How to Draw a 3d House in Opengl

I assume that you have some knowledge of OpenGL. Otherwise, read "Introduction to OpenGL with 2D Graphics".

Example 1: 3D Shapes (OGL01Shape3D.cpp)

This example is taken from Nehe OpenGL Tutorial Lesson # 5 (@ http://nehe.gamedev.net/), which displays a 3D color-cube and a pyramid. The cube is made of of 6 quads, each having different colors. The hallow pyramid is made up of 4 triangle, with different colors on each of the vertices.

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#include <windows.h> #include <GL/glut.h> char title[] = "3D Shapes"; void initGL() { glClearColor(0.0f, 0.0f, 0.0f, 1.0f); glClearDepth(1.0f); glEnable(GL_DEPTH_TEST); glDepthFunc(GL_LEQUAL); glShadeModel(GL_SMOOTH); glHint(GL_PERSPECTIVE_CORRECTION_HINT, GL_NICEST); } void display() { glClear(GL_COLOR_BUFFER_BIT | GL_DEPTH_BUFFER_BIT); glMatrixMode(GL_MODELVIEW); glLoadIdentity(); glTranslatef(1.5f, 0.0f, -7.0f); glBegin(GL_QUADS); glColor3f(0.0f, 1.0f, 0.0f); 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); glColor3f(1.0f, 0.5f, 0.0f); 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); glColor3f(1.0f, 0.0f, 0.0f); 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); glColor3f(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, -1.0f); glVertex3f( 1.0f, 1.0f, -1.0f); glColor3f(0.0f, 0.0f, 1.0f); 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); glColor3f(1.0f, 0.0f, 1.0f); 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(); glLoadIdentity(); glTranslatef(-1.5f, 0.0f, -6.0f); glBegin(GL_TRIANGLES); glColor3f(1.0f, 0.0f, 0.0f); glVertex3f( 0.0f, 1.0f, 0.0f); glColor3f(0.0f, 1.0f, 0.0f); glVertex3f(-1.0f, -1.0f, 1.0f); glColor3f(0.0f, 0.0f, 1.0f); glVertex3f(1.0f, -1.0f, 1.0f); glColor3f(1.0f, 0.0f, 0.0f); glVertex3f(0.0f, 1.0f, 0.0f); glColor3f(0.0f, 0.0f, 1.0f); glVertex3f(1.0f, -1.0f, 1.0f); glColor3f(0.0f, 1.0f, 0.0f); glVertex3f(1.0f, -1.0f, -1.0f); glColor3f(1.0f, 0.0f, 0.0f); glVertex3f(0.0f, 1.0f, 0.0f); glColor3f(0.0f, 1.0f, 0.0f); glVertex3f(1.0f, -1.0f, -1.0f); glColor3f(0.0f, 0.0f, 1.0f); glVertex3f(-1.0f, -1.0f, -1.0f); glColor3f(1.0f,0.0f,0.0f); glVertex3f( 0.0f, 1.0f, 0.0f); glColor3f(0.0f,0.0f,1.0f); glVertex3f(-1.0f,-1.0f,-1.0f); glColor3f(0.0f,1.0f,0.0f); glVertex3f(-1.0f,-1.0f, 1.0f); glEnd(); glutSwapBuffers(); } void reshape(GLsizei width, GLsizei height) { if (height == 0) height = 1; GLfloat aspect = (GLfloat)width / (GLfloat)height; glViewport(0, 0, width, height); glMatrixMode(GL_PROJECTION); glLoadIdentity(); gluPerspective(45.0f, aspect, 0.1f, 100.0f); } int main(int argc, char** argv) { glutInit(&argc, argv); glutInitDisplayMode(GLUT_DOUBLE); glutInitWindowSize(640, 480); glutInitWindowPosition(50, 50); glutCreateWindow(title); glutDisplayFunc(display); glutReshapeFunc(reshape); initGL(); glutMainLoop(); return 0; }

GLUT Setup - main()

The program contains a initGL(), display() and reshape() functions.

