Hi anthony,

> edit; here are some pictures using a combination of blend and
> network commands. its really close to what i'm going for but
> it is off sligthly. i'm thinking a higher level of continuity is the
> missing link.

If that's close to what you want, then you can probably use Rhino to help you get the final step, which would be by using the MatchSrf command in Rhino to edit the surfaces to make them have a particular continuity to their neighbors.

That is also another area that I want to improve in Moi as well - those continuity tools are not really a mature area of MoI right now, MoI is much more focused on making a lot of the more simple stuff like just generating stuff from 2D curves more fluid and quick to use. It's not as sophisticated in the areas of advanced surface modeling with continuity yet.

So to do that kind of stuff right now you will likely need to use another program in combination with MoI instead of only MoI totally all by itself. Rhino makes a good companion for that kind of stuff.

- Michael

thanks michael,

i think you nailed it. i believe that is exactly the issue. its almost perfect but the continuity is off some because i can't select the whole chain of curves.

here are a few more picks. it kind of develops some peaks along the way. the suction and pressure sides i can do with blend at .7 the le and te i have to use network. so it develops the problem areas. but its close. the whole blade is then unioned into a solid.

i used to use rhino until i discovered moi. i would never go back to rhino. i love moi. i am only evaluating it however for users of PROP_DESIGN. i'm retired now so other than evaluating moi for use with PROP_DESIGN, I don't do any other modeling. moi is the best choice as far as i'm concerned. this little problem area doesn't worry me too much because users of PROP_DESIGN would have to move on to much more expensive tools like FEA. moi is a great transition between PROP_DESIGN and FEA. You get the base blade geo and that is all that is really needed at this step. i was just experimenting some and I guess I ran into a small limit here. no worries.

your planned changes should fix the issue i'm having. thanks for the explanation. i won't keep beating on this now that i know i'm not doing something wrong.

i do hope that something like 64-bit precision can be added someday though. that should help with my trailing edge radius problem. but again that can be cleaned up in a downstream program. just another nicety.

give 'em an inch and they want a mile :-)

Hi anthony - MoI actually does already use double-precision floating point numbers which are 64-bit values. The issue about your little segment combined to a much larger profile is not a problem due to non-64-bitness, it's a problem of finding a good tolerance value for fitting operations that does not fit things with too dense or too loose results. Making a mix of a much smaller sized segment along with a larger one for the same surface operation makes it difficult to use one tolerance value for the whole operation. And stuff like using multiple tolerance values for different parts of a calculation can quickly lead to very complex UIs where you do things like tag a bunch of things with esoteric tolerance values for every step that you try to do things...

Anyway - re: continuity and this particular shape you're building - really I would recommend doing things that you want to be smooth like this all as one big long surface rather than trying to do it in chunks like you are doing now.

If you did it as one long sweep or network instead of one bit at a time, it would avoid any need to mess with continuity related stuff at all...

Usually for stuff that you want to be broadly smooth and look like one continuous shape, building it as a larger piece all in one surfacing operation is the best method.

- Michael

hi michael,

thanks for the clarification. i believe i understand some more about the trailing edge being a tolerancing issue.

as far as the sweep i may have confused you. the blade is made by a sweep. then i cut away the hub portion using a Boolean op. the circular part of the blade is also made the same way. the transition is the part that i need moi for. in the old days we used to just put a note on the drawing that said blend. it was left to the guys grinding the blades out or making the dies to figure the blend out (depending on construction methods). so only that one piece is where there is a loft/blend/network type op. everything else is made with sweep.

Hi anthony re: sweep - what I mean is instead of building it in 2 separate sections that then necessitates putting in a blend connection piece, you might instead try to build the entire length of the whole final result just as one long sweep with several cross sections in it.

Right now you're building it out of different separate pieces and that then leaves you with the problem of connecting them together smoothing with a transition piece.

If you were instead to build the entire object out of one longer sweep then you could get a shape that was all smooth throughout it without ever having to worry about the continuity of separate surfaces, because you'd have just one long surface instead of several little adjacent ones.

Often times if you want something to have a smooth continuous quality to its shape, it tends to be best to build the whole thing that you want to be smooth out of one big surface instead of building one smaller section of it separately.

- Michael

One thing you might try is to rebuild the aerofoil curve before you do the sweep. That way the curve will be one piece and will blend with a circle.

hi steve,

is that possible. that is something i wondered about but i don't know how to get it to be one curve. it is created with four different curves and all i can seem to do with it is join it. is there something else that can make it one curve?

Hi anthony, if the pieces are smooth where they touch then join them together so that they are one connected curve and then run the Rebuild command on it:
http://moi3d.com/2.0/docs/moi_command_reference10.htm#rebuild

That can reconstruct a curve that is made up of multiple smooth sub segments into one single piece.

