Prims - the building blocks of Second Life™

Everything you see in Second Life, except the avatars, some of their clothes and the land, is built of prims. They are shaped, prodded, textured, tortured, flexi-ed and so forth, but they're all prims. For that reason, they really are the building blocks of Second Life. (For the pedantic, some of the things you see may be particles - sprites produced by script and only consisting of textures - sprays of water, fog, smoke and the like are often particles.)

Building things used to be entirely the provenance of the in-world editor. There were a few scripted solutions to taking 3D models made in other pieces of software and importing them, but they were hugely expensive in terms of prims - one such bit of code was used to create an accurate model of an academic building. At over 1,600 prims per floor it was very good in terms of the accuracy but except in very specific circumstances unusable in terms of the build.

Sculpted prims (Sculpties) have changed this: you can use software such as Wings3D or Blender, up to Maya, to create a sculpty-map and apply it to a prim in Second Life. This will allow things like the production of 2-prim staircases, interesting 1-prim chairs etc.

However the bulk of the building work is still done with the in-world editing tools and the (mostly) geometric prims available. The entire list of available prims is:

NOTES:

  1. ‡ Spheres can be dimpled as well as cut and hollowed
  2. * These items have an advanced set of manipulations available
  3. † These items can only be rezzed on a parcel by the parcel owner
  4. Torii have circular cross sections, rings have triangular cross sections.

The create menu offers additional options - cones, pyramids, hemispheres etc. These are pre-formed from the base prims listed above. For example a cone is cylinder that has already been tapered to 0 on both x and y axes.

Available building manipulations:

  1. Available to all items:
    • Size - can stretch between 0.01m and 10.000m on any axis;
    • Rotation - can freely rotate around any axis or combination of axes;
    • Path cut - a cut in two local z-orientated planes allowing notched items etc. Often used with values 0.125 and 0.625 or 0.375 and 0.875 to make 1-prim doors.
    • Hollow - again parallel to the local z-axis. You have the choice of rectangular, triangular or circular holes. They are always central to the prim and a percentage of the distance between the centre and nearest edge.
    • Twist - again parallel to the local z-axis. You can twist the top, the bottom or both relative to the default position.
    • Taper - the top or bottom (local z-axis) of the prim can be pinched towards the centre from either the x or y axis direction, or both. You can pinch one in one direction, the other in the other. Range is 0 to 100% in 5% steps (0.05)
    • Top shear - slides the top (local z) over the bottom in either the x or y direction or both. You can only shear half the size of the base - so the centre of the top can be over a corner of the base at the most extreme.
  2. For spheres only:
    • Dimple - similar to cut, but dimple removes cones aligned along the z-axis from the top or bottom or both of the sphere. A combination of dimple and hollow will let you make a simple jar with a circular opening at the top for example.
  3. Advanced options:
    • Hole Size - rings, torii and tubes all have a hole in them. You can play with the size of the hole relative to the remainder of the prim - for example making a bangle could be as simple as a torus with a large hole.
    • Profile Cut - similar to the normal cut above, but cutting around an axis following a circle normal to the z-axis and at half the height of the prim
    • Radius - this allows you to alter the radius of curvature of the prim
    • Revolutions - allows you to make the prim complete more than 1 circuit - for example a torus with hole size x=0.35, y=0.05, revolutions=4.0 gives a good approximation to a spring

Other features of prims

Prims can have textures and colours applied to them. This can be from the very simple - applying a blank texture and a colour can give a pleasing, if plain, colour effect, to the complex - textures with shadows, partial alpha transparency and the, then tinted further in Second Life to give complex appearing shapes. Textures can also be scaled, rotated, the number of repeats controlled, and even planar mapped if necessary. This really represents the area where the builder and the texturer/photoshopper have to work closely together. Full texture control is available on the Texture tab.

Also on the texture tab, and of general use, is the Fullbright option. This causes the texture to render as if lit at noon at all times. People with visual deficits related to poor night vision may find this helpful. It is often useful for signs as it ensures they can be read at all times with relative ease. Fullbright ONLY affects the appearance of the texture(s) it is applied to - the surrounding textures are not affected.

In addition prims can contain one or more scripts, and if necessary additional textures and objects within them to enhance interactivity. The builder must here interact with the scripter for the best results. All of these can be controlled via the Contents tab.