The main() program:

1. glutInit(&argc, argv);
   Initializes the GLUT.
2. glutInitWindowSize(640, 480);
   glutInitWindowPosition(50, 50);
   glutCreateWindow(title);
   Creates a window with a title, initial width and height positioned at initial top-left corner.
3. glutDisplayFunc(display);
   Registers display() as the re-paint event handler. That is, the graphics sub-system calls back display() when the window first appears and whenever there is a re-paint request.
4. glutReshapeFunc(reshape);
   Registers reshape() as the re-sized event handler. That is, the graphics sub-system calls back reshape() when the window first appears and whenever the window is re-sized.
5. glutInitDisplayMode(GLUT_DOUBLE);
   Enables double buffering. In display(), we use glutSwapBuffers() to signal to the GPU to swap the front-buffer and back-buffer during the next VSync (Vertical Synchronization).
6. initGL();
   Invokes the initGL() once to perform all one-time initialization tasks.
7. glutMainLoop();
   Finally, enters the event-processing loop.

One-Time Initialization Operations - initGL()

The initGL() function performs the one-time initialization tasks. It is invoked from main() once (and only once).

glClearColor(0.0f, 0.0f, 0.0f, 1.0f);
glClearDepth(1.0f);

glClear(GL_COLOR_BUFFER_BIT | GL_DEPTH_BUFFER_BIT);
Set the clearing (background) color to black (R=0, G=0, B=0) and opaque (A=1), and the clearing (background) depth to the farthest (Z=1). In display(), we invoke glClear() to clear the color and depth buffer, with the clearing color and depth, before rendering the graphics. (Besides the color buffer and depth buffer, OpenGL also maintains an accumulation buffer and a stencil buffer which shall be discussed later.)

glEnable(GL_DEPTH_TEST);
glDepthFunc(GL_LEQUAL);
We need to enable depth-test to remove the hidden surface, and set the function used for the depth test.

glShadeModel(GL_SMOOTH);
We enable smooth shading in color transition. The alternative is GL_FLAT. Try it out and see the difference.

glHint(GL_PERSPECTIVE_CORRECTION_HINT, GL_NICEST);
In graphics rendering, there is often a trade-off between processing speed and visual quality. We can use glHint() to decide on the trade-off. In this case, we ask for the best perspective correction, which may involve more processing. The default is GL_DONT_CARE.

Defining the Color-cube and Pyramid

OpenGL's object is made up of primitives (such as triangle, quad, polygon, point and line). A primitive is defined via one or more vertices. The color-cube is made up of 6 quads. Each quad is made up of 4 vertices, defined in counter-clockwise (CCW) order, such as the normal vector is pointing out, indicating the front face. All the 4 vertices have the same color. The color-cube is defined in its local space (called model space) with origin at the center of the cube with sides of 2 units.

Similarly, the pyramid is made up of 4 triangles (without the base). Each triangle is made up of 3 vertices, defined in CCW order. The 5 vertices of the pyramid are assigned different colors. The color of the triangles are interpolated (and blend smoothly) from its 3 vertices. Again, the pyramid is defined in its local space with origin at the center of the pyramid.

Model Transform

The objects are defined in their local spaces (model spaces). We need to transform them to the common world space, known as model transform.

To perform model transform, we need to operate on the so-called model-view matrix (OpenGL has a few transformation matrices), by setting the current matrix mode to model-view matrix:

glMatrixMode(GL_MODELVIEW);

We perform translations on cube and pyramid, respectively, to position them on the world space:

glLoadIdentity();
glTranslatef(1.5f, 0.0f, -7.0f);

glLoadIdentity();
glTranslatef(-1.5f, 0.0f, -6.0f);

View Transform

The default camera position is:

gluLookAt(0.0, 0.0, 0.0, 0.0, 0.0, -100.0, 0.0, 1.0, 0.0)

That is, EYE=(0,0,0) at the origin, AT=(0,0,-100) pointing at negative-z axis (into the screen), and UP=(0,1,0) corresponds to y-axis.

OpenGL graphics rendering pipeline performs so-called view transform to bring the world space to camera's view space. In the case of the default camera position, no transform is needed.

