OpenGL：为什么当物体旋转时照明会改变？

Question

下面的代码围绕它们的局部轴旋转金字塔和立方体（我相信）。为什么使用此代码两个对象都会变亮和变暗？光肯定会来自一个固定的位置（比如太阳），而物体本身不会完全变暗和变亮吗？

是什么导致了这种行为？这是旧的即时模式。我是OpenGL新手

看来光源是 "glued" 每个物体的一个面。当每个物体旋转时，它的光源也会随之旋转。

#include <windows.h>                            // Header File For Windows
#include <gl\gl.h>                              // Header File For The OpenGL32 Library
#include <gl\glu.h>                             // Header File For The GLu32 Library
#include <GL\glut.h>

static void resize(int width, int height)
{
    const float ar = (float) width / (float) height;

    glViewport(0, 0, width, height);
    glMatrixMode(GL_PROJECTION);
    glLoadIdentity();
    glFrustum(-ar, ar, -1.0, 1.0, 2.0, 100.0);

    glMatrixMode(GL_MODELVIEW);
    glLoadIdentity() ;
}

GLfloat     rtri  = 0.0;                          // Angle For The Triangle 
GLfloat     rquad = 0.0;                        // Angle For The Quad     
GLfloat     xPosCube = 0.0;

//--------------------------------------------------------------------------------------------
// DrawPyramid
//--------------------------------------------------------------------------------------------
void DrawPyramid( void )
{
    glLoadIdentity();
  glTranslatef( -1.5f, 0.0f, -6.0f );                   // Move Left 1.5 Units And Into The Screen 6.0

  glRotatef( rtri, 0.0f, 1.0f, 0.0f );              // Rotate The Triangle On The Y axis

    glBegin( GL_TRIANGLES );                    // Start Drawing The Pyramid

    // Face 1
      glVertex3f( 0.0f, 1.0f, 0.0f );           // Top Of Triangle (Front)
      glVertex3f( -1.0f, -1.0f, 1.0f );         // Left Of Triangle (Front)
      glVertex3f( 1.0f, -1.0f, 1.0f );          // Right Of Triangle (Front)

    // Face 2
      glVertex3f( 0.0f, 1.0f, 0.0f);            // Top Of Triangle (Right)
      glVertex3f( 1.0f,-1.0f, 1.0f);            // Left Of Triangle (Right)
      glVertex3f( 1.0f,-1.0f, -1.0f);           // Right Of Triangle (Right)

    // Face 3
      glVertex3f( 0.0f, 1.0f, 0.0f);            // Top Of Triangle (Back)
      glVertex3f( 1.0f,-1.0f, -1.0f);           // Left Of Triangle (Back)
      glVertex3f(-1.0f,-1.0f, -1.0f);           // Right Of Triangle (Back)

    // Face 4
      glVertex3f( 0.0f, 1.0f, 0.0f);            // Top Of Triangle (Left)
      glVertex3f(-1.0f,-1.0f,-1.0f);            // Left Of Triangle (Left)
      glVertex3f(-1.0f,-1.0f, 1.0f);            // Right Of Triangle (Left)
    glEnd();
}

//--------------------------------------------------------------------------------------------
// DrawCube
//--------------------------------------------------------------------------------------------
void DrawCube( void )
{
  glLoadIdentity();
    glTranslatef( 1.75f /* + xPosCube*/ ,0.0f,-7.0f + xPosCube);                // Move Right And Into The Screen

  glRotatef(rquad,1.0f,1.0f,1.0f);          // Rotate The Cube On X, Y & Z

    glBegin(GL_QUADS);

        glVertex3f( 1.0f, 1.0f,-1.0f);          // Top Right Of The Quad (Top)
        glVertex3f(-1.0f, 1.0f,-1.0f);          // Top Left Of The Quad (Top)
        glVertex3f(-1.0f, 1.0f, 1.0f);          // Bottom Left Of The Quad (Top)
        glVertex3f( 1.0f, 1.0f, 1.0f);          // Bottom Right Of The Quad (Top)

