Most likely you have some of  your normals reversed. When a facet  has the normal on the wrong side it will look transparent from out side and opaque from the opposite (inside) side. A quick fix would be to apply double sided normals to all your facets. This would insure all facets would have a surface rendering both inside your object and outside your object. Hope this helps, Reuben

Dear  Mr. Reuben, first of all thanks a lot for your reply...what i currently have is 2 arrays..the first one is for the faces it contains the vertices you need for each face....the other one is for the vertices and also contains the normal of each vertex.... the thing is i'm quite new with Marching Cubes and the extraction of Isosurfaces...so practicly I don't have any idea of what you meant by reversed normals or double sided normals....when i extracted the normals i ended up with three values for each vertex I thought that if i apply the corresponding normal to each drawn vertex everything will be good...and that's it....the other thing is that whitout even applying any normals the object drawn is never closed...the triangles appear to be all parallel and if you look at the object from the front or the back you can see square holes that goes directly to the other side....how can i fix this???? Thanks again for your time

You dont need the normals if you list the vertices in the right order and in 3s. The problem with marching cubes is sometimes the facets get flipped. To fix this you can duplicate each facet in the reverse order. Your example has alternating vertices and normals. You dont need the normals as the normal is defined by the order of the vertices.  If you just use 3 vertices for each facet this will create one surface. Next reverse the order to insure both sides are rendered (this will double the size). Here is an example for a simple 4 sided pyramid.

void Render()

{

//clear color and depth buffer

glClear(GL_COLOR_BUFFER_BIT | GL_DEPTH_BUFFER_BIT);

glLoadIdentity();//load identity matrix

glColor3f(0.0f,0.0f,1.0f); //blue color

// draw 4 sided pyramid

glBegin(GL_TRIANGLES); //start drawing triangles

glVertex3f(0.0f,1.7f,1.4f); //triangle 1 first vertex

glVertex3f(-1.5f,-1.5f,1.4f); //triangle 1 second vertex

glVertex3f(1.5f,-1.5f,1.4f); //triangle 1 third vertex

//drawing 2nd triangle

glVertex3f(1.5f,-1.5f,1.4f); //triangle 2 first vertex

glVertex3f(-1.5f,-1.5f,1.4f); //triangle 2 second vertex

glVertex3f(0.0f,0.0f,-1.4f); //triangle 2 third vertex

//drawing 3rd triangle

glVertex3f(0.0f,1.7f,1.4f); //triangle 3 first vertex

glVertex3f(1.5f,-1.5f,1.4f); //triangle 3 second vertex

glVertex3f(0.0f,0.0f,-1.4f); //triangle 3 third vertex

//drawing 4th triangle

glVertex3f(-1.5f,-1.5f,1.4f); //triangle 4 first vertex

glVertex3f(0.0f,1.7f,1.4f); //triangle 4 second vertex

glVertex3f(0.0f,0.0f,-1.4f); //triangle 4 third vertex

// now reverse the order to draw the inside

glVertex3f(0.0f,1.7f,1.4f); //triangle 1 first vertex

glVertex3f(1.5f,-1.5f,1.4f); //triangle 1 third vertex

glVertex3f(-1.5f,-1.5f,1.4f); //triangle 1 second vertex

//drawing 2nd triangle

glVertex3f(1.5f,-1.5f,1.4f); //triangle 2 first vertex

glVertex3f(0.0f,0.0f,-1.4f); //triangle 2 third vertex

glVertex3f(-1.5f,-1.5f,1.4f); //triangle 2 second vertex

//drawing 3rd triangle

glVertex3f(0.0f,1.7f,1.4f); //triangle 3 first vertex

glVertex3f(0.0f,0.0f,-1.4f); //triangle 3 third vertex

glVertex3f(1.5f,-1.5f,1.4f); //triangle 3 second vertex

//drawing 4th triangle

glVertex3f(-1.5f,-1.5f,1.4f); //triangle 4 first vertex

glVertex3f(0.0f,0.0f,-1.4f); //triangle 4 third vertex

glVertex3f(0.0f,1.7f,1.4f); //triangle 4 second vertex

glEnd();//end drawing of triangles

}

Note when reversing the order all facets are duplicated and the second vertex and third vertex are swapped.

You can also check your results from the Marching Cubes if it outputs a standard 3D file. You can load the output into MeshLab software (free) if your file format is STL or PLY or OBJ or Vrml or X3D to make sure your code is correct. MeshLab can also fix any flipped facets but you will need to read the documentation. If you use MeshLab to fix the flipped facts you will not need to make duplicate reversed facets. For ISO surfaces we use VTK and/or ParaView.

Hope this helps, Reuben

PS: attached are 5 files that have the same pyramid that can be viewed with MeshLab.

The likely reason why some of your triangles are missing is back face culling. With normal settings back face culling is on (which means that triangles where you look from the back are not rendered). Back face culling is based on the order of the vertices. The normal mode is that vertices of a triangle have to be provided counter-clockwise. This behavior can also be changed to clockwise. Still, your mesh needs to be consistent. If you want to see the mesh from both the front and the back, turn off back face culling.

BTW, using glBegin(...) and glEnd() is deprecated. As you have mentioned that you already have the vertices in an array, you should have a look at vertex buffers. Search the internet about VAO (vertex array objects) and OpenGL to learn about them. Maybe have a look at http://www.opengl-tutorial.org/

Thanks a lot Mr. Rueben and Mr. Simon....i' ve been using MeshLab for a while now...i'll try to use both your suggestions....greatings from Cuba

hello again friends....turns out that the problem persists....but this time i have more information for you....I created an obj file from my isosurface with just the faces and vertices, no normals....and then loaded it with MeshLab and it was perfect so at least I'm extracting the isosurface correctly...then i realize one thing...seing the object from the inside rendered in Wireframe with Meshlab you can see all the triangle faces from the inside like it should be...but if you do it in my OpenGL application all you can see is a bunch of lines going from all sides...so I  think that probably what is happening is that OpenGL is mixing all my vertices....what can I do?????

If this is the case it sounds like an indexing error. You should post a little more code because otherwise it is hard to help. Maybe you can start by posting your real code from glBegin(GL_TRIANGLES) to glEnd(). Even more code might be required, though.

I don´t have the code with me right now because i'm writing from the university but i can write you some of the lines I consider are the problem.....I was already suspecting it is an indexing problem that could be probably corrected by using VAOs....but i still haven't changed the code.

please excuse me for the poor quality of the code as I said I have no experience with Marching Cubes and all i'm trying is to render the isosurface in the simplest way....

struct Vertex {float x,float y,float z};

struct Face {int v1, int v2, int v3};

Face faces[38000];

Vertex vertices[19000];//these number are aproximated I don't remember the real ones

glBegin(GL_TRIANGLE_STRIP);

    { for (int i=0; i

Looks like you x z z and it should be x y z ... just an observation.