How to draw N sided polygon using VBOs?

beginners
#1

Hi,

I am looking into resources, sample code, anything related on how to draw a N sided polygon using VBOs in 2D

Any help will be much appreciated

Cheers
rS

#2

Well, it looks like the best thing for me is to learn how to draw using vertex array first instead of jumping straight to VBO, so I did but I have a problem, the code draw a n side polygon, but there is always a vertice at [0,0] and I am not sure how to fix it

  
#define NUM_SIDES 4  
  
typedef struct {  
	GLfloat x;  
	GLfloat y;  
	GLfloat z;  
} Vertex3D;  
  
Vertex3D vertices[NUM_SIDES];  
  
GLfloat colors[] = {  
					1.0, 0.0, 0.0, 1.0,  
					0.0, 1.0, 0.0, 1.0,  
					0.0, 0.0, 1.0, 1.0,  
					1.0, 1.0, 1.0, 1.0  
					};  
  
float radius = 100.0;  
  
  
//--------------------------------------------------------------  
void testApp::setup(){  
	ofBackground(0.0, 0.0, 0.0);  
	  
	// set vertices values  
	const float K = TWO_PI / NUM_SIDES;  
	for (int i = 0; i < NUM_SIDES; i++) {  
		vertices[i].x = cos(i * K) * radius;  
		vertices[i].y = sin(i * K) * radius;  
		vertices[i].z = 0.0;  
	}  
}  
  
//--------------------------------------------------------------  
void testApp::draw(){  
	glPushMatrix();  
	glTranslatef(ofGetWidth() * 0.5, ofGetHeight() * 0.5, 0.0);  
	  
	glEnableClientState(GL_VERTEX_ARRAY);  
	glEnableClientState(GL_COLOR_ARRAY);  
	  
	glVertexPointer(3, GL_FLOAT, 0, &vertices);  
	glColorPointer(4, GL_FLOAT, 0, &colors);  
	glDrawArrays(GL_LINE_LOOP, 0, ((sizeof vertices) / 2) * NUM_SIDES);  
	  
	glDisableClientState(GL_VERTEX_ARRAY);  
	glDisableClientState(GL_COLOR_ARRAY);  
	  
	glPopMatrix();  
}

Any help will be much appreciated

Cheers
rS

#3

I think your call to glDrawArrays is wrong –

http://www.opengl.org/sdk/docs/man/xhtm-…-Arrays.xml

seems to me that the number of vertices to draw should be 4, not ((sizeof vertices) / 2) * NUM_SIDES.

If I plug in this, it seems to work fine for me:

  
glDrawArrays(GL_LINE_LOOP, 0, NUM_SIDES);

take care!
zach

#4

BOOMBASTIC!