[Bug 11956] New: drawing a line 2.0px in width has incorrect offset

http://bugs.freedesktop.org/show_bug.cgi?id=11956

           Summary: drawing a line 2.0px in width has incorrect offset
           Product: Mesa
           Version: unspecified
          Platform: x86 (IA32)
        OS/Version: Linux (All)
            Status: NEW
          Severity: normal
          Priority: medium
         Component: Mesa core
        AssignedTo: mesa3d-dev@...
        ReportedBy: ideasman42@...


Hi, when using blender3d with Mesa's libGL.so I notice a display problem that
isnt present with NVidia's or ATI's drivers.

http://members.optusnet.com.au/cjbarton/mesabug.png

This looks to be a bug in Mesa's line drawing function since blender uses it to
draw the outline.

Note that mesa has done this for a long time, and older versions had this
problem also.

The example below uses  glLineWidth(2.0) and glDepthMask(0) and I assume line
drawing with glLineWidth(2.0) to be the problem.

is the function in blender that draws the outline around the mesh -
drawobject.c:1938

/* Mesh drawing routines */

static void draw_mesh_object_outline(Object *ob, DerivedMesh *dm)
{

        if(G.vd->transp==0) {   // not when we draw the transparent pass
                glLineWidth(2.0);
                glDepthMask(0);

                /* if transparent, we cannot draw the edges for solid select...
edges have no material info.
                   drawFacesSolid() doesn't draw the transparent faces */
                if(ob->dtx & OB_DRAWTRANSP) {
                        glPolygonMode(GL_FRONT_AND_BACK, GL_LINE);
                        dm->drawFacesSolid(dm, set_gl_material);
                        glPolygonMode(GL_FRONT_AND_BACK, GL_FILL);
                }
                else {
                        dm->drawEdges(dm, 0);
                }

                glLineWidth(1.0);
                glDepthMask(1);
        }
}


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------- Comment #1 from khaqq@...  2007-08-12 07:52 PST -------

glLineWidth(2.0);

I have the impression that NVIDIA drivers do not draw a 2 pixel wide line in
this case... But Mesa does it.

However, Mesa's output is not really good looking in this case.


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------- Comment #2 from ideasman42@...  2007-08-12 19:49 PST -------
The image shows a 1px line around a 3d object, but this only works because the
line drawn is 2px wide, and half of it is masked out by the 3d cube.

Also, this works for ATI cards as well, which makes me think its not an nvidia
quirk.

A default install of blenders will show this problem if you want to test. Is
there any other way to validate the OpenGL drawing?

Here are some screenshots of opengl outlines rendering correctly.
http://www.zeronave.it/zn/wp-content/uploads/2007/04/blender1.png
http://www.lordfreedom.com/Pagine/Grafica/Download/Immagini/Blender.jpg
http://upload.wikimedia.org/wikibooks/en/0/03/Screenshot-5.png
http://www.siteduzero.com/uploads/fr/files/8001_9000/8249.png - with mesa only
the top and right lines show.
http://www.webalice.it/fulvio.peruggi/public/Pegasos&MorphOS/Articles/06_MorphOS_TLOS/Blender.jpg


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------- Comment #3 from ideasman42@...  2007-08-12 21:21 PST -------
Note, the bug is not that the line drawn by mesa is not 2.0px, I expect both
NVidia drivers and Mesa do this, the bug is that Mesa draws the Offset of that
line differently. you can see that the line is offset to the top right of the
model.

Open blender with teh default scen and middle ckick drag in the 3d view to
reproduce this.


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------- Comment #4 from brian.paul@...  2007-08-13 02:56 PST -------
I think the root problem is Mesa uses a different rasterization method for
lines and polygon edges that hits different pixels.  So, things don't match up
exactly.  Note that Mesa's lines and polygon egdes are conformant so nothing's
really incorrect.  With some work, Mesa's lines could be drawn differently.

One experiment to try would be to enable GL_LINE_SMOOTH and see what happens
since AA lines are drawn in a manner very similar to polygon edges.


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------- Comment #5 from ideasman42@...  2007-08-13 06:08 PST -------
enabled GL_LINE_SMOOTH and GL_BLEND and in this case Mesa gives very similar
results to NVidea

http://members.optusnet.com.au/cjbarton/mesa_bug.png


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--- Comment #6 from Campbell Barton <ideasman42@...>  2008-07-04 18:22:14 PST ---
Is it hard to change this? - It seems that offsetting the line would not be a
big change.


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