Optical Illusion

I created a texture with PShop and uploaded it as a PNG. It had no Alpha channel. I have used it on the spokes of a wheel which I constructed by CTRL Dragging, then hitting CTRL+Z and finally changing the rotation to position it.
When I cam in and out the spokes appear to shift out of position. Some times crossing in the centre and then not. I was told that there must be an Alpha channel in the texture but I have checked.

What can be the problem?

PNG supports both simple transparency and alpha transparency. At the end of the day, they produce the same results in SL. The only difference is your work flow while you're making the image. After it's uploaded, it's all the same.

If your image has transparency in it, then it has an alpha channel in it, whether you deliberately made one or not. If so much as a single pixel in your PNG is less than 100% opaque, then your image will upload as a 32-bit texture.

The only way to avoid the alpha sorting glitch is to make sure 32-bit textures don't overlap at all. If your texture doesn't need transparency, make sure you save it as 24-bit. You can do that either by using a format that doesn't support transparency (24-bit TGA or BMP), or by unchecking transparency in the PNG Save For Web dialog (assuming by "PShop" you meant Photoshop). If your texture does need transparency, then make sure the other textures around it are 24-bit. If that means redesigning your build, then so be it. If you overlap 32-bit textures you will have sorting problems, no exceptions.



So you know, the term "alpha sorting" is a bit of a misnomer. It doesn't really have anything to do with the alpha channel itself. It's all about the presence or absence of transparency. Whether the transparency is mapped to a channel (alpha transparency), or embedded as a color (simple transparency) makes no difference whatsoever. More properly, it should be called "transparency sorting". But simple transparency didn't come along until well after the term was coined, so we still call it "alpha sorting", even though it's not a totally accurate descriptor in all cases.

Thanks for that Chosen. A great help.

one known edge case exception... since the sorting is done by relative position, prims with the same center will generally sort correctly (well at least logically). this effect can be seen in a variety of plants made in SL (and a few items like the two prim table I described in a post ages back) it's a neat effect to incorporate into a filigree sphere, the iner textured on the outside, and the outer textured on the inside, size so that the inner sphere is the exact size of the hollow of the outer.