Line joins
Fernando Cacciola
I don't use OpenGL myself... this is a question from a customer:
When they draw the outline of a polygon/polyline, with a significantly thick
pen, the lines are not joined with any kind of corner. Exactly as if each
straight line segment where drawn independently.
This happens using LINE_LOOP or a Stencil. Only with a PolygonOffset they can
get mittered joins at the corners.
Is this so, or is there a way to have mittered corners without a polygon offset?
Here the code they're using:
GLfloat top, bottom, left, right;
glClearColor( 0.0f, 0.0f, 0.0f, 0.0f );
glClear( GL_COLOR_BUFFER_BIT );
glLineWidth(10);
glPushMatrix();
//
//line loop method
//
glColor3f(1.0f, 0.0f, 0.0f);
glBegin( GL_LINE_LOOP );
glVertex2f( 100.0f, 100.0f );
glVertex2f( 100.0f, 200.5f );
glVertex2f( 200.0f, 200.5f );
glVertex2f( 200.0f, 100.5f );
glEnd();
glBegin( GL_LINE_LOOP );
glColor3f(0.0f, 0.0f, 1.0f);
glVertex2f( 400.0f, 150.0f );
glVertex2f( 450.0f, 100.5f );
glVertex2f( 425.0f, 200.5f );
glVertex2f( 400.0f, 225.5f );
glEnd();
//
// stencil buffer method
//
left = 100; right = 200; top = 250;
bottom = 350;
glPushAttrib( GL_ALL_ATTRIB_BITS );
glClearStencil(0);
glClear(GL_STENCIL_BUFFER_BIT);
glEnable(GL_STENCIL_TEST);
glStencilFunc(GL_ALWAYS, 1, -1);
glStencilOp(GL_KEEP, GL_KEEP, GL_REPLACE);
glPolygonMode(GL_FRONT_AND_BACK, GL_FILL);
glColor3f(1.0f, 1.0f, 0.0f);
glBegin(GL_POLYGON);
glVertex2f(left, top);
glVertex2f(right, top);
glVertex2f(right, bottom);
glVertex2f(left, bottom);
glEnd();
glStencilFunc(GL_NOTEQUAL, 1, -1);
glStencilOp(GL_KEEP, GL_KEEP, GL_REPLACE);
glPolygonMode(GL_FRONT_AND_BACK, GL_LINE);
glColor3f(0.0f, 1.0f, 1.0f);
glBegin(GL_POLYGON);
glVertex2f(left, top);
glVertex2f(right, top);
glVertex2f(right, bottom);
glVertex2f(left, bottom);
glEnd();
glPopAttrib();
//
//glPolygonOffset method
//
left = 350; right = 450; top = 250;
bottom = 350;
glPushAttrib( GL_ALL_ATTRIB_BITS );
glEnable(GL_POLYGON_OFFSET_FILL);
glPolygonOffset(-2.5f, -2.5f);
glPolygonMode(GL_FRONT_AND_BACK, GL_LINE);
glColor3f(1.0f, 1.0f, 1.0f);
glBegin(GL_POLYGON);
glVertex2f(left, top);
glVertex2f(right, top);
glVertex2f(right, bottom);
glVertex2f(left, bottom);
glEnd();
glPolygonMode(GL_FRONT_AND_BACK, GL_FILL);
glColor3f(1.0f, 0.0f, 0.0f );
glBegin(GL_POLYGON);
glVertex2f(left, top);
glVertex2f(right, top);
glVertex2f(right, bottom);
glVertex2f(left, bottom);
glEnd();
glPopAttrib();
glPopMatrix();
TIA
--
Fernando Cacciola
SciSoft Consulting, Founder
http://www.scisoft-consulting.com
fungus
wrote:
> Hi people,
>
> I don't use OpenGL myself... this is a question from a customer:
>
> When they draw the outline of a polygon/polyline, with a significantly thick
> pen, the lines are not joined with any kind of corner. Exactly as if each
> straight line segment where drawn independently.