Viewport Transform

void reshape(GLsizei width, GLsizei height) {
glViewport(0, 0, width, height);
The graphics sub-system calls back reshape() when the window first appears and whenever the window is resized, given the new window's width and height, in pixels. We set our application viewport to cover the entire window, top-left corner at (0, 0) of width and height, with default minZ of 0 and maxZ of 1. We also use the same aspect ratio of the viewport for the projection view frustum to prevent distortion. In the viewport, a pixel has (x, y) value as well as z-value for depth processing.

Projection Transform

GLfloat aspect = (GLfloat)width / (GLfloat)height;
glMatrixMode(GL_PROJECTION);
glLoadIdentity();
gluPerspective(45.0f, aspect, 0.1f, 100.0f);
A camera has limited field of view. The projection models the view captured by the camera. There are two types of projection: perspective projection and orthographic projection. In perspective projection, object further to the camera appears smaller compared with object of the same size nearer to the camera. In orthographic projection, the objects appear the same regardless of the z-value. Orthographic projection is a special case of perspective projection where the camera is placed very far away. We shall discuss the orthographic projection in the later example.

To set the projection, we need to operate on the projection matrix. (Recall that we operated on the model-view matrix in model transform.)

We set the matrix mode to projection matrix and reset the matrix. We use the gluPerspective() to enable perspective projection, and set the fovy (view angle from the bottom-plane to the top-plane), aspect ratio (width/height), zNear and zFar of the View Frustum (truncated pyramid). In this example, we set the fovy to 45°. We use the same aspect ratio as the viewport to avoid distortion. We set the zNear to 0.1 and zFar to 100 (z=-100). Take that note the color-cube (1.5, 0, -7) and the pyramid (-1.5, 0, -6) are contained within the View Frustum.

The projection transform transforms the view frustum to a 2x2x1 cuboid clipping-volume centered on the near plane (z=0). The subsequent viewport transform transforms the clipping-volume to the viewport in screen space. The viewport is set earlier via the glViewport() function.

Example 2: 3D Shape with Animation (OGL02Animation.cpp)

Let's modify the previous example to carry out animation (rotating the cube and pyramid).

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#include <windows.h> #include <GL/glut.h> char title[] = "3D Shapes with animation"; GLfloat anglePyramid = 0.0f; GLfloat angleCube = 0.0f; int refreshMills = 15; void initGL() { glClearColor(0.0f, 0.0f, 0.0f, 1.0f); glClearDepth(1.0f); glEnable(GL_DEPTH_TEST); glDepthFunc(GL_LEQUAL); glShadeModel(GL_SMOOTH); glHint(GL_PERSPECTIVE_CORRECTION_HINT, GL_NICEST); } void display() { glClear(GL_COLOR_BUFFER_BIT | GL_DEPTH_BUFFER_BIT); glMatrixMode(GL_MODELVIEW); glLoadIdentity(); glTranslatef(1.5f, 0.0f, -7.0f); glRotatef(angleCube, 1.0f, 1.0f, 1.0f); glBegin(GL_QUADS); glColor3f(0.0f, 1.0f, 0.0f); 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); glColor3f(1.0f, 0.5f, 0.0f); 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); glColor3f(1.0f, 0.0f, 0.0f); 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); glColor3f(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, -1.0f); glVertex3f( 1.0f, 1.0f, -1.0f); glColor3f(0.0f, 0.0f, 1.0f); 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); glColor3f(1.0f, 0.0f, 1.0f); 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(); glLoadIdentity(); glTranslatef(-1.5f, 0.0f, -6.0f); glRotatef(anglePyramid, 1.0f, 1.0f, 0.0f); glBegin(GL_TRIANGLES); glColor3f(1.0f, 0.0f, 0.0f); glVertex3f( 0.0f, 1.0f, 0.0f); glColor3f(0.0f, 1.0f, 0.0f); glVertex3f(-1.0f, -1.0f, 1.0f); glColor3f(0.0f, 0.0f, 1.0f); glVertex3f(1.0f, -1.0f, 1.0f); glColor3f(1.0f, 0.0f, 0.0f); glVertex3f(0.0f, 1.0f, 0.0f); glColor3f(0.0f, 0.0f, 1.0f); glVertex3f(1.0f, -1.0f, 1.0f); glColor3f(0.0f, 1.0f, 0.0f); glVertex3f(1.0f, -1.0f, -1.0f); glColor3f(1.0f, 0.0f, 0.0f); glVertex3f(0.0f, 1.0f, 0.0f); glColor3f(0.0f, 1.0f, 0.0f); glVertex3f(1.0f, -1.0f, -1.0f); glColor3f(0.0f, 0.0f, 1.0f); glVertex3f(-1.0f, -1.0f, -1.0f); glColor3f(1.0f,0.0f,0.0f); glVertex3f( 0.0f, 1.0f, 0.0f); glColor3f(0.0f,0.0f,1.0f); glVertex3f(-1.0f,-1.0f,-1.0f); glColor3f(0.0f,1.0f,0.0f); glVertex3f(-1.0f,-1.0f, 1.0f); glEnd(); glutSwapBuffers(); anglePyramid += 0.2f; angleCube -= 0.15f; } void timer(int value) { glutPostRedisplay(); glutTimerFunc(refreshMills, timer, 0); } void reshape(GLsizei width, GLsizei height) { if (height == 0) height = 1; GLfloat aspect = (GLfloat)width / (GLfloat)height; glViewport(0, 0, width, height); glMatrixMode(GL_PROJECTION); glLoadIdentity(); gluPerspective(45.0f, aspect, 0.1f, 100.0f); } int main(int argc, char** argv) { glutInit(&argc, argv); glutInitDisplayMode(GLUT_DOUBLE); glutInitWindowSize(640, 480); glutInitWindowPosition(50, 50); glutCreateWindow(title); glutDisplayFunc(display); glutReshapeFunc(reshape); initGL(); glutTimerFunc(0, timer, 0); glutMainLoop(); return 0; }

