how to draw concave polygon in jit.gl.sketch ?

have the same problem here. found out, that we have to do tessellation. found exhamles, written in c, but i have no idea how to do it in jitter :-(
any gl nerds out there, who can help us ?
drawing a filled polygon with jit.gl.sketch and put a texture on it.

thanx in advance !!

Quoting from the redbook:
"OpenGL polygons must be convex, meaning that they cannot have indentations.
Stated precisely, a region is convex if, given any two points in the
interior, the line segment joining them is also in the interior."

Guess you have to use triangles or triangle_strip for your primitive?

pelado

http://www.pelado.co.uk

On Thu, Nov 6, 2008 at 4:09 PM, Mathieu Chamagne
> wrote:

> Hi list,
>
> I need to draw concave polygon in jit.gl.sketch.
> is there a way to do it ?...

Mathieu, I suppose the answer lies in how you intend to use the shape after
it is drawn. There can be many ways to draw a surface, using triangles,
tri_fan, quads etc. Some things to remember:
a) the order that the vertices are drawn is important; b) solid shapes have
two sides;
c) if you draw vertices in a counter clockwise direction then the surface is
'front facing';
d) if you want to draw a non-convex polygon, then you have to break the
surface down into convex polygons, eg triangles;
e) if you want to draw the outline of your surface (poly_mode lines) then
you might get the outline of a triangle that you don't want seen, in which
case you have to use gledgeflags to tell openGL which lines to draw.

Below are some quick examples that might help show some of the differences
and get you on your way:

pelado

thanks very much, Pelado !

your example helped me understand the problem...
I must admit that I didn't took the time to read carefully all the red
book, so I miss some basic opengl concepts...

but my problem is not solved at all, and the solution seems much
complicated than I imagined !

first, let me explain more precisely what 'm trying to do :
I want to be able to draw freely a shape on screen (with mouse, for
ex) ; when mouse button is released, a close shape is created, joining
all the points the cursor passed by.
so :
- I can get a non predictable number of points (between 3 and...)
- if the shape is convex, the polygon method is fine
but, the shape can be ..anything, so...

I want to be able to draw closed and filled shapes (and eventually be
able to apply it a texture afterward...)

example patch :

________________
Mathieu Chamagne
mathieu.chamagne@gmail.com
http://mathieuchamagne.com
http://www.maxobjects.com

Hi!

I thought that the standard way to deal with this was to use a GLU
tesselator (a la red book chapter 11) however, it apparently is not
supported by jit.gl.sketch (based on a brief reading of the help
patcher).. People who know more, please correct me.. I didn't look
much at the patches attached, but assume they're replicating this
functionality in the patch.

I also wondered if maybe jit.gl.lua had wrappers for GLU/GLUT but
couldn't determine that quickly..

Best,
David

Mathieu Chamagne wrote:
> thanks very much, Pelado !
>
> your example helped me understand the problem...
> I must admit that I didn't took the time to read carefully all the red
> book, so I miss some basic opengl concepts...
>
> but my problem is not solved at all, and the solution seems much
> complicated than I imagined !
>
> first, let me explain more precisely what 'm trying to do :
> I want to be able to draw freely a shape on screen (with mouse, for ex)
> ; when mouse button is released, a close shape is created, joining all
> the points the cursor passed by.
> so :
> - I can get a non predictable number of points (between 3 and...)
> - if the shape is convex, the polygon method is fine
> but, the shape can be ..anything, so...
>
> I want to be able to draw closed and filled shapes (and eventually be
> able to apply it a texture afterward...)
>
> example patch :

..snip..

>
>
>
> ________________
> Mathieu Chamagne
> mathieu.chamagne@gmail.com
> http://mathieuchamagne.com
> http://www.maxobjects.com
>
>
>
>

Mathieu, I don't know the answer to your problem, sorry. One thing that
crosses my mind is that as a user draws a shape in the window, they are not
really describing a 'surface', instead they are defining the outline of a
surface that gets filled according to the openGL rules for the primitive
used. So, if I draw a doughnut with polygons the outline would describe the
doughnut but the surface would appear more like a circle (a 2d shape viewed
from the screen). Therefore, to accurately fill-in a surface described by
your user, I would think you would need to have the outline described first
and then to fill it in afterwards?
David, as far as I'm aware you are correct that jit.gl.sketch doesn't
support tesselation, which would help Mathieu with a solution. I don't know
about jit.gl.lua.

pelado

http://www.pelado.co.uk

On Fri, Nov 7, 2008 at 2:54 PM, Mathieu Chamagne
> wrote:

> thanks very much, Pelado !
>
> your example helped me understand the problem...
> I must admit that I didn't took the time to read carefully all the red
> book, so I miss some basic opengl concepts...
>
> but my problem is not solved at all, and the solution seems much
> complicated than I imagined !
>
> first, let me explain more precisely what 'm trying to do :
> I want to be able to draw freely a shape on screen (with mouse, for ex) ;
> when mouse button is released, a close shape is created, joining all the
> points the cursor passed by.
> so :
> - I can get a non predictable number of points (between 3 and...)
> - if the shape is convex, the polygon method is fine
> but, the shape can be ..anything, so...
>
> I want to be able to draw closed and filled shapes (and eventually be able
> to apply it a texture afterward...)
>

thanks again Pelado,

yes, I arrived to the same conclusion ; so the problem is now : how to
fill a shape described by points ? (in opengl...)
maybe I will try drawing in a matrix instead of opengl, and then
something like "close the blob" to fill the shape, and send it to a
texture to display it...
it will probably look worse than pure opengl drawing !
.. I didn't imagine that such a simple task would become so
complicated !
I will think about it this week end...

thanks for your help.

Mathieu
________________
Mathieu Chamagne
mathieu.chamagne@gmail.com
http://mathieuchamagne.com
http://www.maxobjects.com