        glVertex3f( 1.0f,-1.0f, 1.0f);          // Top Right Of The Quad (Bottom)
        glVertex3f(-1.0f,-1.0f, 1.0f);          // Top Left Of The Quad (Bottom)
        glVertex3f(-1.0f,-1.0f,-1.0f);          // Bottom Left Of The Quad (Bottom)
        glVertex3f( 1.0f,-1.0f,-1.0f);          // Bottom Right Of The Quad (Bottom)

        glVertex3f( 1.0f, 1.0f, 1.0f);          // Top Right Of The Quad (Front)
        glVertex3f(-1.0f, 1.0f, 1.0f);          // Top Left Of The Quad (Front)
        glVertex3f(-1.0f,-1.0f, 1.0f);          // Bottom Left Of The Quad (Front)
        glVertex3f( 1.0f,-1.0f, 1.0f);          // Bottom Right Of The Quad (Front)

        glVertex3f( 1.0f,-1.0f,-1.0f);          // Bottom Left Of The Quad (Back)
        glVertex3f(-1.0f,-1.0f,-1.0f);          // Bottom Right Of The Quad (Back)
        glVertex3f(-1.0f, 1.0f,-1.0f);          // Top Right Of The Quad (Back)
        glVertex3f( 1.0f, 1.0f,-1.0f);          // Top Left Of The Quad (Back)

        glVertex3f(-1.0f, 1.0f, 1.0f);          // Top Right Of The Quad (Left)
        glVertex3f(-1.0f, 1.0f,-1.0f);          // Top Left Of The Quad (Left)
        glVertex3f(-1.0f,-1.0f,-1.0f);          // Bottom Left Of The Quad (Left)
        glVertex3f(-1.0f,-1.0f, 1.0f);          // Bottom Right Of The Quad (Left)

        glVertex3f( 1.0f, 1.0f,-1.0f);          // Top Right Of The Quad (Right)
        glVertex3f( 1.0f, 1.0f, 1.0f);          // Top Left Of The Quad (Right)
        glVertex3f( 1.0f,-1.0f, 1.0f);          // Bottom Left Of The Quad (Right)
        glVertex3f( 1.0f,-1.0f,-1.0f);          // Bottom Right Of The Quad (Right)

    glEnd();                        // Done Drawing The Quad
}

//--------------------------------------------------------------------------------------------
// DrawPyramidHiddenLinesPolygonOffset()
//--------------------------------------------------------------------------------------------
void DrawPyramidHiddenLinesPolygonOffset( void )
{
  // Draw cube - Hidden Line Removal with Polygon Offset.. 
  glEnable(GL_DEPTH_TEST);
  glPolygonMode(GL_FRONT_AND_BACK, GL_LINE);
  glColor4f(1.0f,0.0f,0.0f, 1.0f);          // Set The Color To Violet

  DrawPyramid();

  glPolygonMode(GL_FRONT_AND_BACK, GL_FILL);
  glEnable(GL_POLYGON_OFFSET_FILL);
  glPolygonOffset(1.0, 1.0);
  glColor4f(0.5f,0.0f,0.0f, 0.1f);          // Set The Color To Violet

  DrawPyramid();

  glDisable(GL_POLYGON_OFFSET_FILL);
}

//--------------------------------------------------------------------------------------------
// DrawCubeHiddenLinesPolygonOffset()
//--------------------------------------------------------------------------------------------
void DrawCubeHiddenLinesPolygonOffset( void )
{
  // Draw cube - Hidden Line Removal with Polygon Offset.. 
  glEnable(GL_DEPTH_TEST);
  glPolygonMode(GL_FRONT_AND_BACK, GL_LINE);
  glColor4f(1.0f,0.0f,1.0f, 1.0f);          // Set The Color To Violet

  DrawCube( FALSE );

  glPolygonMode(GL_FRONT_AND_BACK, GL_FILL);
  glEnable(GL_POLYGON_OFFSET_FILL);
  glPolygonOffset(1.0, 1.0);
  glColor4f(0.5f,0.0f,0.5f, 0.1f);          // Set The Color To Violet

  DrawCube( FALSE );

  glDisable(GL_POLYGON_OFFSET_FILL);
}

//--------------------------------------------------------------------------------------------
// DrawGLScene()
//--------------------------------------------------------------------------------------------
void DrawGLScene(void)
{
  glClear(GL_COLOR_BUFFER_BIT | GL_DEPTH_BUFFER_BIT);

  // Do camera rotation first..

