How to move triangle on plane in opengl - opengl

Question

I want to move triangle on plane.How will i do it.I am using opengl "glut" and also i want to keep co-ordinate position of triangle.first time it will start at point (0,0,-6) and then when i will press "a" this will move left and when i will press "d" this will move "right".I know the keyoperation definition in opengl of "w" and "s".But my main concern is with movment and knwoing of co-ordinate points.
Thankie

Answer 1

Use global variables to store your programs state. One of these states is the triangle position. Modify this variable in the keyboard handler, then call glutPostRedisplay. In the display function draw the scene according to the program's state.
Important hints: OpenGL is "merely" a sophisticated rasterizer API, which means it "lives for the moment", i.e. you just draw triangles with it, and after you sent the rendering command and they've been processed OpenGL has no "persistency" about the sent geometry and the only trace left are the changes in the framebuffer.
If what you're looking for is more along the lines of "I want to describe a scene consisting of geometric objects and those shall interact through some black box mechanism" you're looking for a so called scene graph, which OpenGL is not (but many scene graphs use OpenGL as a backend).
EDIT: Full example source code
#include <GL/gl.h>
#include <GL/glut.h>
#define ORTHO_SCALE 10.
GLfloat triangle_vertices[] = {
-0.5, 0.0,
0.5, 0.0,
0.0, 1.0
};
struct {
struct {
struct {
GLfloat x, y;
} pos;
GLfloat rot;
} triangle;
} sceneinfo;
void display(void);
void keyboard(unsigned char key, int x, int y);
void special(int key, int x, int y);
int main(int argc, char *argv[])
{
glutInit(&argc, argv);
glutInitDisplayMode(GLUT_RGBA | GLUT_DOUBLE);
glutCreateWindow("simple triangle test");
glutDisplayFunc(display);
glutKeyboardFunc(keyboard);
glutSpecialFunc(special);
glutMainLoop();
return 0;
}
void display(void)
{
GLuint win_width, win_height;
GLfloat win_aspect;
win_width = glutGet(GLUT_WINDOW_WIDTH);
win_height = glutGet(GLUT_WINDOW_HEIGHT);
win_aspect = (float)win_width/(float)win_height;
glViewport(0, 0, win_width, win_height);
glClear(GL_COLOR_BUFFER_BIT | GL_DEPTH_BUFFER_BIT);
glMatrixMode(GL_PROJECTION);
glLoadIdentity();
glOrtho(-win_aspect * ORTHO_SCALE,
win_aspect * ORTHO_SCALE,
-ORTHO_SCALE,
ORTHO_SCALE,
-1., 1.);
glMatrixMode(GL_MODELVIEW);
glLoadIdentity();
glPushMatrix();
glTranslatef(sceneinfo.triangle.pos.x, sceneinfo.triangle.pos.y, 0.);
glRotatef(sceneinfo.triangle.rot, 0, 0, 1.);
glEnableClientState(GL_VERTEX_ARRAY);
glVertexPointer(2, GL_FLOAT, 0, triangle_vertices);
glDrawArrays(GL_TRIANGLES, 0, 3);
glPopMatrix();
glutSwapBuffers();
}
void keyboard(unsigned char key, int x, int y)
{
switch(key) {
case '+':
sceneinfo.triangle.rot += 2.;
break;
case '-':
sceneinfo.triangle.rot -= 2.;
break;
}
glutPostRedisplay();
}
void special(int key, int x, int y)
{
switch(key) {
case GLUT_KEY_LEFT:
sceneinfo.triangle.pos.x -= 0.2;
break;
case GLUT_KEY_RIGHT:
sceneinfo.triangle.pos.x += 0.2;
break;
case GLUT_KEY_UP:
sceneinfo.triangle.pos.y += 0.2;
break;
case GLUT_KEY_DOWN:
sceneinfo.triangle.pos.y -= 0.2;
break;
}
glutPostRedisplay();
}