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问题:

I am working on an assignment to draw wire frame GLUT standard objects. This seems simple but we are told we cannot use gluAtLook(), glOrtho(), glFrustrum but we must use glTranslate(), glScale() and glRotate. How to you project the object without using these functions in glMatrixMode(GL_PROJECTION)?

This is what I have so far:

#include "stdafx.h"
#include <iostream>
#include <cstdlib>
#include <cmath>
#include <math.h> // for sqrt()
#include <glut.h>


void init(void)
{
    glClearColor(0.0, 0.0, 0.0, 0.0);
    glShadeModel(GL_FLAT);
}

void display(void)
{
    glClear(GL_COLOR_BUFFER_BIT);
    glColor3f(1.0, 1.0, 1.0);
    glLoadIdentity();             /* clear the matrix */
    /* viewing transformation  */
    glTranslatef(0.0, 0.0, -5.0);
    glScalef(1.0, 2.0, 1.0);      /* modeling transformation */
    glutWireCube(1.0);
    glFlush();

}

void reshape(int w, int h)
{
    glViewport(0, 0, (GLsizei)w, (GLsizei)h);
    glMatrixMode(GL_PROJECTION);
    glLoadIdentity();
    glFrustum(-1.0, 1.0, -1.0, 1.0, 1.5, 20.0);
    //glTranslatef(0.0, 0.0, -5.0);
    glMatrixMode(GL_MODELVIEW);
    glLoadIdentity();
}

int main(int argc, char** argv)
{
    glutInit(&argc, argv);
    glutInitDisplayMode(GLUT_SINGLE | GLUT_RGB);
    glutInitWindowSize(500, 500);
    glutInitWindowPosition(100, 100);
    glutCreateWindow(argv[0]);
    init();
    glutDisplayFunc(display);
    glutReshapeFunc(reshape);
    glutMainLoop();
    return 0;
}

Can someone please help with this?

回答1:

we are told we cannot use gluAtLook(), glOrtho(), glFrustrum but we must use glTranslate(), glScale() and glRotate

gluLookAt sets up the world to view space transformation, glOrtho does view to orthographic projection space transformation and glFrustum does view to perspective projection space transformation. When you say your tutor hasn't allowed to use it, it clearly means that the intention is to understand how these functions work in the first place.

There're many resources on the internet which teach you that. Here's one by a renowned UC Berkeley professor Dr. Ravi Ramamoorthi. SongHo has good articles which will help you do the same thing.

I can demonstrate a simple case in 2D. Say we've a world defined with objects (for simplicity we take a point P); we want the camera to be at (3, 3) with its X and Y axes pointing in directions opposite to world's X and Y axes. For simplicity we'll assume both frames have the same scaling factor i.e. 1 unit in both X and Y directions measure the same distance (magnitude) for both systems. So the two frames differ only by orientation and origin location (W₀ and V₀ are the symbols denoting them).

We need to derive M_world->view i.e. the matrix which maps points in world space to view space. This is what the now-deprecated gluLookAt function calculates and multiplies with GL_MODELVIEW matrix stack. This matrix will be used to get a view of the world from the camera's viewpoint.

We know that M_world->view = T_view->world. The matrix which maps points of frame A to frame B will also be the matrix which transforms B's frame into A's frame. The derivation goes like this

The point P in world has (1, 2) = P_w as coordinates, we're effectively finding a matrix, which when multiplied with P_w will give P_v i.e. the same point's coordinates in view frame. The point is written as a 3D point since homogeneous extension of a 2D point would be a 3D point; the homogeneous coordinate would be 1 since it's a point; had it been a vector, it'd be 0.

Step one is rotation; rotating view's frame by -180° (right-handed system where +ve rotation is counter-clockwise); now the axes are along the same direction for both frames. We've to tackle the origin difference, this is done by translation, which is step 2. Multiplying both will give the required matrix. Note that each step transforms the view's frame more closer into world's frame by post-multiplying. Also each transformation is based on that current local frame we're in and not based on the starting global (world) frame.

The same idea can be extended to 3D too, with some more effort. In the above derivation all I needed were just rotation matrix, translation matrix and matrix multiplication; no gluLookat.The links I gave you should help in calculating the same for 3D. The projection matrix derivation is a bit more involved. However, you can still achieve the result without using glOrtho; the links I gave above has the formula for the final matrix; you can compose a matrix using that and multiply that to the GL_PROJECTION matrix stack.

