What are the rules...

for getting prims to line up properly?

I am an absolute beginner at graphics. Don't bite me. I've got ZERO experience with this stuff, but I've been using computers for a long time, so I'm not afraid to get into something new.

Here's my problem: I can't find what I need to know anywhere in the tutorials or written materials. I'm one of those people who learn best from reading and doing - lectures and tutorials don't work well for me. My project is a skybox.

What I want to know:

1. How do I get two prims to line up exactly, with no gap or overlap between them?

2. What are the starting points for prim measurements? I have two prims that must make a wall. One is 10m wide, and the other is 7m wide. When the first prim starts at 16m and the second starts at 24m, they don't show a gap. When I change the X of the second prim to 26, I get a huge gap. Obviously, I can't just assume that Prim A starts at 16, and that I can start Prim B at 10m in from that point to line up with the right edge of Prim A. So, where does the measurement start on a prim, and how can I use this information to work with my database-wonk mentality?

3. I get the whole snap-to-grid concept, 'cause it's used in making forms, but I can't see how the grid gives me any kind of reference point to snap TO. How can I use this to line things up, particularly when the objects are large?

Again, I've seen the tutorials. They make perfect sense right up to the point where I try to use what I thought I saw. I need some basic rules to work with. It's a computer, it's gotta have rules.

I use the copy option then adjust the size of the second prim accordingly

Or eyeball it

Eew! Eyeballs are sticky!

I figured the graphics-savvy crowd wouldn't need the kind of tool I'm looking for. But my eyeballs don't work that well.

My skybox currently has a floor and roof made of four 10x10 prims, each 0.5m thick. When I cam around, I get that weird slicing effect. This has got to be a PEBKAC issue*.

/Pollyanna mode ON

The learning curve is driving me bats, but if other people can learn it, so can I.

/Pollyanna mode OFF

Or I can just dive off the skybox and make sure I land on my head.



PEBKAC = Problem Exists Between Keyboard And Chair

Oh so like tiles? If they are all the same there is a nice way to make those! Move one out where you've got room to work and get its rotation nice and neat to all 0. In the edit window you can click on the magic wand create thingo and check the copy selection and center copy boxes, and then you click on a side of the tile and yayness! And if that's no good somehow you can type numbers in for the position, easier than mousing sometimes.

The grid secret is you have to move your mouse away from your prim and right onto the ruler and then you can see the little arrows in their notches. The position is the middle of the prim where all the arrows cross so figure in half its length for lining up stuff!

The distance between the centres of the two prims is 8.5m - 5m from the centre to the edge of the 10m prim plus 3.5 from the edge to the centre of the 7m prim. So at 16 and 24 they overlap by 0.5m and at 16 and 26 they are 1.5m apart at the edges.

The correct separation is 8.5m so if one is at 16, the other needs to be at 24.5.

Remember the co-ordinates are for the centre of the prim, not a "starting" point. I only ever align prims by eye if they are awkward shapes or at angles, and I have never got comfortable with using the grid. I align all my prims by working out the numbers, makes it a lot easier if you get your first prim rotated to 0,0,0 and at a point where the X,Y,Z co-ords are whole numbers. Once you've finished building you can move/rotate the whole thing to where you want it. Oh, and have a calculator to hand and a notepad for writing down co-ordinates etc, either real ones or the ones on your pc, saves enormous amounts of time and cuts out a lot of the errors.

There are many ways to do this. Here are a few:

1. Use simple math. I'll use the measurements from your example below to illustrate. If one prim is 10M wide, and the other is 7M wide, then the distance between their centers if they're perfectly adjacent and flush will be 8.5M. How do I know that? Because the center point of any object is always half way between its edges. Half of 10 is 5, and half of 7 is 3.5. Add the two together, and you get 8.5. So, wherever first object is, add or subtract 8.5 to its location value on whichever axis is the one in question, and you'll know where to put the second one. Very simple.

-OR-

2. Use the on-screen rulers. In the editor, turn on Use Grid, and set the ruler mode to Local. Now, select and object and click on one of the arrows of the manipulator tool, as if you were gong to move the object, but don't actually move it (yet). Notice a white ruler appears on-screen. Let go of the mouse, and you'll see the ruler disappears. This is because the ruler is there to help you snap things into place as you're moving them. It has no function when the system doesn't think you're about to move something.

So how do you use the ruler for what you're trying to do? OK, select your object again, hold down the shift key, and drag the object in any direction. Notice the object not only moves, but also leaves a copy of itself behind. This is called "shift-dragging", as you might have guessed. It's nice, easy method of duplication.

