OpenGL Programming Guide

Chapter 11Evaluators and NURBS

Evaluators

Example: Using Evaluators for Textures

Example 11-4 enables two evaluators at the same time: The first generates three-dimensional points on the same Bézier surface as Example 11-3 , and the second generates texture coordinates. In this case, the texture coordinates are the same as the u and v coordinates of the surface, but a special flat Bézier patch must be created to do this.

The flat patch is defined over a square with corners at (0, 0), (0, 1), (1, 0), and (1, 1); it generates (0, 0) at corner (0, 0), (0, 1) at corner (0, 1), and so on. Since it's of order 2 (linear degree plus one), evaluating this texture at the point (u, v) generates texture coordinates (s, t). It's enabled at the same time as the vertex evaluator, so both take effect when the surface is drawn. See Figure J-26 . If you want the texture to repeat three times in each direction, change every 1.0 in the array texpts[][][] to 3.0. Since the texture wraps in this example, the surface is rendered with nine copies of the texture map.

Example 11-4 : Using Evaluators for Textures: texturesurf.c

#include <GL/gl.h>
#include <GL/glu.h>
#include "aux.h"
#include <math.h>

GLfloat ctrlpoints[4][4][3] = {
    {{ -1.5, -1.5, 4.0}, { -0.5, -1.5, 2.0}, 
        {0.5, -1.5, -1.0}, {1.5, -1.5, 2.0}}, 
    {{ -1.5, -0.5, 1.0}, { -0.5, -0.5, 3.0}, 
        {0.5, -0.5, 0.0}, {1.5, -0.5, -1.0}}, 
    {{ -1.5, 0.5, 4.0}, { -0.5, 0.5, 0.0}, 
        {0.5, 0.5, 3.0}, {1.5, 0.5, 4.0}}, 
    {{ -1.5, 1.5, -2.0}, { -0.5, 1.5, -2.0}, 
        {0.5, 1.5, 0.0}, {1.5, 1.5, -1.0}}
};

GLfloat texpts[2][2][2] = {{{0.0, 0.0}, {0.0, 1.0}}, 
        {{1.0, 0.0}, {1.0, 1.0}}};

void display(void)
{
    glClear(GL_COLOR_BUFFER_BIT | GL_DEPTH_BUFFER_BIT);
    glColor3f(1.0, 1.0, 1.0);
    glEvalMesh2(GL_FILL, 0, 20, 0, 20);
    glFlush();
}

#define   imageWidth 64
#define   imageHeight 64
GLubyte   image[3*imageWidth*imageHeight];

void loadImage(void)
{
    int i, j;
    float ti, tj;

    for (i = 0; i < imageWidth; i++) {
        ti = 2.0*3.14159265*i/imageWidth;
        for (j = 0; j < imageHeight; j++) {
            tj = 2.0*3.14159265*j/imageHeight;
            image[3*(imageHeight*i+j)] = 
                (GLubyte) 127*(1.0+sin(ti));
            image[3*(imageHeight*i+j)+1] = 
                (GLubyte) 127*(1.0+cos(2*tj));
            image[3*(imageHeight*i+j)+2] = 
                (GLubyte) 127*(1.0+cos(ti+tj));
        }
    }
}

void myinit(void)
{
    glMap2f(GL_MAP2_VERTEX_3, 0, 1, 3, 4,
        0, 1, 12, 4, &ctrlpoints[0][0][0]);
    glMap2f(GL_MAP2_TEXTURE_COORD_2, 0, 1, 2, 2, 
        0, 1, 4, 2, &texpts[0][0][0]);
    glEnable(GL_MAP2_TEXTURE_COORD_2);
    glEnable(GL_MAP2_VERTEX_3);
    glMapGrid2f(20, 0.0, 1.0, 20, 0.0, 1.0);
    loadImage();
    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_MAG_FILTER, 
                     GL_NEAREST);
    glTexParameterf(GL_TEXTURE_2D, GL_TEXTURE_MIN_FILTER, 
                     GL_NEAREST);
    glTexImage2D(GL_TEXTURE_2D, 0, 3, imageWidth, imageHeight, 
                     0, GL_RGB, GL_UNSIGNED_BYTE, image);
    glEnable(GL_TEXTURE_2D);
    glEnable(GL_DEPTH_TEST);
    glEnable(GL_NORMALIZE);
    glShadeModel (GL_FLAT);
}

void myReshape(GLsizei w, GLsizei h)
{
    glViewport(0, 0, w, h);
    glMatrixMode(GL_PROJECTION);
    glLoadIdentity();
    if (w <= h)
        glOrtho(-4.0, 4.0, -4.0*(GLfloat)h/(GLfloat)w, 
               4.0*(GLfloat)h/(GLfloat)w, -4.0, 4.0);
    else
        glOrtho(-4.0*(GLfloat)w/(GLfloat)h, 
               4.0*(GLfloat)w/(GLfloat)h, -4.0, 4.0, -4.0, 4.0);

    glMatrixMode(GL_MODELVIEW);
    glLoadIdentity();
    glRotatef(85.0, 1.0, 1.0, 1.0);
}


int main(int argc, char** argv)
{
    auxInitDisplayMode (AUX_SINGLE | AUX_RGBA | AUX_DEPTH);
    auxInitPosition (0, 0, 300, 300);
    auxInitWindow (argv[0]);
    myinit();
    auxReshapeFunc (myReshape);
    auxMainLoop(display);
}

OpenGL Programming Guide