overlapping alphas do weird things

[AngelTheTickleSuccubus]

hello. so I have a bug and was wondering if anyone knows how to fix it.  Its pretty obvious what is happening but im not sure how to fix it.  the two objects are overlapping in the camera view and its drawing the outer one over the inner one. 

 

 

[Lancewae Barrowstone]

If it was me, I would hit Ctrl T, to see what that is and click/edit it and see who is the owner, then you can figure out what to do about it.  Ctrl T makes transparent things visible, you see them in red.  You can then easily click it to explore what it is, and if you have perms to move it.  It could be a single transparent prim that projects a light onto your building, for instance.  Any number of reasons for it. But it definitely looks like a transparent prim. 

[Rowan Amore]

1 hour ago, AngelTheTickleSuccubus said:

hello. so I have a bug and was wondering if anyone knows how to fix it.  Its pretty obvious what is happening but im not sure how to fix it.  the two objects are overlapping in the camera view and its drawing the outer one over the inner one. 

 

 

What's the blue thing on the ground?

[Chaser Zaks]

This can be fixed by using the cheaper method of alpha known as Alpha Masking. You can find this setting in the Build menu under the "Textures" tab.

Blending is an expensive process, so often, assumptions are made on the calculations side of things. Alpha masking is much cheaper, as instead of comparing various ranges between 0 and 255(technically speaking, 0.0 to 1.0, but in floating points, which there will be way more than 256 values), alpha masking will use 1s and 0s. Computers love binary, not so much floating point math.

You can also choose to use "None" as the alpha mode. This is specifically useful for when a solid texture(has no intended transparencies) has a alpha channel. Despite being fully opaque, some textures will still have the alpha channel, which can result in the issue you are seeing.

Edited August 27, 2020 by Chaser Zaks

[Qie Niangao]

What you're seeing is often called the "alpha-sorting problem." It comes up all the time.

If the palm tree is modifiable, see if you can live with the foliage Texture being set to Alpha Mode: Alpha Masking with some suitable Mask cutoff (which is totally texture-specific, so maybe start at 127 and tweak from there).

That's all assuming you want the background to have a (blended) alpha channel; I'm not quite sure what that background is, exactly, that it should fade continuously to transparency like that, but any such gradient is only possible with blended alpha.

[Rolig Loon]

30 minutes ago, RowanMinx said:

What's the blue thing on the ground?

It's pseudo-water.  The OP seems to be building in a skybox with a scene surround and has simulated water around his house by painting the prim platform under the house blue.  The scene is displayed on the inside of a two-piece cylinder, using a 32-bit texture on the upper part and either a 24-bit texture or a 32-but texture with no alpha on the bottom. part.  That's a pretty common way to build scenic surrounds.  It allows the upper part to fade gradually to the natural sky behind it instead of leaving a hard, obvious edge at the top.  The downside, as he has discovered, is that if you have any tall objects with blended alpha between you and the upper part of the surround, they glitch.  There's not any comfortable way to avoid it other than not planting tall palm trees.  Alpha masking might work, as Qie points out, but it's probably going to look ugly.

[Rowan Amore]

16 minutes ago, Rolig Loon said:

It's pseudo-water.  The OP seems to be building in a skybox with a scene surround and has simulated water around his house by painting the prim platform under the house blue.  The scene is displayed on the inside of a two-piece cylinder, using a 32-bit texture on the upper part and either a 24-bit texture or a 32-but texture with no alpha on the bottom. part.  That's a pretty common way to build scenic surrounds.  It allows the upper part to fade gradually to the natural sky behind it instead of leaving a hard, obvious edge at the top.  The downside, as he has discovered, is that if you have any tall objects with blended alpha between you and the upper part of the surround, they glitch.  There's not any comfortable way to avoid it other than not planting tall palm trees.  Alpha masking might work, as Qie points out, but it's probably going to look ugly.

All too technical for me.  I just use a Mirror Box.  Sorta, kinda lazy.

[Rolig Loon]

56 minutes ago, RowanMinx said:

All too technical for me. 