As you're shift-dragging the object, move the mouse onto the ruler. You'll now notice that as the object moves, it will snap to the ledger lines on the ruler. Since we've got the ruler set in local mode, the measurement units are the size of the object. You can now easily snap the moving object into place, exactly adjacent and flush to the stationary one that was left behind.

From there, if you want the two objects to be different sizes, simply stretch them. Just make sure Stretch Both Sides is turned off, so you can move one side or the other, without destroying your joint.

If you want to snap one or the other, or both, to exact sizes, the reference grid is handy for that. Set the ruler mode to Reference. Create a new object, and make it the exact size you want one of the other objects to be, by typing in the numbers. This will be your "reference object". With the reference object selected, press shift-G (make sure chat is closed first). This will tell the rulers to refer to your reference object. The grid units will become the size of the reference object, and will stay that way until you decide to change them. Now stretch the object you want to resize, and move the mouse onto the ruler while you're doing it. You'll be able to snap the sizing, the same way you snapped the location a minute ago.

The rulers are really handy. For all SL's shortcomings as a 3D modeling application, the rulers are the one thing I wish a lot of other programs would borrow from it.

-OR-

3. As has been mentioned, use Copy Selection. Select a prim, and then select the Create tool (the wand) to make a new prim (but don't make it just yet). In the editor, turn on Copy Selection and Center Copy. Click on any side of your selected prim. A copy of the selected prim will now come into being, flush and perfectly aligned with the side you clicked on.


Those are three relatively simple methods for perfect alignment. There are lots of others.

As for whether or not you'll see a visible gap, that can depend on how good your video card is, and how you've got it set. Assuming it's not a total piece of junk, in the settings for your card, assign SL a profile that includes some degree of anti-aliasing. Without that, the edges of objects will always appear jagged, and you'll see gaps all over the place, even in places where no gaps actually exist. There's just no way to get all those jags to align perfectly. Anti-aliasing smooths the jags, this eliminating the problem.



The origin of a prim is the location of its center point. As I said above, for your particular example, you'd want a distance between the centers of 8.5 meters. So, if the first prim is located at 16M on a particular axis, you want the second one to be at 24.5 on the same axis. Simple.


It all depends on the ruler mode you're using. If it's set to World, then the ruler units are global, just like the numbers you might type into the editor. If an object is currently at, say, 50 on any particular axis, and you snap it over one unit, it will be at 51 (assuming your grid unit divisions are set to one). I already described the Local and Reference modes for you above.

Effective use of the rulers depends on switching between the three modes all the time, depending the needs of the task at hand.

I'd like to thank everyone for the advice. It did fill in a few gaps in my information.

Just one more question for now:

I've got a floor made of four prims, each 10x10 and 0.5 thick. The prims have a marble texture, set to one repeat per face. When I stand on the floor and use the arrow keys to turn in a circle, I can see the narrow edge of the prims pop up in sequence, then fade away. I haven't seen this in builds done by other people, and it doesn't happen in a similar floor that has a water texture. (I like walking on water. Go figure.)

Is this an issue with sizing and alignment, the choice of texture and number of repeats, or is it simply a video card issue? Can using a texture improperly have any effect besides being ugly?

i personally use the ruler and sometimes by eye, best thing to do is go check out the torley linden movies, that nut has a wealth of information on almost every subject

EDIT: and to answer your last question thats texture flicker, its your prims overlapping, 2 ways to get around this are try to align your prims so there only just touching or make one slightly thinner than the other. (there are probably other ways as well)

-Crunch

It also could be that you are working with what looks to be a solid marble texture that was wrongly uploaded as a 32 bit targa with an alpha channel. There are some freebie marble textures out there with this problem.

You may have hit on it. It IS a freebie texture, although I don't know how it was uploaded. But it looks good.

Here's one last post about the flicker issue, since folks have given me the tools I needed for everything else.

First, I used the "Create copy" option to make a new floor from scratch. The texture still flickered. Then I set each of the four prims to blank texture and white color. After that, I copied the marble texture to each prim's contents tab and created a new script.

By adding the code: llSetTexture("[texture name]",0); to each prim, I set the top of the floor only to have the marble texture.

At this point, I still had the original floor a few meters below the new floor, and camming around revealed a very strange effect as though there was a rectangular window in the new floor, letting the old floor show through. This was probably the alpha element that Isablan mentioned.

It stopped when I deleted the old floor.

So I am a very happy camper, with my new floor floating above the house. Now I can lower it into position and find a whole new problem to learn from.

Thanks, all!

