CODEWITHSUNDEEP
opengl
Jeff Storey
Jeff Storey

Reputation: 57162

OpenGL filled polygon color "bleeding"

I'm trying to draw a 5 pointed star shape in openGL (Java) that is both filled and outlined. The code i'm using to draw it 

gl.glPushAttrib(GL.GL_ALL_ATTRIB_BITS);
try {
   // set attributes for filling the shape
   Color fillColor = Color.GREEN;
   float[] rgb = fill.getRGBColorComponents(null);
   gl.glColor4f(rgb[0], rgb[1], rgb[2], 1f);
   gl.glEnable(GL.GL_POLYGON_SMOOTH);
   gl.glPolygonMode(GL.GL_FRONT_AND_BACK, GL.GL_FILL);                    
   gl.glEnable(GL.GL_POLYGON_OFFSET_FILL);
   gl.glCallList(this.glListId);

   // set attributes for outlining the shape
   gl.glEnable(GL.GL_LINE_SMOOTH);
   Color outline = Color.RED;
   float[] rgb = outline.getRGBColorComponents(null);
   gl.glColor4f(rgb[0], rgb[1], rgb[2], 1f);
   gl.glPolygonMode(GL.GL_FRONT_AND_BACK, GL.GL_LINE);
   gl.glLineWidth(5);                 
   gl.glCallList(this.glListId);
}
finally {
    gl.glPopAttrib();
}

The creation of the call list just looks like:

gl.glNewList(this.glListId, GL.GL_COMPILE);
gl.glBegin(GL.GL_POLYGON);
for (int j = 0; j < xCoords.length; j++) {
   // 2d polygon
   gl.glVertex2d(xCoords[j], yCoords[j]);
}
gl.glEnd();

When I do this, the outline of the star draws perfectly, but the fill color bleeds outside the edges of the outline (if I draw it without the outline, the polygon looks does not look crisp either). Any ideas why?

EDIT: I added a screenshot showing the problem

enter image description here

thanks, Jeff

Upvotes: 2

Views: 3068

Answers (1)

genpfault
genpfault

Reputation: 52084

Depending on what you mean by "star shape", remember that GL_POLYGON can only render convex polygons correctly:

GL_POLYGON:

Draws a single, convex polygon. Vertices 1 through N define this polygon.

Upvotes: 8

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