Topic Summary:

Let's Start Building Class 4: Use the Grid Rulers to Position and Align Prims


This week we are going to learn how to use the Grid rulers and Ruler Mode Tools to help us position and align prims.

Before we learn how to work with positions and aligning prims, there is some important background information you need to know that will assist you in understanding how prim positions are calculated in SL and how you can then start to move and align them accurately.

Position of a Prim

The position of a prim is determined by X, Y and Z co-ordinates. These position co-ordinates are located in the Position fields that are located in the Object tab, left hand side, X, Y and Z numbers.

Position Numbers on Object Tab

When you move a prim in any direction, these numbers will change depending of the direction of movement.

You can move a prim by dragging it along the red, green or blue position lines - and see the numbers change to show the prim's new position. Or, you can enter the prim position numbers into these fields by hand if you prefer.

What do these numbers mean?

First thing to remember is that the position numbers for a prim do not refer to the size or outside edges of the prim, but in fact , the point where the 3 lines: red, green and blue, intersect. The intersection point of these 3 lines is ALWAYS the centre of the prim. So, the X, Y, Z numbers that you see in the Position fields refer to the location of the centre of the prim. When you move that prim, you are actually moving the centre point of that prim, not the edges.

Another thing to keep in mind is that if you have cut or tapered or hollowed your prim, SL still calculates the position of that prim as if it was full sized.

To see what a mean, try this quick demo: Rez a box. In the Object Tab, make the following changes:

Have a look at your prim: the red, green and blue lines still intersect the prim as if it was the original size. Keep this in mind when you are trying to position your prim. There will be a faint shadow showing you where the missing part of the cut prim was located that SL is using to determine the position of the prim.

You can drag the prims and align them by eye-balling them or you can position prims by manually entering position numbers in the Position fields.

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Position via Eyeballing

Eye-balling means just that - aligning the prims so they look like they are in position.

Well, this may seem obvious and simple to do, I find it very difficult to use this method when trying to align prims.

Working in 3D space is a skill that you will develop as you practice your building skills. You need to remember that while the prims look aligned face-on, when you look at them from another direction, the gaps between them are revealed.

A common problem: you align the prims face-on, then you walk around and look at them from another angle, only to see that they are actually metres apart. So, you align them from that angle, walk around to another angle, still not aligned .... and so on.

As a beginner builder, I used to get dizzy with all that walking around and around in circles, and never getting it quite right! This method may eventually work for you, but, it will take a LOT of time and patience!

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Position via Manual Entry

This method involves you typing in the X, Y or Z position numbers for the the prim in the Object tab. Once again, this method may sound simple and easy if you only have t position 1-3 prims, but if you have a lot of prims to align, you are going to need some paper and a pen to calculate the location of each prim, then enter the numbers manually into the Object tab.

This method can be useful in some instances and I use it myself quite a lot to make minor changes to the position of several prims, but once again, this method can take up a lot of your time and patience to position prims this way.

Is there a better way? You betcha!

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Use the Grid to Position Prims

One of the most useful tools for builders is the SL Grid Rulers. These Position, Rotate and Stretch rulers help you to position and align your prims precisely by allowing you to snap them to position. Let's look at the rulers first before we start to learn how to snap.

The first thing we need to check is that we have actually activated the option to use the grid. At the top of the Edit window, on the right hand side is a list of 3 tick boxes. The first one is called Use Grid and has a button called Options located just underneath it.

Activate Use Grid option

Tick the Use Grid option to activate it.

Now when we rez a prim and start to move it, we will see 2 white rulers appear on each side of your prim along the direction you are moving the prim in. This is the grid ruler that will help you to position your prims.

View the Grid Rulers

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Grid Options

We can now make changes to the settings of this grid ruler by clicking on the Options button that is located just underneath the Use Grid tick box. The Grid Options pop-up box will appear with all the settings we can change.

Grid Options pop-up Window

The option we can adjust in this Option window is the Grid Unit (meters) - that is - how far apart, or close do we want the lines on the ruler to be. The Grid Units are the longer lines on the ruler that have a number next to them; the smaller lines in between the larger units are called Sub Units

The smaller the number, the closer the grid lines will be on your grid ruler, the larger the number; the further apart they will be. Lets have a look at this in action.

Change the number in the Grid Unit field to 0.500. This means that we want the ruler to show Grid Units that are half a metre apart. Now move your prim again and look at the changes to the grid ruler unit markers on the image on the left. Change the grid number to 0.100 and look at your ruler again in the image on the right.

Note, I have de-selected all the other grid options in these examples to make the rulers easier to read.

Grid Units at 0.5 metre Grid Units at 0.1 metre

Remember, there is no right or wrong number with these settings; you can select what-ever setting you feel is useful to you. You can change these settings whenever you like as you build.

Example: if you are working with large prims, you may like to have the grid settings further apart as fine-level sub-units are not that helpful with a larger prim. If you are working with smaller prims, you may like to use small grid settings and zoom in so that you can make very precise movements along the ruler.

Let's look at the other settings in the Grid Options box.

The Grid Extents setting allows you to set how long you want the ruler to be, based on the location of your cursor. A Grid Extent of 5.000 means that the Ruler will be 5 metres in length, 2.5 metres on either side of the prim your are currently moving. Move your prim and notice the the location of the ruler changes so that it is always visible near the current position of your cursor as you move your prim.

