# Concave/convex question

## Recommended Posts

Okay, here I go with another uninformed newb question. I've seen a lot of references in this forum and others to concave and convex polygons. Can someone explain what this means with regard to 3D object creation? I understand the greater meaning of the terms, but am not sure how they would apply to, say, a ground poly.thanks,

##### Share on other sites
 Help AVSIM continue to serve you!Please donate today!

Convex is an 'outie' and concave is an 'innie'.A concave shape can usually be split into 2 or more sub-shapes to make an 'outie' ( convex ).Convex = goodConcave = bad

##### Share on other sites

Wow, that's easier than I thought, Dick. Thanks for the explanation! It's just the sort of thing that one misses when diving into a program like gmax with no prior 3D experience. So, using your above illustrations as a reference, how about these:Is poly 1 convex? How about poly 2? Just for clarification.thanks,

##### Share on other sites

Hi Bill.Actually both shapes have concave elements.Any shape that could be rotated to form a cup, has a concave area.If it holds water, it's concave. All concave shapes can eventually be broken down into convex sub-shapes... The triangle is the most basic convex sub-shape.Some aspects of FS are pretty forgiving about concave shapes. The newer TDF polygons are very forgiving.Dick

##### Share on other sites

Gotcha. Good analogy with the cup.

##### Share on other sites

You should be using gmax by now anyway. It seems like FS2004 will only support triangles (which gmax does)...The reason for only using convex polygons is that graphics renderers (like opengl or directx) only can deal with convex polygons. If you use concave polygons they look wrong. Previous FS versions have dealt with concave polygons, by 'tesselating' them (ie split them into triangles). This costs valuable rendering time, so just using triangles is the best way...The exception are the new lwm and vtp masks. These aren't polygons in a classical sense but rather texture masks that get blended over terrain. Using concave polygons for this scenery shouldn't cost you any framerates, in fact the less polygons you have, the better your framerates (this is just an educated guess, but would make sense)...Cheers, Christian

##### Share on other sites

Yeah, because I'm a newcomer to scenery design I have the advantage of having started with Gmax... nothing to relearn! But some of the terminology is still lost on me, hence the question.

##### Share on other sites

Hi there Bill one thing to keep in mind a single "simple" polygonmade up of two faces(triangles) is NOT concave (or convex) the most it could be is "V" shaped as the building blocks (faces) in polygonal meshes are always flat surfaces made up of 3 vertices (points) and the lines that connect them like AB,BC and CA. So to get something close to a curve let alone a concave surface takes a lot of poly's all made up of the above mentioned triangular faces. I just thought I would mention the above so people understand that we have to be carefull about using curved surfaces as your poly count can skyrocket if you use curved surfaces with out thought as to how complex concave shapes really are. Also it might be possable by the time 2004 is out that we will be able to use patch modeling in addition to polygonal modeling (something else to think about). Dan

##### Share on other sites

Dan, can you explain what "patch" modelling is?Bob B

##### Share on other sites

Hi guys,Let's make this easy...If you trace a line on a poly(in all possible directions...) and you only cross 2 lines, then this is a Convex poly, otherwise is Concave.Rafael Sanchez

##### Share on other sites

Well sorry but what if those poly's are "flat" one could cross manyedges and never get any amount of curve. What I was trying to get across sir is that in standard polygonal modeling the building blocks(faces) are always flat. Of course just about angle one cares to mention is either concave or convex but all I was trying to get across was to be carefull about using curved surfaces that's all.And maybe let some people here gain a little knowledge on the geommetry behind what we all love to do. Dan

##### Share on other sites

Sure Bob Patch modeling is kinda half way between poly modeling andNURBS modeling (the math that underlies patch modeling is very different than with poly modeling.) To keep things getting out of hand here is that patch modeling is based on surfaces called "Patch Objects" that allow one to build very complex curved surfaces form"flat" patch objects buy pulling on vertexes(one way for example). I will post a pic in a little while. I beleve that gmax "does" Patch modeling BTW I know that 3DS max does(and several other progies as well). I would suggest that you play with patch objects in gmax (it's great fun). BTW you can NOT use patch objects in 2002. Hope this helps you Dan