Small point of clarification, to say something was "uploaded as" 32-bit or not is a potentially confusing way to phrase it, I think. And by the sound of it, Senga (no offense) does seem a little confused now as to what it means. I'll try to help clear this up.

An image will be SAVED AS 32-bit or not by whomever made it, at the time of its creation. After that, if it happens to end up getting uploaded to SL, its bit depth can't change by the uploading process. There are no "upload as" options.

So what do terms like "32-bit" and "bit depth" and such actually mean? If you don't know, please read the Transparency Guide at the top of the texturing forum. This post is already gonna be a bit long. I don't wish to make it any longer by explaining concepts here that are already fully detailed in the sticky.

Senga, here's what's going on with your floor. There's a very simple explanation for why you were seeing what you're referring to as "flicker" when all sides of your floor prims were textured with that marble, and why it went away when you changed some of the sides of the prims to have a blank texture instead.

The "flicker", as you put it, is the result of what's known as the alpha sorting glitch. When two or more 32-bit-textured surfaces overlap each other in close proximity in 3D space, the renderer has trouble determining which surface to draw as "in front". The surface whose center point is physically closest to the camera at any given moment will usually be the winner. Often this has the effect of bringing surfaces to "the front" that we as human beings would consider in the real world to be "in back". The computer doesn't know anything about the real world, or about how we perceive 3D objects.

As you move your camera around, the distances between it and each surface change. The movement can cause surfaces to appear to flip-flop, or leap-frog, back and forth in 3D space. One moment surface A is "in front" with surface B "behind" it; the next moment, the opposite.

This phenomenon only affects surfaces with 32-bit textures. 24-bit textures are naturally immune, which is why the problem stopped when you changed the hidden faces of the prims to have the blank texture instead of that botched marble. The blank is 24-bit.

For more on bit depth, and on the alpha sorting glitch, again, see the Transparency Guide at the top of the texturing forum. There's a full explanation of what channels are, how they determine the bit depth of an image, and how alpha channels are employed to create transparency.

So you know, the glitch is present in all OpenGL applications, from virtual worlds, to most video games, to ultra high end professional 3D modeling and animation programs costing thousands of dollars. The only reason you don't notice it in video games is because game artists know enough to work around it. It's still there; pro artists just make it a point not to build in such a manner that they would expose it. But since SL content is created mostly by amateurs, people in SL stumble across the glitch all the time.

The glitch has existed pretty much since the dawn of 3D graphics, and will continue to exist until computers become powerful enough that an entirely new paradigm can be employed. We're still years away from reaching that point. All we as artists can do for now is what we've always done, live with it, and work around it.

Anyway, if you want to check if a texture is 32-bit or 24-bit, apply it to a prim, and press ctrl-alt-shift-T. Green text will appear on your screen, describing the texture as "alpha" if it's 32-bit or "opaque" if it's not.

If you want to fix that marble texture, it's easy to do (assuming you have full permissions on it). Here's how:

1. Double click it in your inventory to open it in the vewer.

2. Go File -> Save Texture As... and save it to your local hard drive.

3. Open the file in a good image editor, such as Photoshop, Paintshop Pro, or GIMP.

4. Delete the alpha channel, and save either as 24-bit TGA or as BMP.

5. Upload your corrected version of the texture (this will cost you L$10), and use yours instead of the old one at all times.


And by the way, just so you know, while your use of a script to texture the individual faces of your prims obviously works, there are much simpler ways to accomplish the same result. The quickest thing to do is simply to drag the texture from inventory onto the face you want to affect. You'll texture that face instantly, without changing the other faces. You can also turn on Select Texture in the editor, select the face you want to affect by clicking on it (shift-click to add any desired additional faces to the selection) and then use the texture picker to assign a texture to your selection. The script works too, of course, but it's total overkill for such a simple task.


On a side note, It's pretty incredible that those messed up marble textures are still floating around. They've been making the rounds for at least the past 4 years. It's amazing to think about how one person's little mistake in save-options 4 or 5 years ago has created confusion and frustration for so many people. Out of all the hundreds of thousands of textures now in existence in SL, somehow those particular ones still find their way to people. Crazy.

If it's a full-perms texture you can just "save to disk," convert it to .bmp, then re-upload it.

I MEANT to say "saved"

Yeah, those marble textures are still out there in freebie texture collections widely available. It's kind of a rite of passage for a new builder to get bitten by them as virtually everyone starts out using freebie textures. Those marble textures have caused more grief than a thousand self-replicating flying penii attacks....

Yeah I remember a whole week of spending every evening trying to work out what I was doing wrong with those textures on one of my first builds. Then I found out about the alpha sorting bug.