  // Now draw and rotate objects..
  DrawCubeHiddenLinesPolygonOffset();
  DrawPyramidHiddenLinesPolygonOffset();


    rtri  += 0.022f;                        // Increase The Rotation Variable For The Triangle 
    rquad -= 0.045f;                        // Decrease The Rotation Variable For The Quad     
  //xPosCube -= 0.0005f;

  glutSwapBuffers();
}

const GLfloat light_ambient[]  = { 0.0f, 0.0f, 0.0f, 1.0f };
const GLfloat light_diffuse[]  = { 1.0f, 1.0f, 1.0f, 1.0f };
const GLfloat light_specular[] = { 1.0f, 1.0f, 1.0f, 1.0f };
const GLfloat light_position[] = { 2.0f, 5.0f, 5.0f, 0.0f };

const GLfloat mat_ambient[]    = { 0.7f, 0.7f, 0.7f, 1.0f };
const GLfloat mat_diffuse[]    = { 0.8f, 0.8f, 0.8f, 1.0f };
const GLfloat mat_specular[]   = { 1.0f, 1.0f, 1.0f, 1.0f };
const GLfloat high_shininess[] = { 100.0f };



void IdleFunc()
{
  glutPostRedisplay();
}


void KeyboardFunc( BYTE key, int x, int y )
{
  switch (key)
  {
     case 27: // Escape key
      //glutDestroyWindow( Win.id );
      exit (0);
      break;
  }
}


/* Program entry point */
int main(int argc, char *argv[])
{
    glutInit(&argc, argv);
    glutInitWindowSize(640,480);
    glutInitWindowPosition(10,10);
    glutInitDisplayMode(GLUT_RGB | GLUT_DOUBLE | GLUT_DEPTH);

    glutCreateWindow("Programming Techniques - 3D Spheres");

    glutKeyboardFunc( KeyboardFunc );
    glutReshapeFunc(resize);
    glutDisplayFunc( DrawGLScene );
    glutIdleFunc( IdleFunc );

    glClearColor(1,1,1,1);
    glEnable(GL_CULL_FACE);
    glCullFace(GL_BACK);

    glEnable(GL_DEPTH_TEST);
    glDepthFunc(GL_LESS);

    glEnable(GL_LIGHT0);
    glEnable(GL_NORMALIZE);
    glEnable(GL_COLOR_MATERIAL);
    glEnable(GL_LIGHTING);

    glLightfv(GL_LIGHT0, GL_AMBIENT,  light_ambient);
    glLightfv(GL_LIGHT0, GL_DIFFUSE,  light_diffuse);
    glLightfv(GL_LIGHT0, GL_SPECULAR, light_specular);
    glLightfv(GL_LIGHT0, GL_POSITION, light_position);

    glMaterialfv(GL_FRONT, GL_AMBIENT,   mat_ambient);
    glMaterialfv(GL_FRONT, GL_DIFFUSE,   mat_diffuse);
    glMaterialfv(GL_FRONT, GL_SPECULAR,  mat_specular);
    glMaterialfv(GL_FRONT, GL_SHININESS, high_shininess);

    glutMainLoop();

    return EXIT_SUCCESS;
}

Answer 1

您没有提供任何表面法线，但光照依赖于它。结果，你所有的顶点和面都有相同的法线（GL的初始默认值是（0,1,0），iirc），这意味着每个面都被照亮，就好像它们与光源的角度都相同。当您应用旋转时，此法线会发生变化。

OpenGL：为什么当物体旋转时照明会改变？

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