与glFeedback一起使用时glEvalCoord无法提供预期的输出 [英] glEvalCoord when used with glFeedback is not giving output as expected

问题描述

我正在尝试从glEvalCoord读取值，但是没有获得我应该获取的确切值.我的代码是

I am trying to read the value from glEvalCoord, but not getting the exact values which I should get. My code is

GLfloat ctrlpoints[4][3] = {
    { -4.0, -4.0, 0.0}, { -2.0, 4.0, 0.0}, 
    {  2.0, -4.0, 0.0}, {  4.0, 4.0, 0.0}};

void display()
{
    glClear(GL_COLOR_BUFFER_BIT | GL_DEPTH_BUFFER_BIT );
    glEnable(GL_LIGHTING);
    glMap1f(GL_MAP1_VERTEX_3, 0.0, 1.0, 3, 4, &ctrlpoints[0][0]);
    glEnable(GL_MAP1_VERTEX_3);
    glMatrixMode(GL_MODELVIEW);
    glLoadIdentity();

    GLint size;
    GLfloat feedBuffer[1024];

    glFeedbackBuffer (1024, GL_3D, feedBuffer);
    glRenderMode (GL_FEEDBACK);
    glBegin (GL_POINTS);
    for (int i=0; i<=30; ++i)
    {
        GLfloat t = GLfloat(i)/30;
        glEvalCoord1f(t);   
    }
    glEnd();
    size = glRenderMode (GL_RENDER);
    cerr<<size<<endl;
}

现在，我不确定，但是不应该为曲线的x，y和z坐标中的每一个给我30 * 3的值吗?但是我只得到7 * 3的值.输出的大小为28.

Now, I am not sure but shouldn't it give me 30*3 values for each of the x, y and z coordinates of the curve?? But I am getting only 7*3 values. And the output of size is 28.

推荐答案

我认为您的size是28，因为您的projection/modelview对正在删减某些点.

I think your size is 28 because your projection/modelview pair is clipping out some points.

在反馈模式下，将被栅格化的每个基元 ...生成一个值块，该值块已复制到反馈数组中."

尝试一下:

#include <GL/glut.h>
#include <iostream>

using namespace std;

void display()
{
    glClear( GL_COLOR_BUFFER_BIT );

    glMatrixMode( GL_PROJECTION );
    glLoadIdentity();
    double w = glutGet( GLUT_WINDOW_WIDTH );
    double h = glutGet( GLUT_WINDOW_HEIGHT );
    double ar = w / h;
    glOrtho( -5 * ar, 5 * ar, -5, 5, -1, 1 );

    glMatrixMode( GL_MODELVIEW );
    glLoadIdentity();

    GLfloat ctrlpoints[4][3] = 
    {
        { -4.0, -4.0, 0.0 }, { -2.0, 4.0, 0.0 }, 
        {  2.0, -4.0, 0.0 }, {  4.0, 4.0, 0.0 }
    };

    glMap1f(GL_MAP1_VERTEX_3, 0.0, 1.0, 3, 4, &ctrlpoints[0][0]);
    glEnable(GL_MAP1_VERTEX_3);

    GLfloat feedBuffer[1024];
    glFeedbackBuffer (1024, GL_3D, feedBuffer);
    glRenderMode (GL_FEEDBACK);

    glPointSize( 5 );
    glColor3ub( 255, 255, 255 );
    glBegin (GL_POINTS);
    for (int i=0; i<=30; ++i)
    {
        GLfloat t = GLfloat(i)/30;
        glEvalCoord1f(t);   
    }
    glEnd();

    GLint size = glRenderMode (GL_RENDER);
    cerr << size << endl;

    glutSwapBuffers();
}

int main( int argc, char **argv )
{
    glutInit( &argc, argv );
    glutInitDisplayMode( GLUT_RGBA | GLUT_DOUBLE );
    glutInitWindowSize( 640, 480 );
    glutCreateWindow( "GLUT" );
    glutDisplayFunc( display );
    glutMainLoop();
    return 0;
}

我添加了一个稍大的投影矩阵.

I added a slightly larger projection matrix.

这样我得到124的size:

• 31分*每点3个浮点数= 93
• 31分*每点1 GL_POINT_TOKEN = 31

• 31 points * 3 floats per point = 93
• 31 points * 1 GL_POINT_TOKEN per point = 31

31 + 91 = 124

31 + 91 = 124

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