Note: The above derivation assumes column vectors and thus transformation matrices (like rotation) and multiplication order are done based on that. If you assume row vector convention then transpose all the matrices and reverse the order of multplication since

(AB)^T = B^T A^T

回答2:

What you need to do is compute your own projection matrix and model-view matrix. After that you can load them using glLoadMatrix right after you call glLoadIdentity.

回答3:

#include "stdafx.h"
#include <iostream>
#include <cstdlib>
#include <cmath>
#include <math.h> // for sqrt()
#include <glut.h>

static GLfloat Xvalue = 0.0f;
static GLfloat Yvalue = 0.0f;
static GLfloat xRot = 0.0f;
static GLfloat yRot = 0.0f;
static GLfloat zRot = 0.0f;
static GLfloat xScale = 1.0f;
static GLfloat yScale = 1.0f;
static GLfloat zScale = 1.0f;

void init(void)
{
glClearColor(1.0, 1.0, 1.0, 1.0);
glShadeModel(GL_FLAT);
}

void display(void)
{
glClear(GL_COLOR_BUFFER_BIT);
glColor3f(0.0, 0.0, 0.0);
glLoadIdentity(); //clear the matrix 
glTranslatef(Xvalue, Yvalue, -3.0); // viewing transformation 
glScalef(xScale, yScale, zScale); // modeling transformation
glRotatef(xRot, 1.0f, 0.0f, 0.0f);
glRotatef(yRot, 0.0f, 1.0f, 0.0f);
glRotatef(zRot, 0.0f, 0.0f, 1.0f);
glutWireCube(1.0);
glFlush();
}

void reshape(int w, int h)
{
GLfloat identity[16] = { 1.0, 0.0, 0.0, 0.0, 0.0, 1.0, 0.0, 0.0, 0.0, 0.0, 1.0, 0.0, 0.0, 0.0, 0.0, 1.0 };

if (h == 0) // don't want a divide by zero
{
    h = 1;
}

glViewport(0, 0, (GLsizei)w, (GLsizei)h);

glMatrixMode(GL_PROJECTION);
glLoadMatrixf(identity);
GLfloat fovy = 52.0f; // in degrees
GLfloat c = 1.0 / (GLfloat)tan(fovy / 4.0), a = (GLfloat)w / (GLfloat)h, n = 1.0f, f = 1000.0f;
GLfloat projection[16] = { c / a, 0.0, 0.0, 0.0,
    0.0, c, 0.0, 0.0,
    0.0, 0.0, -(f + n) / (f - n), -1.0,
    0.0, 0.0, -2.0*f*n / (f - n), 0.0 };
glMultMatrixf(projection);

glMatrixMode(GL_MODELVIEW);
glLoadIdentity();
}

void keyInput(unsigned char key, int x, int y)
{ // Keyboard input processing routine.
switch (key)
{
case 'b': // move left
    Xvalue -= 0.1;
    glutPostRedisplay();
    break;
case 'B': // move right
    Xvalue += 0.1;
    glutPostRedisplay();
    break;
case 'c': // move down
    Yvalue -= 0.1;
    glutPostRedisplay();
    break;
case 'C': // move up
    Yvalue += 0.1;
    glutPostRedisplay();
    break;
case 'e': // scale down
    zScale -= 0.1;
    xScale -= 0.1;
    yScale -= 0.1;
    glutPostRedisplay();
    break;
case 'E': // scale up
    zScale += 0.1;
    xScale += 0.1;
    yScale += 0.1;
    glutPostRedisplay();
    break;
case 'f': // rotate x axis CW
    xRot -= 5.0f;
    glutPostRedisplay();
    break;
case 'F': // rotate x axis CCW
    xRot += 5.0f;
    glutPostRedisplay();
    break;
case 'g': // rotate y axis CW
    yRot -= 5.0f;
    glutPostRedisplay();
    break;
case 'G': // rotate y axis CCW
    yRot += 5.0f;
    glutPostRedisplay();
    break;
case 'h': // rotate z axis CW
    zRot -= 5.0f;
    glutPostRedisplay();
    break;
case 'H': // rotate z axis CCW
    zRot += 5.0f;
    glutPostRedisplay();
    break;
}
}


int main(int argc, char** argv)
{
glutInit(&argc, argv);
glutInitDisplayMode(GLUT_SINGLE | GLUT_RGB);
glutInitWindowSize(500, 500);
glutInitWindowPosition(100, 100);
glutCreateWindow(argv[0]);
init();
glutDisplayFunc(display);
glutReshapeFunc(reshape);
glutKeyboardFunc(keyInput);
glutMainLoop();
return 0;
}