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Newbie problems: face 'gaps' and flickering 'shadows'


Vulpinus
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I've made a mesh entrance to replace my prim build, and I have two issues I need advice on...

 

First, if I look at just the right angle, aligned with where two faces meet, I get a thin, flickering line as if there is the tiniest gap between the faces, reminiscent of two prims being off by a micron or so and why we texture the hidden, insides of prims where they abut. I've tried to capture it in the second picture below. It moves rapidly with the slightest change in view and looks much worse that the static photo shows.

The faces terminate on the same vertices (no 'doubles'), although when I checked each one, for some reason some were off by a tiny amount. 7.50002 instead of 7.5 for instance. I put them all to 7.5 and reuploaded but could still see the flickering line. How do I stop this happening?

 

Second, and worse, is what appears like a flicking 'shadow' around where my window frames protrude from the wall. This is not a shadow that should be there from a light source, but it appears nevertheless and flickers like crazy with the slightest movement. I've tried to capture that with the animated gif below, but again it looks much worse in the viewer. I think I've seen similar in other peoples mesh builds in SL. Looking closely at the prim version, I can see similar effects there but nowhere near as noticeably bad.

Is there anything I can do about this?

 

These problems happen on the mesh with or without a texture being applied; the photos have none because it's easier to see the issues.

 

 



[Editied to remove now-redundant link to my files]

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Vulpinus wrote:

The faces terminate on the same vertices (no 'doubles'), although when I checked each one, for some reason some were off by a tiny amount. 7.50002 instead of 7.5 for instance. I put them all to 7.5 and reuploaded but could still see the flickering line. How do I stop this happening?

Yes, the "prim gap" issue also exist in other 3D applications. Nowadays we use 64 bit floats while SL is still old enough to use 32 bit ones. That reduces the problem significantly but doesn't completely eliminate it.

One solution that usually works is to merge the vertices and then split them again. The snap function in Blender can also be very helpful: first open the Display section in the palette to the right and change grid scale to a very low number - 0.001 or 0.002 perhaps. Then select all and Snap Selection to Grid in the Mesh menu.

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The construction of your mesh has a few oddities. Its not easy to explain in words, but maybe a picture can help. I have moved the 3 selected faces backwards. Now you see that these faces are connected to the rest of the wall only by single vertices:



Here is a closeup:



Actually the problem (flickering) is caused by duplicated edges in your construction.

You can get rid of the flickering by reworking the walls a bit by connecting faces by edges. Here is a proposal. I am not sure if this is the best that can be done, but it may give you direction:



 This wall needs one more quad but it works for me.

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Thanks for your input Gaia, it's much appreciated.

So, is having a face connected to another like I have, with only a single vertex, potentially not a good thing? Looking at that another way, is there some significance to having faces connected by a common edge?

What probably looks like superfluous faces in my design was because I was planning to use a different texture on those places. However, I've rethought that somewhat, removed two materials and redesigned it similar to your example. I've also added extra faces either side of the quad above the door so that it isn't just connected by the lone vertices at the top corners.

Surprisingly (to me at least), the import to SL came in fractionally lower than the original in download cost despite the extra vertices. I guess the fewer materials makes a bigger difference. I've made a lower detail version for the medium LOD and a physics shape, and it comes in at 2.984 LI. Nice! Even better, I can link the mesh doorbell I made (it IM's me if anyone presses it and records any visitors to my parcel) and the double doors and it's still only 3 LI. I like mesh!

 

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Vulpinus wrote:

Thanks for your input Gaia, it's much appreciated.

So, is having a face connected to another like I have, with only a single vertex, potentially not a good thing? Looking at that another way, is there some significance to having faces connected by a common edge?

It's a safety measure. Usually it makes no difference but sometimes just aligning the vertices to an edge is not enough.

 


Vulpinus wrote:

I've made a lower detail version for the medium LOD and a physics shape, and it comes in at 2.984 LI. Nice!

Yes, nice :)

Now, get it down to 1 LI! :P

It is certainly possible even without sacrificing LOD.

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ChinRey wrote:

Now, get it down to 1 LI!
:P

It is certainly possible even without sacrificing LOD.

... 1 LI??? OK, you've got me thinking (that's sometimes dangerous, lol)

I thought it might be by reducing the number of materials. I reduced it to just two, from six, and it only dropped the download cost from 2.984 to 2.962. Not that then.

It's just occurred to me that I might not need the inside planes of the medium LOD model - I'll never get so far away that it will be needed, even in my mansion. So that can go. In fact, if that's the case, the outside of the full LOD model has fewer vertices than the whole medium model I made.

Right, I'm off to experiment. I'll be back...

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1.2 LI  - Yes!

I'll have to check later on the beta grid (this laptop I'm on is not so good) but I think this should do the job.

Removing the inside of the medium LOD model and simplifying the outside (but actually better visually than my first attempt) has got me down to 1.2 download cost. With the size of the thing, by the time anyone gets far enough away to drop down the LOD it'll probably be beyond their draw distance anyway.

Of course, that might go up now when I link my other bits to it, but... 1.2 !!!

Thank you for making me think about it further. (I doubt I've thought of everything though...)

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Vulpinus wrote:

... 1 LI???


Maybe that's a bit too much but certainly well below 2.9.

 

You can easily get the download weight down well below 1.0 simply by splitting it up into four separate meshes, wall, two window frames and door frame. That would increase the server weight to 2.0 though. Whether that's a good idea depends on the weight balance of the rest of the house.


Vulpinus wrote:

It's just occurred to me that I might not need the inside planes of the medium LOD model - I'll never get so far away that it will be needed, even in my mansion.


The theoretical switch points for this particular mesh are 35, 140 and 280 m for mid, low and lowest res model respectively, a little bit lower (32, 129 and 258) if the mesh has four or five faces. That means you have to be a bit careful not to reduce the mid model too much even on the inside but you can simlify quite a bit. The low and lowest models you can just thrash. (Me, I would actually have kept fairly detailed models of the outer wall even at those LOD levels but I may be a bit over-sensitive to LOD issues.)

For the mid model, aim for 68 tris. You may have to go up to 76 but hopefully not. (You will need to do some simple retopology for that, not just delete polys.)

There may be a little bit to gain by improving the file's compressability. Even something as simple as aligning the door opening with some of the edges of the door frame may improve the LI noticeably. Your model is fairly well optimized already though so there may not be that much to gain here, unless you want to edit the dae file manually that is. ;)

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Yeah, I've been thinking about the balance of the three factors when linking. I need to start thinking on a larger scale now I'm getting at least a little of the hang of this stuff. Also, if I was designing for other people, I quite agree that I would keep more detail to account for potentially lower graphics settings. Since it's just for me, and I don't get many visitors, I'm happy with it.

I noticed that the LI in-world went up to 2 when I made the physics shape Prim, but a quick optimisation of that shape got the physics cost down to, and staying at, 0.6. That's without analysing the shape in the uploader, which made it better in the first case but worse in my better optimised version.

So, the final version...

The better medium LOD model I made looks just like the orthographic projection; I flattened it and removed the needless bits. Even when it switches at 30-ish metres, it's hard to tell the difference if I miss the slight texture change on the frames.

I made the medium LOD model double-sided again; it's only 44 tris per side. Like that, the mesh comes in at 2.1 download cost. The single-sided version is 1.2. I also discovered a loose vertex and edge or two and the useful 'select loose geometry' tool :)

 

Thanks to some very good help here, things are coming along nicely!

 

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