Polygon 2D

Hi. Can anyone tell me why my polygon is drawing with extensions…

Like the picture:

There is my code:


                glPushMatrix(); 

                glPolygonMode(GL_FRONT_AND_BACK, GL_FILL); 

                CMTObject::Draw(); 
                 
                glBegin(GL_POLYGON); 
                for(CMTPolygon::iterator it = begin(); it != end(); it++) 
                { 
                        glColor3f(1,0,0); 
                        glVertex2d(it->x,it->y); 
                } 
                glEnd(); 
                 
                glVertexPointer(2, GL_DOUBLE, 0, data()); 
         
                glColor3f(0,0,0); 
                glDrawArrays(GL_LINE_LOOP,0,size()); 

How I can paint my polygon interior (inside lineloop)?

looks to me like a GL_POLYGON is internally drawn as a GL_TRIANGLE_FAN.
if you switch to glPolygonMode(GL_FRONT_AND_BACK, GL_LINE), you should see that all triangles start at one single point.
and since your polygon is not konvex, some triangles overlap.

btw, which country is that?

[QUOTE=RigidBody;1251205]looks to me like a GL_POLYGON is internally drawn as a GL_TRIANGLE_FAN.
if you switch to glPolygonMode(GL_FRONT_AND_BACK, GL_LINE), you should see that all triangles start at one single point.
and since your polygon is not konvex, some triangles overlap.

btw, which country is that?[/QUOTE]

Its a city limits from Blumenau - Santa Catarina - Brasil…

This is a non convex polygon…

OpenGL only guarantees drawing convex polygons correctly with glBegin/glEnd. It states this here: http://www.opengl.org/sdk/docs/man2/xhtml/glBegin.xml
The bit that covers in in the compatibility spec is:

What is the best way to do this “painting” inside the non convex polygon?

You can draw filled concave polygon with the stencil buffer. See this thread and this link.

Edit: with chromium on linux , open the link in a new tab otherwise the link bring me at the bottom of the web page.

Sorry trinitrotoluene… It’s no work. :stuck_out_tongue:

Im trying to use Tess, but steal not working, can you help me?


GLvoid tcbBegin(GLenum type)
	{
		glBegin(type);
	}

	GLvoid tcbVertex(GLvoid *vertex)
	{
		glVertex3dv((GLdouble*)vertex);
	}

	GLvoid tcbEnd()
	{
		glEnd();
	}

	GLvoid combineCallback(GLdouble coords[3], GLdouble *vertex_data[4], GLfloat weight[4], GLdouble **dataOut )
	{
		GLdouble *vertex;
		vertex = (GLdouble *) malloc(3 * sizeof(GLdouble));
		vertex[0] = coords[0];
		vertex[1] = coords[1];
		vertex[2] = coords[2];
		*dataOut = vertex;
	}

	GLvoid errorCallback(GLenum errorCode)
	{
		const GLubyte *estring;
		estring = gluErrorString(errorCode);
		fprintf(stderr, "Tessellation Error: %s
", estring);
	}

void draw()
{
double* dData = reinterpret_cast<double*>(data());

GLuint startList = glGenLists(2);

GLUtesselator* wall1 = gluNewTess();
gluTessCallback(wall1, GLU_TESS_VERTEX, (GLvoid (APIENTRY *) ()) &tcbVertex);
gluTessCallback(wall1, GLU_TESS_BEGIN, (GLvoid (APIENTRY *) ()) &tcbBegin);
gluTessCallback(wall1, GLU_TESS_END, (GLvoid (APIENTRY *) ()) &tcbEnd);
gluTessCallback(wall1, GLU_TESS_COMBINE, (GLvoid (APIENTRY *) ()) &combineCallback);
gluTessCallback(wall1, GLU_TESS_ERROR, (GLvoid (APIENTRY *) ()) &errorCallback);

glNewList(startList, GL_COMPILE);
glShadeModel(GL_SMOOTH);
gluTessProperty(wall1, GLU_TESS_WINDING_RULE, GLU_TESS_WINDING_POSITIVE);

gluTessBeginPolygon(wall1, NULL);
	gluTessBeginContour(wall1);
	for (int i = 0; i < size(); i+=3)
	{
		gluTessVertex(wall1, &dData[i], &dData[i]);
	}
	gluTessEndContour(wall1);
gluTessEndPolygon(wall1);

glEndList();

gluDeleteTess(wall1);
}

Try that with a simple concave polygon first:

  

//INIT CODE
glEnable(GL_DEPTH_TEST);


