Finding Outer Vertices of Shapes
Quote:With your method, none of them are computed correctly.I just inputted all your examples and the shadows were fine for all of them.
One thing that I'm wondering about though... in your code, you use s._points.at(i) to get the point at index i, but in mine, you use s._points[i] for some reason. Does that matter?
at() is rangechecked; [] is not. That's the only difference, I think.
The dot product method is the fastest and easiest to do. I think the bug in your code is that you are taking the dot product between the edge direction and direction to one of the vertices. You should use the edge normal instead. The normal is (y, x) or (y, x) depending on how you choose to use it . Also not need to normalize if you are just test if the edge is backfacing or not.
Also note that this method requires you to always have same winding of the polygons otherwise the dot product test will fail. The easiest way to check the polygon winding is to calculate the area and if it is negative, reverse you array of points.
Code:
size_t n = m_verts.size();
for(size_t i = 0; i < n; ++i)
{
size_t j = (i+1) % n;
Vec2i edge(m_verts[j].x  m_verts[i].x, m_verts[j].y  m_verts[i].y);
Vec2i norm(edge.y, edge.x);
Vec2i dir(m_verts[i].x  m_x, m_verts[i].y  m_y);
if(Dot(norm, dir) > 0)
{
// font facing
dc.MoveTo(m_verts[i].x, m_verts[i].y);
dc.LineTo(m_verts[j].x, m_verts[j].y);
}
}
Also note that this method requires you to always have same winding of the polygons otherwise the dot product test will fail. The easiest way to check the polygon winding is to calculate the area and if it is negative, reverse you array of points.
Thanks memon! That worked perfectly. I realized yesterday I was using the edge instead of the edge normal, but when I switched it even my rectangles didn't work right. Your code works perfectly for all my shapes so far. Thanks.
Possibly Related Threads...
Thread:  Author  Replies:  Views:  Last Post  
finding a training center  Heeeelp  Glauter  1  2,933 
Dec 26, 2011 12:30 PM Last Post: zenkimoto 

Adding lots of static shapes in Chipmunk  performance question  TomorrowPlusX  3  7,305 
Jul 11, 2011 01:39 PM Last Post: Skorche 

Finding an object by name  markhula  2  4,547 
Mar 29, 2011 08:08 AM Last Post: markhula 

Game rectangles & OpenGL vertices  mk12  2  2,744 
Sep 7, 2010 07:45 PM Last Post: mk12 

Kind of a noob question  finding artwork within my .app??  Nethfel  2  2,791 
Mar 8, 2010 08:00 AM Last Post: AndyKorth 