    float one = 1;
    glBegin(GL_QUADS);
    glTexCoord2f(0,one);   glVertex3f( 0,0,0); 
    glTexCoord2f(one,one); glVertex3f(10,0,0); 
    glTexCoord2f(one,0);   glVertex3f(10,5,0); 
    glTexCoord2f(0,0);     glVertex3f( 0,5,0); 
    glEnd();

    glPopAttrib();
    glFlush();
    glutSwapBuffers();
}

/* ================================================================ */
int main(int argc, char **argv) {
    init_check_flag();
    glutInit(&argc, argv);
    glutInitDisplayMode(GLUT_DOUBLE | GLUT_RGB | GLUT_DEPTH);
    twInitWindowSize(500, 500);
    glutCreateWindow(argv[0]);
    glutDisplayFunc(display);
    twBoundingBox(0,10,0,5,-1,1);
    twMainInit();
    twKeyCallback('u',toggleFlag,"toggle which flag to show");
    glutMainLoop();                   
}