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Thread: How to draw on top of image

  1. #1
    Junior Member Newbie
    Join Date
    Jun 2014
    Posts
    4

    How to draw on top of image

    This is my first time using opengl and I am experimenting with adding/drawing polygons/points/etc. on top of a PPM image. The image is already loaded when the application runs. My attempt to draw a square is from lines 30 - 35. The program runs but the square is not present. Just the image. Any help is appreciated.

    Code :
    #include<windows.h>
    #include <stdio.h>
    #include <stdlib.h>
    #include<Gl/gl.h>
    #include<GL/glut.h>
     
    int n;
    int m;
     
    int *image;
     
    void myInit (void)
    {
         glClearColor ( 1.0, 1.0, 1.0 , 1.0);
         glColor3f ( 1.0f, 0.0f, 0.0f );
         glPointSize ( 4.0 );
         glMatrixMode ( GL_PROJECTION );
         glLoadIdentity ( );
         gluOrtho2D ( 0.0, 400.0, 1.0, 400.0 );
    }
     
     
    void display(void)
    {
     
        glClear ( GL_COLOR_BUFFER_BIT );
        glRasterPos2i(0,0);
        glDrawPixels(n,m,GL_RGB, GL_UNSIGNED_INT, image);
        glPointSize(10.0);
        glBegin(GL_POLYGON);
                 glVertex2f(-0.5, -0.5);
                 glVertex2f(-0.5, 0.5);
                 glVertex2f(0.5, 0.5);
                 glVertex2f(0.5, -0.5);
        glEnd();
        glFlush ( );
     
    }
     
    void myreshape(int h, int w)
    {
    	glMatrixMode (GL_PROJECTION);
    	glLoadIdentity();
    	gluOrtho2D(0.0, (GLfloat) n, 0.0, (GLfloat) m);
    	glMatrixMode(GL_MODELVIEW);
    	glLoadIdentity();
    	glViewport(0,0,h,w);
    }
     
    int main(int argc, char**argv)
    {
    	FILE *fd;
    	int k, nm;
    	char c;
    	char b[70];
    	float s;
    	char red, green, blue;
    	int x, y;
     
    	fd = fopen("map.ppm", "r");
    	if(fd == 0)
    	{
    		exit(0);
    	}
     
     
    	fscanf(fd, "%s", b);
    	if((b[0] != 'P') || (b[1] != '6'))
    	{
    		printf("%s is not a PPM file!\n", b);
    		exit(0);
    	}
     
    	fscanf(fd, "%c", &c);
     
    	fscanf(fd, "%c", &c);
    	while(c == '#')
    	{
    		fscanf(fd, "%[^\n]", b);
    		printf("%s\n", b);
    		fscanf(fd, "%c", &c);
    		printf("%c", c);
    	}
     
    	ungetc(c,fd);
     
    	fscanf(fd, "%d %d %d", &n, &m, &k);
     
    	printf("%d rows  %d colums  max value = %d\n", n, m, k);
     
    	nm = n*m;
     
    	image = (int*)malloc(3*sizeof(GLint)*nm);
     
    	s = 255./k;
     
    	for(x = 0; x < m; x++)
    	{
    		for(y = n-1; y >= 0; y--)
    		{
               fscanf(fd, "%c", &red);
        	   fscanf(fd, "%c", &green);
    	       fscanf(fd, "%c", &blue);
     
    		   image[3*nm - 3*(x*n +y) -3] = green;
    		   image[3*nm - 3*(x*n +y) -2] = blue;
    		   image[3*nm - 3*(x*n +y) -1] = red;
     
    		}
    	}
     
     
    	glutInit(&argc, argv);
    	glutInitDisplayMode(GLUT_SINGLE | GLUT_RGB);
    	glutInitWindowSize(n,m);
    	glutInitWindowPosition(0,0);
    	glutCreateWindow("Little BigHorn");
    	glutReshapeFunc(myreshape);
    	glutDisplayFunc(display);
    	glPixelTransferf(GL_RED_SCALE, s);
    	glPixelTransferf(GL_GREEN_SCALE, s);
    	glPixelTransferf(GL_BLUE_SCALE, s);
    	glPixelStorei(GL_UNPACK_SWAP_BYTES, GL_TRUE);
    	glClearColor(1.0, 1.0, 1.0, 1.0);
    	myInit ( );
        glutMainLoop ( );
    }

  2. #2
    Advanced Member Frequent Contributor
    Join Date
    Apr 2010
    Posts
    870
    Your chosen projection and the coordinates of your polygon's vertices don't fit together.

    Code :
    gluOrtho2D ( 0.0, 400.0, 1.0, 400.0 );

    This makes vertices with x in [0,400] and y in [1,400] project to your screen, but your vertex positions are in [-0.5,0.5].

  3. #3
    Junior Member Newbie
    Join Date
    Jun 2014
    Posts
    4
    Wooooooowwwww....I feel stupid. Thank you very much!

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