>
Correct.
> Only with a PolygonOffset they can
> get mittered joins at the corners.
>
??
> Is this so, or is there a way to have mittered corners without a polygon offset?
>
Draw a circle at the line ends, with the radius the
same as the line width.
> Fernando Cacciola
> SciSoft Consulting, Founderhttp://www.scisoft-consulting.com
Do I get part of your consulting fee?
--
<\___/>
/ O O \
\_____/ FTB.
http://www.topaz3d.com/ - New 3D editor for real time simulation
Vladimir Jovic
> I don't use OpenGL myself... this is a question from a customer:
>
> When they draw the outline of a polygon/polyline, with a significantly
> thick pen, the lines are not joined with any kind of corner. Exactly as
> if each straight line segment where drawn independently.
>
http://en.wikipedia.org/wiki/A_picture_is_worth_a_thousand_words
--
ultrasound www.ezono.com
Fernando Cacciola
Fair enough... I figured that the issue were so common that I could spare the
trouble of uploading the image somewhere, etc... (I don't have an account on
Flickr or any such thing FWIW)
Anyway, I wanted to confirm that OpenGL does indeed know nothing of line joins.
That seesm to be the case.
Best
Fernando Cacciola
> On Dec 11, 4:15 pm, Fernando Cacciola <fernando.cacci...@gmail.com>
> wrote:
>> Hi people,
>>
>> I don't use OpenGL myself... this is a question from a customer:
>>
>> When they draw the outline of a polygon/polyline, with a significantly thick
>> pen, the lines are not joined with any kind of corner. Exactly as if each
>> straight line segment where drawn independently.
>>
>
> Correct.
>
So no line joins in OpenGL?
>> Only with a PolygonOffset they can
>> get mittered joins at the corners.
>>
>
> ??
>
See the bottom of the code I posted. They create a closed polygonal chain that
corresponds to the outline of the thick stroke (this is called buffering btw).
>
>> Is this so, or is there a way to have mittered corners without a polygon offset?
>>
>
> Draw a circle at the line ends, with the radius the
> same as the line width.
This won't really work in the sense that resulting geometry is not that of a
mittered corner, but I get the idea: draw the corner manually.
The one way it will work is to draw and fill the tangled parallelogram that
comes from joining the four corners of the line endingds.
>
>
>> Fernando Cacciola
>> SciSoft Consulting, Founderhttp://www.scisoft-consulting.com
>
> Do I get part of your consulting fee?
>
LOL, we can share the same pat in the back I'm getting for doing this.
Best
Vladimir Jovic
>>
>>> Is this so, or is there a way to have mittered corners without a
>>> polygon offset?
>>>
>>
>> Draw a circle at the line ends, with the radius the
>> same as the line width.
>
> This won't really work in the sense that resulting geometry is not that
> of a mittered corner, but I get the idea: draw the corner manually.
>
Would drawing big points at the line ends be ok?
glPointSize to change the point size
--
ultrasound www.ezono.com
fungus
wrote:
>
> So no line joins in OpenGL?
>
No. OpenGL isn't really designed for that sort of
rendering.
> This won't really work in the sense that resulting geometry is not that of a
> mittered corner, but I get the idea: draw the corner manually.
>
Oh, you want miters...I missed that.
> The one way it will work is to draw and fill the tangled parallelogram that
> comes from joining the four corners of the line endingds.
>
That's one way....
The other way is to draw quads instead of lines and
adjust the endpoints (the graphics card is probably
quads anyway when you change the line width).
Fernando Cacciola
> On Dec 11, 5:13 pm, Fernando Cacciola <fernando.cacci...@gmail.com>
> wrote:
>
>
>
> > So no line joins in OpenGL?
>
> No. OpenGL isn't really designed for that sort of
> rendering.
>
> The other way is to draw quads instead of lines and
> adjust the endpoints (the graphics card is probably
> quads anyway when you change the line width).
>
Right... this makes the most sense.
Thanks
Fernando