//DRAW CODE
glClear(GL_COLOR_BUFFER_BIT|GL_STENCIL_BUFFER_BIT|GL_DEPTH_BUFFER_BIT);

glEnable(GL_STENCIL_TEST);
glBindVertexArray(m_VAO);
  //Dont write in the color buffer
  //Dont write in the depth buffer
  glColorMask(GL_FALSE,GL_FALSE,GL_FALSE,GL_FALSE);
  glDepthMask(GL_FALSE);
  glEnable(GL_STENCIL_TEST);
  glStencilFunc(GL_ALWAYS,0x1,0x1);
  glStencilOp(GL_KEEP,GL_KEEP,GL_INVERT);
  glDrawArrays(GL_TRIANGLE_FAN,0,m_Vertices.size()); 
  
  
//Draw the triangle strip a second time but only where the stencil is one
  glDepthMask(GL_TRUE);
  glColorMask(GL_TRUE,GL_TRUE,GL_TRUE,GL_TRUE);
  
  glStencilFunc(GL_EQUAL,0x1,0x1);
//NOT TESTED:  using GL_INVERT as the last parameter permit to clear stencil value to 0 where 
//stencil value is one and to draw multiple concave polygon in the same frame.   
  glStencilOp(GL_KEEP,GL_KEEP,GL_KEEP);
  glDrawArrays(GL_TRIANGLE_FAN,0,m_Vertices.size()); 
  
glDisable(GL_STENCIL_TEST);



It work fine for me.

Also,when you create your rendering context with GLUT for example, you must ask for a depth buffer with a stencil buffer to be present when you call glutInitDisplayMode.

It’s work, but with some pixels error.

How to solve this?

Which algorithm are you using, the one with stencil buffer or the one with GLU tessellation (a deprecated library) ?

I’m using with stencil buffer.


glVertexPointer(3, GL_DOUBLE, 0, data());
		
		glColorMask(GL_FALSE,GL_FALSE,GL_FALSE,GL_FALSE);
		glDepthMask(GL_FALSE);
		glEnable(GL_STENCIL_TEST);
		glStencilFunc(GL_ALWAYS,0x1,0x1);
		glStencilOp(GL_KEEP,GL_KEEP,GL_INVERT);
		glDrawArrays(GL_TRIANGLE_FAN,0,size()); 

		//Draw the triangle strip a second time but only where the stencil is one
		glDepthMask(GL_TRUE);
		glColorMask(GL_TRUE,GL_TRUE,GL_TRUE,GL_TRUE);

		glStencilFunc(GL_EQUAL,0x1,0x1);
		//NOT TESTED:  using GL_INVERT as the last parameter permit to clear stencil value to 0 where 
		//stencil value is one and to draw multiple concave polygon in the same frame.   
		glStencilOp(GL_KEEP,GL_KEEP,GL_KEEP);
		glDrawArrays(GL_TRIANGLE_FAN,0,size()); 

		glDisable(GL_STENCIL_TEST);

		//Desenha contorno
		glColor3f(0,0,0);
		glDrawArrays(GL_LINE_LOOP,0,size());

Can you put your vertices data on a hosting web site and put a link on this thread so I can test your data on my computer.

There is the points…

https://docs.google.com/file/d/0B8B-NIoST12WMjVjVjBvbjBteVE/edit?usp=sharing

I have tried your data and the algorithm with stencil buffer work as is. I think your problem is when you read your data in a file or when you pass the data to OpenGL. Check if the value of your points in the array is all valid.

These points are my debug Trace from my point vector.

Im using opengl with MFC window. Not using glut. May be some init parameter?

Have some settings to gradient color?

Because just setting color and drawvertex polygon it’s painting with a gradient.

To test your data, I have computed the min and max for x and y. Then computed the mean value for both min max for each x,y and subtracted that value to each point. I do so because it easier to hardcode the value for the orthographic projection and I have read on this forum (but I don’t remember well) that when you give large value for translatef, strange things can happen (Your vertices are far away the origin). When you call VertexPointer with GL_DOUBLE most if not all OpenGL implementations convert your data to GL_FLOAT type, but I don’t think it will solve your problem.

I never used MFC so I can’t help for this.

[QUOTE=trinitrotoluene;1251315]To test your data, I have computed the min and max for x and y. Then computed the mean value for both min max for each x,y and subtracted that value to each point. I do so because it easier to hardcode the value for the orthographic projection and I have read on this forum (but I don’t remember well) that when you give large value for translatef, strange things can happen (Your vertices are far away the origin). When you call VertexPointer with GL_DOUBLE most if not all OpenGL implementations convert your data to GL_FLOAT type, but I don’t think it will solve your problem.

I never used MFC so I can’t help for this.[/QUOTE]

My ortho has the min and max x/y…

I will try to do this, subtract the min x and y to each point. And set my ortho do 0 and width of the limit map.

So I’ll tell you what happened.

Trinitroluene, using glut it’s working. Dammit! Thanks for all.