[vtk-developers] OpenGL Line Stippling support ?

[Sebastien BARRE]

At 22/01/2001 08:56, Will Schroeder wrote:

>I think it would be a nice addition. The only minor concern is whether 
>this could be supported by other rendering libraries.

OK, done.

The problem is that OpenGL is kind of very "specific" in the way it handles 
line stippling. The relevant API call is :

     glEnable (GL_LINE_STIPPLE);
     glLineStipple (factor, pattern);

where "pattern" is a 16-bit binary pattern (1 = pixel on, 0 = pixel off), 
and "factor" specifies how many times each bit in the pattern is to be 
repeated.

Thus, I've added the following settings to vtkProperty and vtkProperty2D 
(and modified  vtkOpenGLProperty and vtkOpenGLPolyDataMapper2D accordingly).

   // Set/Get the stippling pattern of a Line, as a 16-bit binary pattern
   // (1 = pixel on, 0 = pixel off).
   // This is only implemented for OpenGL. The default is 0xFFFF.
   vtkSetMacro(LineStipplePattern,int);
   vtkGetMacro(LineStipplePattern,int);

   // Description:
   // Set/Get the stippling repeat factor of a Line, which specifies how
   // many times each bit in the pattern is to be repeated.
   // This is only implemented for OpenGL. The default is 1.
   vtkSetClampMacro(LineStippleRepeatFactor,int,1,VTK_LARGE_INTEGER);
   vtkGetMacro(LineStippleRepeatFactor,int);

I've added the graphics/exampleTcl/TestLineStipple.tcl script, an extended 
version of the squadViewer, where you can change the repeat factor and the 
pattern *interactively*, and watch the resulting line aspect.

Feel free to tell me if this interface is too specific, although we already 
have OpenGL specific functions in vtkProperty (LineWidth, PointSize). The 
problem is (as you stated) that this pattern and repeat factor might not 
have direct equivalent in the other rendering libraries. On the other hand, 
if you play with the testing script, you will see that an abstract 
interface like SetLineStyleToPlain or SetLineStyleToDash provides less 
control and will not guarantee that the line will look "dashed".

Just my 2 cents. Any other compromise is welcome, of course.