OK, here's an example.  My own skybox.  The surround scene is a nighttime cityscape, displayed on two hollow cylinders that are each 50m across and 12m high, stacked on top of each other.  The lower one, which has most of the skyline on it, has no alpha.  That's why the tree that I have planted outside the patio wall doesn't glitch against it.  The upper cylinder, on the other hand, has transparency graded from 0% at the bottom to 100% at the top, where you see that it blends into the sky beyond rather than having an ugly hard edge up there.  (When the sky is really dark, of course, it's all black up there except for the stars.  It's really quite convincing.)   I was careful about planting trees, so I am hardly ever at a spot where I am looking up at a steep enough angle to get the tree overlapping that upper cylinder.  For example, if I am in my work area on the roof, I am always looking down on anything that has an alpha channel. It's viewed against that lower cylinder and never a problem.  Even at "ground" level, I can only get the trees to glitch if I wander outside my patio wall and look up.

[AngelTheTickleSuccubus]

1 hour ago, RowanMinx said:

All too technical for me.  I just use a Mirror Box.  Sorta, kinda lazy.

actually the blue is just the sky. since we are in a skybox and that goes off of our property.  

[AngelTheTickleSuccubus]

2 hours ago, Lancewae Barrowstone said:

If it was me, I would hit Ctrl T, to see what that is and click/edit it and see who is the owner, then you can figure out what to do about it.  Ctrl T makes transparent things visible, you see them in red.  You can then easily click it to explore what it is, and if you have perms to move it.  It could be a single transparent prim that projects a light onto your building, for instance.  Any number of reasons for it. But it definitely looks like a transparent prim. 

so all objects are mine.  what you are seeing is a palmtree where the leaves are not an object but an image with alpha.  The background is a panoramic 'skybox' that circles our land and makes it look like there is stuff in the distance.  the top has alpha so that it blends with the sky. 

[AngelTheTickleSuccubus]

2 hours ago, Qie Niangao said:

What you're seeing is often called the "alpha-sorting problem." It comes up all the time.

If the palm tree is modifiable, see if you can live with the foliage Texture being set to Alpha Mode: Alpha Masking with some suitable Mask cutoff (which is totally texture-specific, so maybe start at 127 and tweak from there).

That's all assuming you want the background to have a (blended) alpha channel; I'm not quite sure what that background is, exactly, that it should fade continuously to transparency like that, but any such gradient is only possible with blended alpha.

'alpha sorting' interesting.  does it have anything to do with the fact that the origin of the panoramic skybox (the pretty background image)  is actually behind the camera, so that the origin of the object of the background is actually closer than the origin of the tree?  anyways either way ill give this a try and let you know here shortly 

[Rolig Loon]

1 minute ago, AngelTheTickleSuccubus said:

actually the blue is just the sky. since we are in a skybox and that goes off of our property. 

Ah, I see, so the bottom edge of the sand is actually the bottom of the surround cylinder, and you didn't put a "floor" on the structure.  You saved a prim, where I decided to use a big "floor" prim to cap the bottom of mine.  

[AngelTheTickleSuccubus]

Just now, Rolig Loon said:

Ah, I see, so the bottom edge of the sand is actually the bottom of the surround cylinder, and you didn't put a "floor" on the structure.  You saved a prim, where I decided to use a big "floor" prim to cap the bottom of mine.  

yes.  though you brought up an idea that I may steal.I could make some sand and extend it out there for continuity

[Rolig Loon]

11 hours ago, AngelTheTickleSuccubus said:

does it have anything to do with the fact that the origin of the panoramic skybox (the pretty background image)  is actually behind the camera, so that the origin of the object of the background is actually closer than the origin of the tree?

Sort of, but it's not quite as simple as that.  If you start playing with alpha textures, you'll find that you can minimize glitching by increasing the distance between surfaces that might glitch, or sometimes by setting them at an angle to each other.  To anthropomorphise shamelessly, I think of my graphics card as getting confused about which surface is in front of the other when both have a degree of transparency.  You have to "unconfuse" it by  playing tricks that make it easier to distinguish one surface from the other.

Google "alpha sorting" to get a good discussion of the problem.  It's very common, and not exclusive to SL.  It's just more frequently an issue here because most builders are amateurs and haven't developed strategies for designing to avoid the glitch.

Edited August 28, 2020 by Rolig Loon
typos. as always.

[Marianne Little]

It is several creators that sell palm trees with Alpha Masking already in place. The textures on them look better than if we change the textures to Alpha Masking ourselves.

[Nova Convair]

That is nothing new and SL is not the only "game" with that problems. In other game environments the scenes are designed though and so this can be easily avoided.
In SL I made my own universe sphere for that reason - with alpha mask. That requires optimized textures. In the scene don't use multiple overlapping transparent faces. Trees with alpha blend textures are a no go.
LL couldn't fix it in the last 15 years and they never will fix it. So it's up to you to not make it happen.