The new codes are:

GLfloat anglePyramid = 0.0f;
GLfloat angleCube = 0.0f;
int refreshMills = 15;
We define two global variables to keep track of the current rotational angles of the cube and pyramid. We also define the refresh period as 15 msec (66 frames per second).

void timer(int value) {
glutPostRedisplay();
glutTimerFunc(refreshMills, timer, 0); //
}
To perform animation, we define a function called timer(), which posts a re-paint request to activate display() when the timer expired, and then run the timer again. In main(), we perform the first timer() call via glutTimerFunc(0, timer, 0).

glRotatef(angleCube, 1.0f, 1.0f, 1.0f);
......
glRotatef(anglePyramid, 1.0f, 1.0f, 0.0f);
......
anglePyramid += 0.2f;
angleCube -= 0.15f;
In display(), we rotate the cube and pyramid based on their rotational angles, and update the angles after each refresh.

Example 3: Orthographic Projection (OGL03Orthographic.cpp)

As mentioned, OpenGL support two type of projections: perspective and orthographic. In orthographic projection, an object appears to be the same size regardless of the depth. Orthographic is a special case of perspective projection, where the camera is placed very far away.

To use orthographic projection, change the reshape() function to invoke glOrtho().

void reshape(GLsizei width, GLsizei height) {          if (height == 0) height = 1;                    GLfloat aspect = (GLfloat)width / (GLfloat)height;          glViewport(0, 0, width, height);          glMatrixMode(GL_PROJECTION);      glLoadIdentity();                                   if (width >= height) {             glOrtho(-3.0 * aspect, 3.0 * aspect, -3.0, 3.0, 0.1, 100);    } else {             glOrtho(-3.0, 3.0, -3.0 / aspect, 3.0 / aspect, 0.1, 100);    }          }

In this example, we set the cross-section of view-volume according to the aspect ratio of the viewport, and depth from 0.1 to 100, corresponding to z=-0.1 to z=-100. Take note that the cube and pyramid are contained within the view-volume.

Example 4: Vertex Array

In the earlier example, drawing a cube requires at least 24 glVertex functions and a pair of glBegin and glEnd. Function calls may involve high overhead and hinder the performance. Furthermore, each vertex is specified and processed three times.

Link to OpenGL/Computer Graphics References and Resources

bodineprall1938.blogspot.com

Source: https://www3.ntu.edu.sg/home/ehchua/programming/opengl/CG_Examples.html

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