##### Share on other sites

Sorry to intercept here:NURBS will also give triangles in the end. The only difference is that the hardware will calculate the triangles for you. If NURBS are supported in the hardware, this saves some storage space since you don't have to save all the triangles, but just a few control points.But, NURBS will also give triangles, so except for the storage place there is no real advantage over a high triangle count model...Cheers, Christian

##### Share on other sites

You are right Christian in regards to NURBS(Gmax does not "do" NURBS)A patch is Like I said halfway between NURBS and polygonal modeling(well sort of)LOL but is much less compute intensive than NURBS ogjects are. I am going to post a pic in a min for those that are interested. Dan

##### Share on other sites

Whew, Gerrish! Let me (hopefully) wrap up this tired old oveburdened thread by saying a big "thanks" for the full explanation. In the end, what I take away from all of this is -- drumroll please -- I use gmax, so it shouldn't be my first concern. I was just really curious about the terminology, and I think I've got a pretty good grounding now, thanks to you guys. BTW, want to hear something funny? You apologized for your SCASM notes above, but wanted to be plain. I'm one of those upstarts that is confused by SCASM, but is starting to get comfy with BGLC. Twisted, ain't it?thanks for the help,

##### Share on other sites

Hi Gerrish, your post cracked me up!Whilst not wanting to lose the thread for Bill Womack either, it looks like he's taken care of..so....... a comment about your first Para.<>It seems to me that patch objects, as they are provided for by GMAX are simply a unique fashion to arrive at a shape....once the shape is present as desired, wouldn't it be as simple as converting the object to editable mesh, for it to become supportable in fs2002?In which case learning about Patch is pretty interesting.Bob B

##### Share on other sites

Yup I have made several objects by patch modeling and then converted them so yes they are usefull in FS2002.Not sure why some people took me to task for bringing up patches they do save time and even whenconverted to meshes are not to poly heavy compared to doing a similar shape polygonaly from the start. Oh one other thing gmax hasan optimize tool no need for hand coding to "clean up" your modelsunless you like hand coding. Anyway thank you Bob for your interest! Dan

##### Share on other sites

My apologies, Dan, for misunderstanding what you were saying about patch objects and NURBS. I got the impression it was being said that one couldn't use them for FS2002, and I was obviously wrong! Any discussion about advanced techniques that we CAN use in gMax for FS2002 is, of course, very worthwhile, even though it had nothing to do with the 'headline' topic of this thread.I'm a bit ignorant about gMax myself because I simply don't need it for my own work. gMax is a fabulous 3D design tool and it's very kind of Microsoft to give us something so sophisticated for use with FS. But we will soon be seeing lots of alternative 3rd-party tools too. For my own work, I prefer to code by hand rather than use 3D tools with GUI interfaces, because my professional background is computer programming (and I'm not involved in designing complex objects!). That's why I prefer to stick with SCASM too. I spent years using assemblers, but those days are long past because there are so much more convenient ways of doing most programming work these days. I certainly won't be moving to BGLC as a tool of choice, thankyou very much, unless I want to do some section 8 'TMF' stuff and Manfred hasn't got round to updating SCASM ... But each to his own, eh.Kind RegardsGerrish

##### Share on other sites

Hi Gerrish, I'm sure there's no problems here, kind of a mutual respect society with only a minor ruffle....In re this post, It's interesting to me...I made the decision to learn scasm shortly before fs2002 came out, and was making headway with hand coded macros also, but gmax gives me a chance to experience the kind of gui used by pro 3d object designers, and that's been fun. I'm so far over my own head with what I could have done with scasm programming, but your work is fab so to each his own!Best,Bob B

##### Share on other sites

Not to worry Gerrish I to do a little coding for this sim as well(mostly to tweek the mapping of ground textures and such) I willattach a pic of a "laker" that uses both a "standard" mesh and somepatch stuff as well. Anyway like many people I use several tools todo our stuff as we (Jm and I)do both mesh scenery and ALL the 3D objects that go into doing a complete scenery package in other wordswe do a lot of hand tweaking!!. Dan

## Create an account

Register a new account

• Tom Allensworth,
Founder of AVSIM Online

• ### Hot Spots

• Flight Simulation's Premier Resource!

AVSIM is a free service to the flight simulation community. AVSIM is staffed completely by volunteers and all funds donated to AVSIM go directly back to supporting the community. Your donation here helps to pay our bandwidth costs, emergency funding, and other general costs that crop up from time to time. Thank you for your support!