- Michael

Thanks guys,

I have been trying various sweep stuff and I tried the rebuild command. I got something almost perfect with sweep. The rebuild didn't work out. It caused a lot of problems. Once I get this just right, I'll post how I did it and include some pics. I do think the upgrade to blend and network would be good. Allowing you to select multiple curves and hopefully do a g3 or higher surface. But the sweep method looks like it is going to work.

Here are some pics of a model that is really close. The only problem is there are problems with some surfaces that I'm trying to work out. All sorts of small slivers are being created in certain areas. I am using a mixture of sweep, network, rebuild, and other commands. Basically the sweep works but its a bit jagged in some places. So I'm trying to extract the curves and do a network. That is almost working. I'll keep working on it. I think it can be done it is just pretty difficult. Also I attached some pics of the small trailing edge radius issue. Right now I am increasing by about 6 times what is needed. The reduces the chord by about 1 percent.

I need to perfect the process to avoid the surface sliver elements but I can tell at this point the final model will look like the pics attached. The rebuild command is useful. I'm learning when and when not to use it. The multi-profile sweep is useful too. I wasn't familiar with it before.

Ideally however I am looking forward to an upgraded blend and network command.

Hi anthony, it's looking good! I was not able to see any of the slivery stuff that you were talking about in those particular screenshots. If it looks just a little triangular, it is possible that you're just seeing some display artifacts because the display mesh just to not have generated a lot of triangles in that particular area. That kind of display issue does not necessarily mean that there is a problem with the actual geometry.

The display system is generally oriented towards trying to do things fast so that you don't have to sit around for too long after doing every operation, so sometimes with this emphasis on speed of the display there is a chance for there to be some imperfections in it as well.

If it looks more messed up than just slightly jaggedy then it could be a problem though. It's hard for me to make a more accurate guess without seeing an example.

- Michael

hi michael,

they were so small i didn't get a screen shot of it. what is happening is the multi-profile sweep isn't giving me quite a smooth enough transition. It gets it exactly right but its rather linear in some places rather than a smooth curve. So I have been getting curves for a network patch by intersecting the results of the sweep with two planes. Then I use those curves and the network command. It looks much smoother but along a few lines it creates all these small surfaces. Too small to really show you. I think it is because of taking the kind of jagged sweep results and getting curves. It's hard for me to say when or why its happening exactly. What I am going to try next is rebuild the curves I need with a low amount of points and then run a spline through them. So make completely new curves. I think it will work. Everything is a band-aid though. It would really be great if the blend worked. I have to go through a lot more steps to basically do multiple network commands because the blend and sweep aren't quite working out.

A long time ago you were saying something about maybe making a tolerance that was adaptive or something. To get around the problem I have with the small trailing edge. I don't understand the issue even though you explained it to me a few times. But do you think there is something that could be created to allow for smaller features to be captured. Also this might play into the sweep issue I'm getting now. When I do the multi-profile sweep its almost perfect but it seems like I have a place where the rate of change is a little outside its comfort zone. So rather than a nice smooth curve I am getting like a piece wise linear deal. I'm envisioning perhaps sliders in dialogues where tolerancing is computed internally. maybe if it was exposed and the user can adjust it, that could possible fix many of the things i'm running into.

overall I think I can get this to work. but i'm just having to use a lot of trial and error and work around various problems that pop up. this has been good though making me get to know moi more. i really like it, even though its kicking my a** for three days now. i know a lot more about it than i did before.

Hi anthony, I'm afraid it's really difficult for me to understand what kind of straightness you were seeing without actually seeing an image.

From what you describe, it is entirely possible that the straightness was just a lack of triangulation in the display mesh in some areas which is just a display artifact to ignore and not necessarily an actual problem in the underlying geometry.

It's really hard for me to know for sure though just by a general written description.

- Michael

hi michael,

understood. here are some pics using the sweep. the back edge that has a ton of curvature appears smooth until you start zooming in and then it becomes piecewise linear. i have tried various mesh angles but this doesn't have an affect on it. the only way i can get rid of it is to go through a bunch of steps to generate curves that i can then use the network command on. it is much easier to use the sweep obviously. i tried rebuild on all the curves and rails prior to the sweep and that helped a lot. i previously hadn't thought to do rebuild on the rails. but i am still left with this piecewise linear problem. it seems like there is some sort of internal setting that the user can't access that is controlling this phenomenon. not sure though. in any event, i can see this working out ok. i just have to pound on it some more.