Beyond this cross-over areas, all prims can be directed to act as point light sources (from the Features tab) and to cast light around them. People on low-end computers or those with their preferences set to not show this will not see this so it can be a dangerous option to rely on. In addition, there is a limit (of 6) to the number of local light sources that will render in Second Life. Many new hair styles come with "facelights" which are prims that act to cast light on your face and upper chest and which will always be closer than the light sources you incorporate to the user and may overwrite your effect.

All prims can be made phantom from the Object tab. Phantom prims do not generate collision events in technical parlance: what that means in practise is that you can walk through them and that objects can move freely through them. (For the scripters there is an exception created by llVolumeDetect(TRUE); which causes a prim to act as if phantom but still register collisions. However this exception is broken if you make the prim phantom via the Object tab.)

All prims except flexi-prims can be made physical (from the Object tab). It is not wise to make objects physical and phantom as they will fall off the bottom of the world and be returned to your inventory. Physical objects are subject to all of the physics engine's effects. This means, most obviously, that they accelerate downwards under gravity and will rotate if they hit obstructions. Under the current Havok 4 physics engine it is hard to crash a sim with physical objects but physical objects in motion are still demanding on the sim and should be kept to a minimum. Vehicles and objects which must move smoothly or which must push avatars (e.g. lifts) are the most common uses of physical prims.

Box and cylinder prims may also be made flexible (from the Features tab), which is most commonly called flexi. These flexible paths can create trees with sway in the breeze (unlike Linden trees that sway anyway), flags, hair that moves organically and the like. Making a flexi-prim prevents it being made physical and makes it phantom.

Problems with prims

There aren't many problems with prims in Second Life.

One major one is called z-sorting. If faces of two prims co-exist (either for all of part of their faces) the client doesn't know which one to draw. This results in the object flashing between the textures of the two faces. If their textures are not the same this is particularly noticable, but you will find your eyes are drawn to it even if the textures are the same because you will react to the flicker.

Prims don't allow subtractive interactions - you can't nibble a sphere out of a cube for example.

Particularly if you are stretching linked objects there are two limiting factors:

  1. The 10m and 0.01m limit applies to all objects in the linked set and will act as a hard (and sometimes unexpected) limit on the stretching.
  2. If the movement (or all or a subset of the prims) results in the link set becoming illegal it will appear to work, and then bounce back when released. This is not always 100% reliable - it rubberbands back rather more often than it should. You can work around this by releasing the object from the linked set, resizing and then relinking but this is not always easy to do and usually catches you by surprise
  3. If you are planning changing things other than texture and scale in Second Life, sculpties are not the tool for you. Sculpties are created out of Second Life, imported and applied. If you want to edit the sculptie it is not done in Second Life, it is done outside, reimported and tried again. That isn't to say you can't have design meetings around sculpted prims, but you will add (and you can only add) detail in normal prims until your sculpty artist can create a new sculptie for you. If may well be better to have a multi-prim object for a design meeting so all the parts of it can be easily and directly edited and changed.

Prims and parcels

Building with fewer prims (primficiency) may seem, at first, to be a rather pointless exercise. An island comes with 15,000 prims available after all. 15,000 prims doesn't go very far despite first appearances. Drexel University opened their island with a curious mix of the primficient and the profligate in their building. Within 2 months they were hitting their prim limits and trying to economise wherever they could.

If the island is parcelled (for example for streaming video or audio to different parcels) but retains the same owner, then the same limit of 15,000 prims remains. In fact you could build with all 15,000 prims in one small parcel and leave the rest of the island bare. If, however, you parcel and sell the parcels (even on a temporary basis), for example to different student groups for their building work, the various land owners will share the available 15,000 prims between them in proportion to the land area they own. If, for example, you have 3 parcels, one half the area, one 3/8ths of the area and one 1/8th of the area they would have 7,500, 5,625 and 1,875 prims available respectively.

How to save prims

There are an almost endless list of tips and tricks that can go in here. However some of the common ones that can be widely applied include:

Building styles

There is no right way to build in Second Life. You will find builders who judge everything by eye, builders who obsessively flick back and forth and build by the numbers, builders who rely on the various in-world visual tools such as the grid, snapping options etc. I don't know of anyone that uses all the tools with equal facility, although as time goes by you will probably find that you use a mixture of all of these systems.

Despite that, there are some things that I don't think you can get away from. These include:


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