special line rendering

OpenGL OpenGL: Basic Coding
1

Hi,

I need to draw wireframe objects like the box in this image:

hilite wireframe box

what I tried was:

for each line,
render a thicker line with background color, and render again the line with the object color, thiner, and scaled a bit.

but does not give the effect… I looked at polygon offset, but looks like there is no something similar with lines…
Any Ideas ??

Thanks!

2

i think it will work if you do the same thing, but draw the lines front to back.

3

sorry, i mean back to front

4

could you tell what it means in code ?, an example perhaps…

Thanks!

5

Polygon offset affects lines too. You just have to enable is separately with glEnable(GL_POLYGON_OFFSET_LINE).

6

I tried that already, but does not do anything different… I’m just drawing lines, not polygons, the lines are arbitrary, they may or not form a wireframe object.

7

Originally posted by memfr0b:
Polygon offset affects lines too. You just have to enable is separately with glEnable(GL_POLYGON_OFFSET_LINE).
No, glPolygonOffset enables for point, line and fill affect your selected glPolygonMode style GL_POINT, GL_LINE and GL_FILL.
PolgonOffset does NOT affect point or line primitives. It needs a face to calculate the offset.

8

mmm… any hints ???, I can’t find a solution yet… :confused:

9

like the previous poster said, polygon offset doesnt affect GL_LINES, so you need to set your polygon mode to GL_LINE and draw the polygon faces of the cube.

GLdouble vert[8][3] = {
	{-.5, -.5, -.5}, {.5, -.5, -.5},
	{-.5, .5, -.5}, {.5, .5, -.5},
	{-.5, -.5, .5}, {.5, -.5, .5},
	{-.5, .5, .5}, {.5, .5, .5}
};

int quads[6][4] = {
	{0, 1, 3, 2}, {4, 5, 7, 6},
	{0, 1, 5, 4}, {2, 3, 7, 6},
	{0, 2, 6, 4}, {1, 3, 7, 5}
};

void display(void)
{
	int i;
	glClear(GL_COLOR_BUFFER_BIT | GL_DEPTH_BUFFER_BIT);
	glPolygonMode(GL_FRONT_AND_BACK, GL_LINE);
	glPolygonOffset(1, 1);
	for (i = 0; i < 6; i++) {
		glEnable(GL_POLYGON_OFFSET_LINE);
		glLineWidth(9);
		glColor3f(0, 0, 0);

		glBegin(GL_QUADS);
		glVertex3dv(vert[quads[i][0]]);
		glVertex3dv(vert[quads[i][1]]);
		glVertex3dv(vert[quads[i][2]]);
		glVertex3dv(vert[quads[i][3]]);
		glEnd();
		glDisable(GL_POLYGON_OFFSET_LINE);

		glLineWidth(1);
		glColor3f(1, 1, 1);
		
		glBegin(GL_QUADS);
		glVertex3dv(vert[quads[i][0]]);
		glVertex3dv(vert[quads[i][1]]);
		glVertex3dv(vert[quads[i][2]]);
		glVertex3dv(vert[quads[i][3]]);
		glEnd();
	}
	glFlush();
}

int main(int argc, char **argv)
{
	glutInit(&argc, argv);
	glutInitDisplayMode(GLUT_RGBA | GLUT_SINGLE | GLUT_DEPTH);
	glutCreateWindow(argv[0]);
	
	glClearColor(0, 0, 0, 0);
	glEnable(GL_DEPTH_TEST);
	glutDisplayFunc(display);
	glutReshapeFunc(reshape);
	glutMainLoop();
	return 0;
}