I just max this setting out to 10.0 and keep the ruler as long as the system will let me. Make it as long, or as short as your like as you build.

When you select the Sub-Unit Snapping option when selected allows you to m ove your prim and snap it to the position of the ruler's sub units as well as the Grid Units. (More on snapping soon!). Don't want, or need this setting? Remove the tick from the box next to this setting.

The Show Cross Sections setting highlights the centre plane of the prim - showing the centre point of the prim in the direction you are moving it. Some people find it useful, others find it annoying and getting in the way of their building. Turn this option on and off as you prefer.

Grid Options settings and Ruler

The Grid Opacity setting lets you set the visibility of the Grid Ruler. Move the slider towards the right hand side to make the Ruler more visible; move it towards the left hand side to make it less visible. If you move the slider all the way to the left, the rulers are no longer visible. Adjust this setting as you wish.

Grid Opacity settings and Ruler

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Snap To Grid

Finally! We get to my favourite and most used feature for positioning prims: Snap To Grid. What does snap mean anyway you may ask?

When you use the Snap to Grid functionality to move your prim, it tries to align itself to the nearest ruler marking, as if the nearest Grid Unit is a magnet, drawing your prim close to it. When you move your prim slowly to exaggerate this effect, its like your prim is stepping along the ruler in even steps.

To use Snap to Grid, we need to slightly adjust the way we move our cursor as we move the prim along the direction of the ruler. If you move your prim too fast you won't see this effect, so we'll move it slowly till you get comfortable using this method.

Select your prim and move it along the blue direction, only this time, while you are holding down your mouse button to move the prim, drag your cursor on top of the grid ruler. A white line will appear between the 2 rulers showing you the position of your prim. Look closely at this line - can you see that each line end has a little white triangle on it touching each side of the ruler? This white line touching both ends of the ruler is what we use to snap the prim.

Snap to Grid Marker line

Move your prim slowly while dragging your mouse over the top of this ruler and see how the prim jumps to the next ruler marking.

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Ruler Modes

So far we have made changes to the Ruler settings to help us positin prims. Ruler Mode is another tool we can use to help us position prims while building. The Ruler Mode settings determine what types of numbers will appear on the Grid Ruler. We can even change the direction of the Grid Rulers in relation to the location of the prim.

The Ruler mode drop down box is located in the top right hand side of the Edit Window with the default World option selected.

Ruler Mode drop down box

The default setting that we have been using so far is World mode. In World mode, the ruler numbers and position co-ordinates are based on the region's co-ordinates. Your prim's position is calculated in relation to the whole region's position and the red, green and blue position lines align to the forward/back, left/right and up/down directions.

When you select the Local Ruler Mode, the numbers on the ruler change. In fact, the alignment of the ruler itself changes! The red, green and blue position lines are now aligned to the position of the prim, not the region, so when you activate the ruler - that oo is aligned to the posiiton of the prim.

Local Ruler Mode

Another feature of the Local Ruler is that the numbers also reflect the location of the prim. The centre of the prim is now always located at position zero and all other numbers on the Grid Ruler are relative to the position of this selected prim. When you move the position of the prim and release it, the ruler resets and when you touch the prim again to move it, the new prim location is now position zero.

Local Ruler Mode

Local Mode also re-aligns the direction (plane) of the position lines to reflect the new position of the prim.

Try this - rotate your prim and now look at the position lines. Move your prim along this new rotated direction. You can even Shift-Drag a copy of the prim in this new direction.

Local mode is very useful when you want to move a prim along an angle - faster than having to zig-zag along the red/green or blue lines to get the prim into position.

The Reference Mode ruler markers are set by the position of a selected prim - the reference prim is at position zero; other prims are moved relative to the location of the reference prim

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Rotation Ruler

So far we have looked at how to use the Position rulers. We can also use the rulers to help us rotate a prim.

Switch to Rotation Mode (top left of Edit window) and use the same techniques we have just learnt to see the circular ruler for Rotation Mode

Rotation Ruler

Practice using Rotation mode to rotate your prim, then switch to position mode to mode it.

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Stretch Ruler

The Stretch Ruler looks similar to the Position Ruler, except it only extends out in one direction.

The direction of the ruler depends on which stretch handle you have selected to pull or push - the ruler will appear in that direction

Stretch Ruler

Switch to Stretch Mode and experiment with this Ruler.

Rotation Ruler

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2D Grid

This is a more advanced topic than the Grid rulers, but we'll have a quick look at it as you may have already accidentally activated this grid and wondered what the heck it was :)

2D Grid

The 2D Grid allows you to move prims in 2 directions at the same time.

The 2D grid will appear to assist you with moving your prims in 2 directions based on the set of arrows you have selected

2D Grid Arriws

And yes, you can also change the alignment of the 2D grid when switching to Local Ruler mode

Rotated 2D Grid

Practice moving your prim using this grid.


Further Reading and Videos on Using the Grid

Torley's Video: How to use the Building Grid

Torley's Video: Reference Ruler and Grid

Second Life Help: Snapping to Grid

Next Class: Class 5: Working With Transparent Prims

Previous Class: Class 3: Copy and Link Multiple Prims

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