如何在使用 triangle_strip 创建的 OpenGL 中对圆柱体进行纹理处理? [英] How do I texture a cylinder in OpenGL created with triangle_strip?

问题描述

这是我创建的圆柱体:

void drawCylinder(float r, float g, float b) {
    setMaterialColors(r, g, b);
    glColor4f(r, g, b, 1.0);
    /* top triangle */
    double i, resolution  = 0.1;
    double height = 1, radius = 0.5;
    glPushMatrix();
    glTranslatef(0, -0.5, 0);
    glBegin(GL_TRIANGLE_FAN);
        glVertex3f(0, height, 0);  /* center */
        for (i = 0; i <= 2 * PI; i += resolution)
            glVertex3f(radius * cos(i), height, radius * sin(i));
    glEnd();

    /* bottom triangle: note: for is in reverse order */
    glBegin(GL_TRIANGLE_FAN);
        glVertex3f(0, 0, 0);  /* center */
        for (i = 2 * PI; i >= 0; i -= resolution)
            glVertex3f(radius * cos(i), 0, radius * sin(i));
        /* close the loop back to 0 degrees */
        glVertex3f(radius, height, 0);
    glEnd();

    /* middle tube */
    glBegin(GL_QUAD_STRIP);
        for (i = 0; i <= 2 * PI; i += resolution)
        {
            glVertex3f(radius * cos(i), 0, radius * sin(i));
            glVertex3f(radius * cos(i), height, radius * sin(i));
        }
        /* close the loop back to zero degrees */
        glVertex3f(radius, 0, 0);
        glVertex3f(radius, height, 0);
    glEnd();
    glPopMatrix();
}

现在，我无法将纹理映射到它.我尝试保持纹理坐标与三角条坐标相同，但没有奏效.

Now, I am unable to map textures to it. I tried keeping texture coordinate same as the triangle strip coordinates, but it didn't work.

PS:我不想使用 gluCylinder.我知道它存在.

PS: I don't want to use gluCylinder. I know it exists.

如果我只能对磁盘进行纹理处理就可以了.四边形不需要纹理.

It's fine if I can only texture the disks. The quad strip needn't be textured.

推荐答案

对于中心:将 i 除以 2 * PI 得到一个在 之间变化的数字>0.0 和 1.0 并将其用作纹理坐标:

For the center: Divide i by 2 * PI to get a number that varies between 0.0 and 1.0 and use that as a texture coordinate:

for (i = 0; i <= 2 * PI; i += resolution)
{
    const float tc = ( i / (float)( 2 * PI ) );
    glTexCoord2f( tc, 0.0 );
    glVertex3f(radius * cos(i), 0, radius * sin(i));
    glTexCoord2f( tc, 1.0 );
    glVertex3f(radius * cos(i), height, radius * sin(i));
}

/* close the loop back to zero degrees */
glTexCoord2f( 0.0, 0.0 );
glVertex3f(radius, 0, 0);
glTexCoord2f( 0.0, 1.0 );
glVertex3f(radius, height, 0);

对于 endcaps:您可以直接使用 sin()/cos() 输出(几乎):

For the endcaps: You can use the sin()/cos() output (almost) directly:

glTexCoord2f( 0.5, 0.5 );
glVertex3f(0, height, 0);  /* center */
for (i = 0; i <= 2 * PI; i += resolution)
{
    // scale sin/cos range (-1 to 1) by 0.5 to get -0.5 to 0.5
    // then shift that range up/right by 0.5 to get 0 to 1:
    glTexCoord2f( 0.5f * cos(i) + 0.5f, 0.5f * sin(i) + 0.5f );
    glVertex3f(radius * cos(i), height, radius * sin(i));
}

完整示例:

#include <GL/glew.h>
#include <GL/glut.h>
#include <cmath>

void cylinder()
{
    const double PI = 3.14159;

    /* top triangle */
    double i, resolution  = 0.1;
    double height = 1;
    double radius = 0.5;

    glPushMatrix();
    glTranslatef(0, -0.5, 0);

    glBegin(GL_TRIANGLE_FAN);
        glTexCoord2f( 0.5, 0.5 );
        glVertex3f(0, height, 0);  /* center */
        for (i = 2 * PI; i >= 0; i -= resolution)

