PolygonMesh behavior

dvonhausen · 5 Oct 2010, 19:01

I am writing an obj exporter. I notice that the normals for each vertice from PolygonMesh is different depending on whether I first run Google's obj exporter using:
"Sketchup::active_model.export(objFileName,false)"

If I run my exporter without running Google's exporter, the normal for all the vertices of a face from PolygonMesh are the same which is just the normal for the face itself producing a faceted render.

However, if I run my exporter after running Google's exporter on the same object, the normal for all the vertices of any face from PolygonMesh are different producing a smooth face. This tells me that Google's exporter takes into account smooth edges that are connected to a vertice when computing a normal for a vertice and stores it somewhere that PolygonMesh can assess but there is nothing in the doc to suggest this.

Does anyone know why this is and where are the normals for vertices stored when exporting? Any doc or examples?

Thanks

thomthom · 5 Oct 2010, 21:37

This is odd...

Are you saying that the face normal doesn't depend on the order which the vertices of the PolygonMesh is added?

When you say it produces a smooth face - in SU? Or in the app that imports the OBJ you export?

AFIK, the only way to smooth a PolygonMesh was to specify Soft+Smooth when you add the mesh to the model. I've not seen any methods of smoothing individual polygons in a mesh - though I'd very much like to have that control.

mitcorb · 5 Oct 2010, 22:20

Pardon my eavesdropping, but exactly how would I visualize a vertex normal? I can visualize a face normal, even an edge normal under some conditions. Or is it sort of like a tesseract?

thomthom · 6 Oct 2010, 06:15

@mitcorb said:

Pardon my eavesdropping, but exactly how would I visualize a vertex normal? I can visualize a face normal, even an edge normal under some conditions. Or is it sort of like a tesseract?

I did that for Vertex Tools, I take the average normal of the faces connected to the vertex.

AdamB · 6 Oct 2010, 10:01

@thomthom said:

I did that for Vertex Tools, I take the average normal of the faces connected to the vertex.

You're missing a step with your averaging there. You need to weight by the contribution each face connected to the vertex makes - ie weight by the interior angle otherwise a cube constructed of triangles will end up with vertex normals not all pointing out exactly at 45 degrees.

soldatino · 15 Nov 2010, 22:56

I am sorry but I use this topic because it seems to be about my question.
I make a solid using the help example for smooth surfaces (6 sides), then I push for the elevation, then I select all the object, then I apply Soften edges angle 79,0 degrees, and the edge appear soft.
But if I get the normals at each point I read always (0, 0, 0), that is not a valid vector.
Sorry, where I am wrong?

 
for np in 1..numpol do
  vertices=mesh.polygon_at np
  numvrtx=vertices.length - 1
  for eachvrtx in 0..numvrtx do
    x = mesh.point_at((vertices[eachvrtx]).abs).x*RatioPov
    y = mesh.point_at((vertices[eachvrtx]).abs).y*RatioPov
    z = mesh.point_at((vertices[eachvrtx]).abs).z*RatioPov


    nx = mesh.normal_at (eachvrtx+1) #index is base 1
    UI.messagebox(nx.to_s)

thomthom · 16 Nov 2010, 07:20

@soldatino said:

I make a solid using the help example for smooth surfaces (6 sides

Which example?

@soldatino said:

But if I get the normals at each point

I think that .normal_at( index ) refer to the normal of a polygon - not point...

soldatino · 16 Nov 2010, 08:33

Error 404 (Not Found)!!1

(code.google.com)

but this is about points "particular index in the mesh."
Or it is for each face? In this case I have to calculate each angle between vertexes of adjacent faces?
Anyway the vector that I get is 0,0,0, but the faces have smooting in SU.
http://sketchup.google.com/support/bin/answer.py?hl=it&answer=114941
the procedure is explained in the yellow window..

thomthom · 16 Nov 2010, 09:15

I think "particular index in the mesh." refer to the index of polygons.

@soldatino said:

Anyway the vector that I get is 0,0,0, but the faces have smooting in SU.

Normals in SketchUp does not relate the smoothing. Normals are always perpendicular to the face/polygon.

To find out of the mesh has smooth edges use PolygonMesh.polygon_at http://code.google.com/intl/it/apis/sketchup/docs/ourdoc/polygonmesh.html#polygon_at or PolygonMesh.polygons http://code.google.com/intl/it/apis/sketchup/docs/ourdoc/polygonmesh.html#polygons

soldatino · 16 Nov 2010, 11:33

yes, thanks, pointing at the face I get the float values of the vector.
Now if it is a offset valid for all the points of each face, I have to find the common points of adjacent faces, and interpolate their offsets to get the vector for the points. A lot of work... do yo think it is right?

thomthom · 16 Nov 2010, 11:37

Think so - SU doesn't provide vertex normals.
When I wrote Vertex Tools I have to work them out manually from the connected faces.

TIG · 16 Nov 2010, 11:47

Look at a vertex.
Get its faces.
Get the normals of the faces [lets say there are 3 of them].
Combine the faces' normals [vectors] using v = v1 + v2 + v3
Now 'v' should be the combined-vector at that vertex [the vertex-normal?]...
Test it using entities.add_line(vertex.position,v.to_a)
which should draw a line at the vertex along the direction of the combined-vector 'v'...

soldatino · 16 Nov 2010, 19:43

thomthom, TIG, thanks. I will attempt to write this in Ruby, even if I dont know much of this language, to do it with a post editor would be very easy, storing the array of all the normals of every face as comments within the ouput of the points ...

note: Vertex.used_by? in a loop of all connected faces ? waiting for the eternity...

thomthom · 16 Nov 2010, 22:28

@soldatino said:

note: Vertex.used_by? in a loop of all connected faces ? waiting for the eternity...

soldatino · 16 Nov 2010, 23:40

@thomthom said:

@soldatino said:

note: Vertex.used_by? in a loop of all connected faces ? waiting for the eternity...