It might sound like a good idea to have a patch that can be G3 continuous at it's edges but looking at you prop design, it would be extremely difficult to use. Solidworks can do this but looking at your model you would need four patches. Now, which one would you do first? The first one is going to have two curves as two of the edges. These can only have G0 (positional ) continuity. Then when you put the next patch in, you face the same problem on at least one edge. The resulting surface may well not be what you want. These type of patches (certainly in my experience with SW) are normally used to close a hole that has been delberately cut out to remove unwanted geometry.

Hi anthony, so from what I see there that does look to be likely just a lack of refinement in the display mesh in those areas.

It's most likely nothing to be concerned about - in order to be displayed the surfaces have to be converted to triangles and those areas of the surface just did not get a whole lot of triangles generated for them.

The display is inherently piecewise linear because video cards only understand how to process triangles. Most of the time MoI does a nice job of making enough triangles to make things have a smooth appearance. In some situations you may not get enough - that's partly because the focus on the display mesh generation is on doing it really quickly so it cuts corners in some areas and it is possible to have a bit of a jagged appearance when that happens.

When that happens it is not something that you need to go through any great lengths to fix - it's not an actual error in your geometry so you should just ignore it and not try to reconstruct your model.

Normally one way to verify that it's only a rough display mesh would be to export to a mesh format and crank the export density way up and look at what that does. The export mesher uses a somewhat different approach than the display mesher and does a much more thorough job of testing and refining things.

But my main advice for what you see there is to just ignore it - as far as I can see it's simply a display artifact and not any actual error. Maybe MoI's typically nice display has inadvertently made things worse in this case, by making you worry too much about seeing some evidence of the slightly lower density triangulation in the display mesh in some spots. I mean what you are showing there seems to be a close zoom in to a small area of the large surface, just doing that kind of closely zoomed in examination will exaggerate what is actually only really a small amount of angularity in the display...

- Michael

The curve display is somewhat different though - curves are diced up dynamically to only deviate by less than a pixel on the current view screen, so the curve display automatically refines as you zoom in.

So that means that what you see on your screen for curves is a really accurate representation of the real curve.

Unfortunately there is too much number crunching work involved in doing that same dynamic refinement process on shaded surfaces, so a surfaces are only broken down to a static set of triangles one time and do not automatically refine as you zoom in. So that's why it is not unusual or an "error" to see some evidence of the triangulation process in the shaded surface display. When you see some slight angularity and it does not look like it is strangely leaking outside of bounds or looks like really scrambled colors (those types of things can be evidence of trim curve problems or self intersections), then you're just seeing some of the silhouettes that the display mesh is made up of. That's nothing to worry about - it's a normal part of the display.

- Michael

thanks guys,

you may be right steve. i have never tried this before so i don't know how to actually do it. it seemed to me that a blend with higher continuity would make sense. however the sweep is doing what i want. i'm not sure how it figures it all out but it is doing it.

i do get a lot of graphics issues with the surfaces. so its good to know that it is just graphics and not a real problem. it may be nice to be able to see the actual surface mesh in moi though based on what you are saying. at least to be able to check the real thing without having to go to another program. however, i've got what i need at this point. again my only purpose was to kick the tires on moi to see if its a good downstream program for PROP_DESIGN. i believe that it is. just a little tricky in areas.

Hi anthony,

> it may be nice to be able to see the actual surface mesh
> in moi though based on what you are saying.

You mean the surface control points? You can do that in MoI by separating out the surface so that it's not joined to anything else and then you can turn on the control point cage with Edit > Show pts.

But that shows you the control polygon hull of the surface - the surface itself is a smooth spline that is influenced by those hull points.

Your video card does not know how to show a mathematically continuous spline directly - any program that you load the surface into is going to need to break it down into triangles in order to display it just like MoI does, and most of the time they do it actually quite a bit rougher than MoI does. That kind of slightly jagged angular type silhouette is a pervasive type of display artifact that's common to pretty much all CAD programs.

It's just not something to really worry about so much - as long as the only thing that looks weird is a slight angularity but things are not leaking grossly outside of boundaries or look like pieces are sticking through each other then it's pretty safe to assume that you're just seeing a slightly coarse display mesh in that particular area.


> at least to be able to check the real thing without having to
> go to another program

Normally the best way to check it in MoI is to export to a polygon mesh format like OBJ for example and then crank the export mesh density way up. The export mesher does not take shortcuts with how it analyzes the mesh and decides to subdivide it, so you can use it to generate a more accurate polygon representation and examine that. But you won't be able to use that method with the "no save" version though, since it relies on going through the process of saving to a poly format.

- Michael

thanks michael,

i posted the final example pics on my website:

http://propdesign.weebly.com/screenshots.html

i mention MoI on my website as well at various places. All good words of course. This does it for me. Fantastic program. Thanks for all the help. I couldn't have done it without you. I couldn't even import the points at first. So the forum support was definitely important.