Mega Prims - The final Decision

I know that there is already plenty of info on this subject in the forum, but I need a definite response on this specific situation, so I can show it to the land owner, so here it is.
Okay, here is the deal, I have a mega prim on a piece of property, and it is perfectly centered and has the dimensions of 256*128*1. It is phantom and nonphysical, and does not go over the sim boundaries. But the owner continues to persist that the mega prims cause more lag then normal prims. The only reason I used a mega prim, is because it is a single texture back drop. But in the end he wants it removed. So I am asking you guys which would cause less lag. 1 mega prim with those settings or 314 normal prims, which would take months if not years of uvmapping to get the texture coordinates right. Input from lindens would be most certainly welcome, as that is probably the only people he will listen to.

The mega issue has still has not be decided yet on how it will work with Havok4.

But I will mention that mega prims as backdrops really don't work well because your draw distance must be set higher than most people use. If the center of the prim is outside your draw distance you don't see the texture. A sim I visit alot I only recently found out they use a megaprim backdrop after setting my draw to 256 to try to find a lost object. Nice backdrop but I would have never seen it if not by accident and probably won't see it often since my normal draw distance is 64 meters.

Not sure about the megaprim - you probably won't get the Lindens to answer here. Having phantom/non-physical is definitely good but I've seen (what seems like) conflicting answers from Lindens on if having megaprims causes increased sim/sim traffic.

Months or years? A little math and a week of learning to script should do the trick, I think.

Still, each individual prim would have to load the texture, and the server would have to send the position and size data to each client for each of the 314 prims. Whereas one big prim would only have to have the data sent once. If the client has their view distance set down to 64 it won’t matter how many prims the backdrop is made out of, they won’t be able to see most of it it anyways, and what they will be able to see will just look like crap. Also it is a phantom prim, so physics are not applied to it.

Also:

Sl can no t unwrap accurate enough to do that. You would have to break the texture into 314 square pieces. Then upload each piece individually at the cost of 10 L, that would come to a nice sum of 3140L, which isn't bad but your defiantly not gaining anything from doing it. Then finish it all off by assigning each texture to the according prim.

Nope. It just loads the texture once.


Yes but it's really not that expensive. Whether or not it's more expensive than one megaprim isn't known. Again, I've heard what seems like conflicting statements from LL about this.


We'll just all get that done tonight for you. I was bored being able to see more than 1/3rd of the way across my land anyway.

I mean each prim load the texture client side.

I just love it when people quote half a sentence and then make a statement about it. Also, it is a lot harder on the graphics card to render 1256 faces, rather then it just rendering 4 faces.

(Also 256/64 =4 therefore its 1/4)

But that is beside the point, I just want to know which causes less lag. Has anyone filled a sim with prims and then replaced them all with one mega prim yet to see which works better?

accoding to Andrew Linden, it's still up in the air as to whether a single NON-PHANTOM mega causes more sim processing than the equivalent in normal prims... and that phantom should not be an issue, relevent posts #42 and 43 here


he also earlier in the same thread say that the main issue that LL has w/ them is griefing potential...

but when it comes down to it, the owner is always right.

Consider this:

If your backdrop screen is 256*128 meters in size, and if you use a 1024 x 512 texture to texture it, every pixel in that texture will be 250 cm on a side. That makes each pixel bigger than an avatar's head. It's a pretty fuzzy picture if you get at all close to the backdrop itself, even when using a huge imported texture.

If you make the screen out of multiple 10M x 10M panels, then yes, you have to do a little math to calculate offsets, or you have to break the picture into more textures and import, BUT you can get a higher resolution on the surface of the screen!

It's pretty simple to use offsets and repeats to tile a single texture across a 3 x 3 array of 9 prims. So use one texture import for each 30M x 30M section of your screen. Now, a 1024 x 1024 texture applied to that section will give you pixels 29 cm on a side, or almost ten times more resolution! (8.6 times, if you want to be picky.)

8 of these 3x3 arrays will give you a 240 M long, 30 M tall wall, that takes 72 prims, and requires only 8 texture imports to texture it.

16 of these 3x3 arrays will give you a 240 M long, 60 M tall wall, that takes 144 prims, and requires only 16 texture imports to texture it.

Either wall will look 10 times better, and requires no megaprims.

I forgot about the fun parts, it’s an alpha blend texture, so you would have to re-alpha all the parts of the back drop what were see through. Also, there is no way that it could look better; at best it would look the same. You would also have to make all the back sides of the prims see through as well or else you could see the tiled texture between each prim if there was an alpha spot there. And yes I am quite aware that it can be done, but that was not my question. And actually you won’t be able to do it because alpha leaves those nontransparent edges, so you would have to screw with that, and when you got done it would just be an ugly mess of overly stretched textures. (keep in mind, sl doesn’t uvmap accurate enough to do it without breaking the image apart).

I've done huge multi prim pictures using texture offsets before, without noticeable lines, but editing each individual prims offsets IS a pain..... I've noticed some oddness in how the viewer translates these though... for example the texture will show seams right after I edit it, but if I leave and come back, it looks seamless...YMMV

Ceera's point about about texture stretching is well put, but the problem is, if you just use smaller bits while using the same texture, it's still stretched out, the solution is to chop up the parts beforehand, and resize them outside of SL... but then you'll have different parts loading at different times.... it may actually be slower to rez because of overhead for each individual texture, but it won't spike bandwidth like a big texture will... on the plus side you can get more detail this way by making an effectively larger texture than SL will allow you to upload by default

I recently did a project requiring a surrounding backdrop and it uses a set of 20x20x"1" megaprims, all set to phantom. While I havent opened it to the public yet, I havent had any physics issues on the systems I have tested on, other than a slight bend in my flight upwards--and that could be due to the flight override I have since its at over 500m. I dont intend to fly up here anyways. Most of the issues I had were with sculpties.

A quick snapshot is on flickr: http://www.flickr.com/photos/17046579@N08/2046549344/

I started with 50x50 tiles, but as Ceera mentioned, the resolution of the texture was too poor. 50m Prims are also hard to work with. I wont use anything larger due to risk of aggravating neighbors not for physics, but for size. 20x20 works well, and i am ready to go to 10x10 shoudl i run into problems.

That aside, if your backdrop texture is on a flat surface, most graphic apps have batch jobs to tile up the image. i broke mine down to 512x512 chunks, and they load quite nicely. It also makes it much easier to do maintenance--basically i can patch a quick fix instead of re-uploading the entire image.

I designed the background in a way specifically to have some repetitive tiles, and no worry about precise borders. This helped immensely, and prevented a lot of headaches. learn to embrace your environment.

In the end, If your landlord doesnt want megaprims on their land, they'll win, so you might want to aim for plan B before you lose a lot of time and gain a lot of frustration.

Hope this helps,

Court

I want the Mythbusters on this.

Retexturing the back and the unseen edge faces is easy.

* Select all the prims in the wall.
* Choose "Select Texture", and all the prim faces will be selected.
* Shift-click the exposed faces on the side that you will use for your poster, which de-selects just that prim face.
* Apply a 100% alpha texture to all the remaining selected faces at once.

Get owner to measure sim performance.
Delete megaprim
Repeat measurements.
Add a lot of smaller replacement prims.
Repeat measurement.