Moving Shape via Mouse in Opengl

问题:

void Rectangle(void) { 

glClear(GL_COLOR_BUFFER_BIT);
glBegin(GL_QUADS);
    glColor3f(1.0,0.0,0.0);
    glVertex2f(m_x-0.25,m_y-0.25);
    glColor3f(1.0,0.0,0.0);
    glVertex2f(m_x-0.25,m_y-0.75);
    glColor3f(1.0,0.0,0.0);
    glVertex2f(m_x-0.75,m_y-0.75);
    glColor3f(1.0,0.0,0.0);
    glVertex2f(m_x-0.75,m_y-0.25);
glEnd();
glutSwapBuffers();

I want to move the rectangle to where pressed mouse. m_x and m_y are global variable.This function has called in main glutDisplayFunc(Rectangle), mouse function like this:

void mouse(int button,int state,int x,int y)  
  switch(button)
{
case GLUT_LEFT_BUTTON:
    if(state==GLUT_DOWN)
    {
        m_x=x;
        m_y=y;
        Rectangle();
        glutSwapBuffers();
    }
    break;

default:
    break;

}

When the application runs ,it draws a rectangle after i pressed mouse the rectangle lost.What is my wrong?

回答1:

Don't render stuff in your mouse handler like that, just update the object position and queue a redraw with glutPostRedisplay():

#include <GL/glut.h>

float objX = 100;
float objY = 100;
float objSize = 50;
bool dragging = false;
void mouse( int button, int state, int x, int y )
{
    if( GLUT_DOWN == state )
    {
        bool colliding = 
            objX - objSize <= x && x <= objX + objSize
            &&
            objY - objSize <= y && y <= objY + objSize;
        if( colliding )
        {
            dragging = true;
            objX = x;
            objY = y;
            glutPostRedisplay();
        }
    }
    else
    {
        dragging = false;
    }
}

void motion( int x, int y )
{
    if( dragging )
    {
        objX = x;
        objY = y;
        glutPostRedisplay();
    }
}

void drawRect( float x, float y, float size )
{
    glPushMatrix();
    glTranslatef( x, y, 0.0f );
    glScalef( size, size, 1.0f );
    glBegin( GL_QUADS );
    glColor3ub( 255, 255, 255 );
    glVertex2f( -1, -1 );
    glVertex2f(  1, -1 );
    glVertex2f(  1,  1 );
    glVertex2f( -1,  1 );
    glEnd();
    glPopMatrix();
}

void display()
{
    glClearColor( 0, 0, 0, 1 );
    glClear( GL_COLOR_BUFFER_BIT | GL_DEPTH_BUFFER_BIT );

    glMatrixMode( GL_PROJECTION );
    glLoadIdentity();
    const double w = glutGet( GLUT_WINDOW_WIDTH );
    const double h = glutGet( GLUT_WINDOW_HEIGHT );
    glOrtho( 0, w, h, 0, -1, 1 );

    glMatrixMode( GL_MODELVIEW );
    glLoadIdentity();

    drawRect( objX, objY, objSize );

    glutSwapBuffers();
}

int main(int argc, char **argv)
{
    glutInit( &argc, argv );
    glutInitDisplayMode( GLUT_RGBA | GLUT_DEPTH | GLUT_DOUBLE );
    glutInitWindowSize( 600, 600 );
    glutCreateWindow( "GLUT" );
    glutDisplayFunc( display );
    glutMouseFunc( mouse );
    glutMotionFunc( motion );
    glutMainLoop();
    return 0;
}