        {
            glTexCoord2f( 0.5f * cos(i) + 0.5f, 0.5f * sin(i) + 0.5f );
            glVertex3f(radius * cos(i), height, radius * sin(i));
        }
        /* close the loop back to 0 degrees */
        glTexCoord2f( 0.5, 0.5 );
        glVertex3f(radius, height, 0);
    glEnd();

    /* bottom triangle: note: for is in reverse order */
    glBegin(GL_TRIANGLE_FAN);
        glTexCoord2f( 0.5, 0.5 );
        glVertex3f(0, 0, 0);  /* center */
        for (i = 0; i <= 2 * PI; i += resolution)
        {
            glTexCoord2f( 0.5f * cos(i) + 0.5f, 0.5f * sin(i) + 0.5f );
            glVertex3f(radius * cos(i), 0, radius * sin(i));
        }
    glEnd();

    /* middle tube */
    glBegin(GL_QUAD_STRIP);
        for (i = 0; i <= 2 * PI; i += resolution)
        {
            const float tc = ( i / (float)( 2 * PI ) );
            glTexCoord2f( tc, 0.0 );
            glVertex3f(radius * cos(i), 0, radius * sin(i));
            glTexCoord2f( tc, 1.0 );
            glVertex3f(radius * cos(i), height, radius * sin(i));
        }
        /* close the loop back to zero degrees */
        glTexCoord2f( 0.0, 0.0 );
        glVertex3f(radius, 0, 0);
        glTexCoord2f( 0.0, 1.0 );
        glVertex3f(radius, height, 0);
    glEnd();

    glPopMatrix();
}

GLuint tex;
void init()
{
    unsigned char data[] =
    {
        128, 128, 128, 255,
        255, 0, 0, 255,
        0, 255, 0, 255,
        0, 0, 255, 255,
    };

    glGenTextures( 1, &tex );
    glBindTexture( GL_TEXTURE_2D, tex );
    glTexParameteri( GL_TEXTURE_2D, GL_TEXTURE_WRAP_S, GL_CLAMP_TO_EDGE );
    glTexParameteri( GL_TEXTURE_2D, GL_TEXTURE_WRAP_T, GL_CLAMP_TO_EDGE );
    glTexParameteri( GL_TEXTURE_2D, GL_TEXTURE_MAG_FILTER, GL_NEAREST );
    glTexParameteri( GL_TEXTURE_2D, GL_TEXTURE_MIN_FILTER, GL_NEAREST );
    glTexImage2D( GL_TEXTURE_2D, 0,GL_RGBA, 2, 2, 0, GL_RGBA, GL_UNSIGNED_BYTE, data );
}

float angle = 0;
void timer( int value )
{
    angle += 6;
    glutPostRedisplay();
    glutTimerFunc( 16, timer, 0 );
}

void display()
{
    glClearColor( 0, 0, 0, 1 );
    glClear( GL_COLOR_BUFFER_BIT | GL_DEPTH_BUFFER_BIT );

    glMatrixMode( GL_PROJECTION );
    glLoadIdentity();
    gluPerspective( 60, 1.0, 0.1, 100.0 );

    glMatrixMode( GL_MODELVIEW );
    glLoadIdentity();
    glTranslatef( 0, 0, -5 );

    glEnable( GL_CULL_FACE );
    glEnable( GL_DEPTH_TEST );

    glRotatef( angle, 0.2, 0.3, 0.1 );

    glEnable( GL_TEXTURE_2D );
    glBindTexture( GL_TEXTURE_2D, tex );
    cylinder();

    glutSwapBuffers();
}

int main(int argc, char **argv)
{
    glutInit( &argc, argv );
    glutInitDisplayMode( GLUT_RGBA | GLUT_DEPTH | GLUT_DOUBLE );
    glutInitWindowSize( 600, 600 );
    glutCreateWindow( "GLUT" );
    init();
    glutDisplayFunc( display );
    glutTimerFunc( 0, timer, 0 );
    glutMainLoop();
    return 0;
}

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