Funky overlapping effect bug thing

Okay, so I'm working on a fallen angel avatar, and today I made the wings, based off of Luis Royo's fallen angels (the female ones) with some cuneiform writing added.

Me wearing the wings

If you look at the right side of the picture though, you can see that the front side of the wing is overlapping the side that is behind, even though it should be the other way around. I took another picture for clarity, where it's easier to see something's not right there.

The wings are both 1 prim, a cut cylinder that's hollowed out with a texture on the outside and inside on the hollowed out part, to create both sides of the wing.

I asked someone for an opinion on the wings and they said they didn't notice the effect at all, so now I'm completely confused. Can anyone explain what's happening here and possibly offer a fix for it?

You've encountered a long-standing bug with overlapping alpha textures. Interpenetrating alpha textures will be drawn with one entirely on top, rather than part of one and part of another. The wings you made look like they're based on a section of a cylinder or sphere, given the curve. In such cases the wrong side is frequently drawn (i.e. the inside drawn over the outside) when the shape is viewed at certain angles. I'll send you an invite for the Society for Proper Rendering of In-world Graphics (SPRIG), a group specifically founded to get the Lindens to address this bug. Beyond lobbying the Lindens, though, there's unfortunately no way to deal with the problem.

Is there any workaround for this? Does it only happen when alpha textures intersect or are on the same prim? Could I split the back and front textures into two seperate prims to fix it?

Not really.. It happens even when the textures are on two separate prims...

for a cheap and easy in-world example using free resources, try putting a zebrea plant (Or any plant for that matter, but the Zebra has a significant amount of alpha space) (Potted... freebie widely available) in front of a glass-block wall...

Because the camera sees both textures, and it doesn't posess the logic to be able to differentiate distance properly, the clipping engine has to decide which goes in front.... often, it is wrong.

I heard a rumor that the alpha-sorting problem is better on 1.7 - one thing I haven't looked at in there yet...
/me goes off to see...

Let us know what you find.

You might try adding some transparent prims or hidden prims. I read this can work in some cases.

When looking at an early release of 1.7 preview, I saw a sim named Texture that seemed built to test the alpha-layer confusion bug fix. It seemed to work properly on my renderer.

I'm concerened. While "fixing" alpha sorting would clear up many problems, it would also create new ones. A great many objects like trees, bushes, fire, etc., are built specifically to take advantage of the "flaws" in alpha sorting. Were overlapping alphas no longer to interfere with eachother, those objects would break, and we'd all have to find new ways to make them.

I'm not sure how to feel about it. I know that the right answer is to fix it, since it is a bug, but it's kind of a "damned if you do, damned if you don't" situation. Fix it and you break many pre-existing objects and techniques. Don't fix it and you continue to make lots of other objects impossible. It seems there's no way to win.

Concerns aside though, if they do fix it then that would make SL quite unique in the graphics world. I have yet to see an 3D application that does not have alpha sorting issues. The objects I mentioned that are built to take advantage of it in SL are also built similarly in many video games for the exact same reasons. Even top of the line, high end apps have the same behavior. For example, I use Maya all the time, which to give you an idea, costs $6000 and is commonly used to create much of the most impressive CGI in the world (like Golumn and all the creatures and armies in Lord of the Rings, for a well known example), and it has the same alpha issues when using hardware texturing that SL has.

It's pretty rediculous that all these programs all have the same glitch in common, especially considering that from what I understand, it's fairly well know among graphics programmers what the cause is, at least from a general standpoint. However, the fact that it is so common has given it a status of expected, and most modelers and texture artists operate according to that expectation. As dumb as it may be that working with a bug is a common practice, it is common all the same. It's a tuff situation.

*idly wonders what the ETA on 1.7 is*

I'm a rotten tester-er, but I went to the preview grid (to my preview home in the island formed by the letter P) and set up a testy-type thingy. I put up a cutout behind a cutout inside a box textured with an alpha gradient, which were behind a window, with another cutout in front. If I looked at it head-on it seemed ok but if I looked from an angle things at the back popped into the front. I should do the same thing here to see if I notice much difference.

So... Would this also explain why a semi-transparent screen in front of a semi-transparent wall doesn't always render properly?

In my home (Japanese traditional styling) the walls are translucent Shojii-screens (Paper and wood lattice). I set up a 4-prim folding screen as a room partition (the freebie 'Japanese Screen', found in many free sets), and that screen is also slightly transparent. Viewed from certain angles, whole panels of the screen vanish, or else the screen seems to poke through the nearby wall. Problems sorting alpha layers might explain that...

i hear you chosen, but my gut says that if we avoid all improvements that break pre-existing objects or clothing, then we will never see progress in SL. We'd simply have to wait for someone else to come around and create a better metaverse from the ground up.

Should be a rule for doing business or creating in SL: when it comes to your creations, be prepared for obsolescence.

I agree with you, Forseti - it is the only way.

Chosen, I acknowledge that fixing the alpha ordering bug might break some existing builds, but I think it would enable many more than it would break. There have been scripting bugs which people found ways to work around, which then broke after updates. With this bug there is no workaround.

It is possible for it to work correctly: create a tree object, focus on it, and move the camera away from it. At some point the tree will switch to a simpler model, which involves intersecting alpha textures. Those textures are rendered properly.

I haven't been in the Preview recently, but last week the alpha problem still persisted. I created a simple texture with one small alpha hole, and I use this texture for testing. If you've got tools to create your own textures, I encourage you to try the same thing. I don't know whether the existence of an alpha channel alone is enough to trigger the problem, but a single transparent pixel will do it. Apply that texture to an hollowed-out cylinder and color the outside differently than the inside for a good demonstration of the bug, or apply it to two intersecting prims. I can only get get bug to appear consistently when the textures actually intersect, or they're on different sides of a curved object that overlaps itself.