I am worried because I think that with large model the time required will be huge.
Addictionally I would not repeat the calculation for the same point when the loop scans the already involved faces, but if I do not want to repeat I have to save an array for all the entities, containing normals already calculated, and to scan it also, which could be worse...

thomthom · 17 Nov 2010, 07:10

What are you doing? An exporter?

thomthom · 17 Nov 2010, 07:14

<span class="syntaxdefault"><br />&nbsp;&nbsp;</span><span class="syntaxkeyword">for&nbsp;</span><span class="syntaxdefault">eachvrtx&nbsp;in&nbsp;0.</span><span class="syntaxkeyword">.</span><span class="syntaxdefault">numvrtx&nbsp;</span><span class="syntaxkeyword">do<br />&nbsp;&nbsp;&nbsp;&nbsp;</span><span class="syntaxdefault">x&nbsp;</span><span class="syntaxkeyword">=&nbsp;</span><span class="syntaxdefault">mesh</span><span class="syntaxkeyword">.</span><span class="syntaxdefault">point_at</span><span class="syntaxkeyword">((</span><span class="syntaxdefault">vertices</span><span class="syntaxkeyword">[</span><span class="syntaxdefault">eachvrtx</span><span class="syntaxkeyword">]).</span><span class="syntaxdefault">abs</span><span class="syntaxkeyword">).</span><span class="syntaxdefault">x</span><span class="syntaxkeyword">*</span><span class="syntaxdefault">RatioPov<br />&nbsp;&nbsp;&nbsp;&nbsp;y&nbsp;</span><span class="syntaxkeyword">=&nbsp;</span><span class="syntaxdefault">mesh</span><span class="syntaxkeyword">.</span><span class="syntaxdefault">point_at</span><span class="syntaxkeyword">((</span><span class="syntaxdefault">vertices</span><span class="syntaxkeyword">[</span><span class="syntaxdefault">eachvrtx</span><span class="syntaxkeyword">]).</span><span class="syntaxdefault">abs</span><span class="syntaxkeyword">).</span><span class="syntaxdefault">y</span><span class="syntaxkeyword">*</span><span class="syntaxdefault">RatioPov<br />&nbsp;&nbsp;&nbsp;&nbsp;z&nbsp;</span><span class="syntaxkeyword">=&nbsp;</span><span class="syntaxdefault">mesh</span><span class="syntaxkeyword">.</span><span class="syntaxdefault">point_at</span><span class="syntaxkeyword">((</span><span class="syntaxdefault">vertices</span><span class="syntaxkeyword">[</span><span class="syntaxdefault">eachvrtx</span><span class="syntaxkeyword">]).</span><span class="syntaxdefault">abs</span><span class="syntaxkeyword">).</span><span class="syntaxdefault">z</span><span class="syntaxkeyword">*</span><span class="syntaxdefault">RatioPov<br /></span>

Change into this

<span class="syntaxdefault"><br />&nbsp;&nbsp;</span><span class="syntaxkeyword">for&nbsp;</span><span class="syntaxdefault">eachvrtx&nbsp;in&nbsp;0.</span><span class="syntaxkeyword">.</span><span class="syntaxdefault">numvrtx&nbsp;</span><span class="syntaxkeyword">do<br />&nbsp;&nbsp;&nbsp;&nbsp;</span><span class="syntaxdefault">pt&nbsp;</span><span class="syntaxkeyword">=&nbsp;</span><span class="syntaxdefault">mesh</span><span class="syntaxkeyword">.</span><span class="syntaxdefault">point_at</span><span class="syntaxkeyword">((</span><span class="syntaxdefault">vertices</span><span class="syntaxkeyword">[</span><span class="syntaxdefault">eachvrtx</span><span class="syntaxkeyword">]).</span><span class="syntaxdefault">abs</span><span class="syntaxkeyword">)<br />&nbsp;&nbsp;&nbsp;&nbsp;</span><span class="syntaxdefault">x&nbsp;</span><span class="syntaxkeyword">=&nbsp;</span><span class="syntaxdefault">pt</span><span class="syntaxkeyword">.</span><span class="syntaxdefault">x</span><span class="syntaxkeyword">*</span><span class="syntaxdefault">RatioPov<br />&nbsp;&nbsp;&nbsp;&nbsp;y&nbsp;</span><span class="syntaxkeyword">=&nbsp;</span><span class="syntaxdefault">pt</span><span class="syntaxkeyword">.</span><span class="syntaxdefault">y</span><span class="syntaxkeyword">*</span><span class="syntaxdefault">RatioPov<br />&nbsp;&nbsp;&nbsp;&nbsp;z&nbsp;</span><span class="syntaxkeyword">=&nbsp;</span><span class="syntaxdefault">pt</span><span class="syntaxkeyword">.</span><span class="syntaxdefault">z</span><span class="syntaxkeyword">*</span><span class="syntaxdefault">RatioPov<br /></span>

Should be a little bit faster.

soldatino · 17 Nov 2010, 08:55

This is a part of my exporter to POV-Ray. Yes, now I apply your suggestion !

soldatino · 17 Nov 2010, 09:20

17/11/2010 10:03:00
//Ending at 17/11/2010 10:03:18

17/11/2010 10:09:00
//Ending at 17/11/2010 10:09:18

28935 faces
Later I will check on largest files...

tomasz · 18 Nov 2010, 07:22

@thomthom said:

Think so - SU doesn't provide vertex normals.

You are wrong Thomas!
Check this :
polygonmesh.normal_at(vertex_index)

Soldatino, your exporter will work faster now
I think that is what you have been looking for, unless you want to do vertex welding.