Viewing Volume is nothing but the region of 3D Cartesian space in that will occupy the window. It is nothing but the minimum and maximum x, y and z values that are inside the window. So if a vertex is outside this range of x, y and z values then they are clipped by OpenGL before rendering can occur.
Z Buffer
The new term we have to deal with in addition to width and height of an object in 3D graphics is depth. The depth of an object is its distance from the viewpoint. The viewpoint is the location from which we are looking at that point. This depth value goes into the depth or Z-buffer. If we are drawing 2 objects that have some pixels that overlap, the first object will after it is rendered have its depth value in the depth buffer. When the next object is rendered, OpenGL will check to see whether the pixel it’s about to draw is in front of (with respect to the viewpoint) any pixel from the first object that’s already drawn. It does this by checking the Z value of the current pixel with the value that is already in the buffer. If the new pixel is closer to the viewpoint, OpenGL places its depth value in the depth buffer. This is how the Z-buffer works.
正交投影和透视投影
One term we need to understand very well to learn 3D Graphics well is projection. Well, computer graphics at its simplest is all about setting a color to a pixel on screen. And a pixel on a screen can have only two dimensions. So 3D graphics is merely an illusion. The 3D coordinates that we specify will have to be projected onto a 2D surface to create this illusion for us. And we have to specify how this projection works. By specifying a projection we specify the clipping or viewing volume.
基本3D图形绘制
1,在CCY457OpenGLView.h中加入下列变量
BOOL m_bLine; //Status of Line
BOOL m_bPolygon; //Status of Polygon
BOOL m_bTriangle; //Status of Triangle
并且在构造函数中初始化
{
m_bPoint = FALSE;
m_bLine = FALSE;
m_bPolygon = FALSE;
m_bTriangle = FALSE;
m_bCube = FALSE;
m_bTorus = FALSE;
m_bTeapot = FALSE;
m_bIcosahedron = FALSE;
m_bSimpleCube = FALSE;
}
2,加入五个菜单项及其对应的事件处理程序。
{//画茶壶
m_bCube = FALSE;
m_bTorus = FALSE;
m_bTeapot = TRUE;
m_bIcosahedron = FALSE;
m_bSimpleCube = FALSE;
InvalidateRect(NULL,FALSE);
}
void CCY457OpenGLView::OnObjectsCube()
{//画立方体
m_bCube = TRUE;
m_bTorus = FALSE;
m_bTeapot = FALSE;
m_bIcosahedron = FALSE;
m_bSimpleCube = FALSE;
InvalidateRect(NULL,FALSE);
}
void CCY457OpenGLView::OnObjectsIcosahedron()
{//画二十面体
m_bCube = FALSE;
m_bTorus = FALSE;
m_bTeapot = FALSE;
m_bIcosahedron = TRUE;
m_bSimpleCube = FALSE;
InvalidateRect(NULL,FALSE);
}
void CCY457OpenGLView::OnObjectsTorus()
{//画环面
m_bCube = FALSE;
m_bTorus = TRUE;
m_bTeapot = FALSE;
m_bIcosahedron = FALSE;
m_bSimpleCube = FALSE;
InvalidateRect(NULL,FALSE);
}
void CCY457OpenGLView::OnObjectsSimplecube()
{//画简单立方体
m_bCube = FALSE;
m_bTorus = FALSE;
m_bTeapot = FALSE;
m_bIcosahedron = FALSE;
m_bSimpleCube = TRUE;
InvalidateRect(NULL,FALSE);
}
3，由于开始绘制3维图形，因此使用第二篇文章的OnSize()函数。
4，在RenderScene中加入具体的绘制代码：
{//绘制函数
if(m_bPoint==TRUE)
{
glPointSize(3.0f);
glBegin(GL_POINTS);
glVertex2f(0.0f,0.0f);
glVertex2f(1.0f,0.0f);
glVertex2f(0.0f,1.0f);
glEnd();
}
if(m_bLine==TRUE)
{
glBegin(GL_LINES);
glVertex2f(0.0f,0.0f);
glVertex2f(1.0f,0.0f);
glEnd();
}
if(m_bTriangle==TRUE)
{
glBegin(GL_TRIANGLES);
glVertex2f(0.0f,0.0f);
glVertex2f(2.0f,0.0f);
glVertex2f(0.0f,2.0f);
glEnd();
}
if(m_bPolygon==TRUE)
{
glBegin(GL_POLYGON);
glVertex2f(0.0f,0.0f);
glVertex2f(3.0f,0.0f);
glVertex2f(4.0f,3.0f);
glVertex2f(1.5f,6.0f);
glVertex2f(-1.0f,3.0f);
glEnd();
}
//Replace the current matrix with Identity Matrix
glLoadIdentity();
glTranslatef(0.0f,0.0f,-5.0f);
glRotatef(-30.0f,1.0f,1.0f,0.0f);
//Draw a Cube
if(m_bCube)
{
glutWireCube(1.0f);
}
//Draw a Torus
if(m_bTorus)
{
glutWireTorus(0.5f, 1.0f, 50, 50);
}
//Draw a Teapot
if(m_bTeapot)
{
glutWireTeapot(1.0f);
}
//Draw a Icosahedron
if(m_bIcosahedron)
{
glutWireIcosahedron();
}
//Draw a cube by specifying the vertices individually
if(m_bSimpleCube)
{
glPolygonMode(GL_FRONT_AND_BACK, GL_LINE);
//Front Face
glBegin(GL_POLYGON);
glVertex3f(-1.0f,-1.0f,0.0f);
glVertex3f( 1.0f,-1.0f,0.0f);
glVertex3f( 1.0f, 1.0f,0.0f);
glVertex3f(-1.0f, 1.0f,0.0f);
glEnd();
//Back Face
glBegin(GL_POLYGON);
glVertex3f(-1.0f,-1.0f,-1.0f);
glVertex3f(-1.0f, 1.0f,-1.0f);
glVertex3f( 1.0f, 1.0f,-1.0f);
glVertex3f( 1.0f,-1.0f,-1.0f);
glEnd();
//Left Face
glBegin(GL_POLYGON);
glVertex3f(-1.0f,-1.0f, 0.0f);
glVertex3f(-1.0f, 1.0f, 0.0f);
glVertex3f(-1.0f, 1.0f,-1.0f);
glVertex3f(-1.0f,-1.0f,-1.0f);
glEnd();
//Right Face
glBegin(GL_POLYGON);
glVertex3f(1.0f,-1.0f, 0.0f);
glVertex3f(1.0f,-1.0f,-1.0f);
glVertex3f(1.0f, 1.0f,-1.0f);
glVertex3f(1.0f, 1.0f, 0.0f);
glEnd();
//Top Face
glBegin(GL_POLYGON);
glVertex3f(-1.0f, 1.0f, 0.0f);
glVertex3f( 1.0f, 1.0f, 0.0f);
glVertex3f( 1.0f, 1.0f, -1.0f);
glVertex3f(-1.0f, 1.0f, -1.0f);
glEnd();
//Botton Face
glBegin(GL_POLYGON);
glVertex3f(-1.0f, -1.0f, 0.0f);
glVertex3f(-1.0f, -1.0f, -1.0f);
glVertex3f( 1.0f, -1.0f, -1.0f);
glVertex3f( 1.0f, -1.0f, 0.0f);
glEnd();
}
}