Prim and mesh differences

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What's different between prim attachments and the avatar mesh?

Let's use hair as an example to get acquainted, then expand from there. In Second Life, there are two "flavors" of hair:

Prim - Attachments made from primitive objects can be attached to (and detached from) various points on your body. Prims can be shaped into just about anything you can conceive, and allow for more flexibility, including with hair — literally! For example, you don't have control over how mesh hair sways in the wind, but flexible prims ("flexiprims") can make long locks swish naturally, or even defy gravity while wiggling around. Since prim attachments can also be scripted, this allows for easy tinting, resizing, and other click-and-change features.
Mesh - Your basic "avatar mesh" — which we'll simply call "mesh" from now on — includes your base appearance "body parts", such as the length of your arms and the size of your tummy. It also includes types of clothing. All mesh settings are controlled in the Appearance Editor, accessed from Edit > Appearance. With the Appearance sliders, there's a certain degree of customization, but it's limited compared to prims.

This quick video tutorial helps you understand the key concepts further:

It makes sense that the vast majority of hair being sold is prim hair due to its customizability and higher potential for pleasing appearance. A lot of this has to do with advances in artistic and technological achievement.

Furthermore, a number of the starter avatars you can begin your Second Life with have prim, instead of mesh, hair. Each one is contained in its own folder in your inventory, in the Library > Clothing folder.

How do I edit my prim attachments?

This can get very detailed. But for starters, it's quite easy:

Right-click the prim attachment on your body.
Select Edit on the pie menu. The build tools appear and you can use them to change various aspects.

Here's another video for your delight:

Tip: If things you expect to change are grayed out, the attachment's permissions may include no-modify. Search for the attachment by name (name is shown under the build tools' General tab) in your inventory to see if it shows "(no modify)" next to it.

Also, certain permissions can't be changed when an object (hair or otherwise) is attached to you. You may need to:

Right-click the attachment and choose Drop from the pie menu.
Edit the object on the ground.
Right-click the attachment again, then choose More > Wear to reattach it to the same place it was before.

Why can't I take off my mesh body parts?

You can remove mesh clothing, you can't remove the four mesh body part types (shape, skin, hair, eyes). You can only change them, as they're specified as being required by your avatar appearance.

Oh look! Another helpful video!

However, you can hide your hair. For example, if you're wearing spiky mesh hair and want to only have elegant prim hair visible, what you might do is wear a bald mesh base that effectively hides your mesh hair, so you can attach the prim hair without embarrassment. A lot of prim hair being sold includes a bald base for your convenience.

Furthermore, there are workarounds like invisiprims and "body crushers" to hide your whole avatar mesh, especially when prims are used to provide a body that doesn't closely resemble the mesh. Think of a dragon or a giant ant. The former uses prims with a special texture to partially or fully obscure the mesh. These are commonly used with "tiny"-style and other surreal avatars. "Body crushers" are scripted objects which animate your avatar into a a tight ball so your mesh can be hidden within. While this would be incredibly painful outside of Second Life, thankfully, you're flexible.

Humanoid avatars tend to show their mesh bodies unless special effects are used. For example, to approximate that scene from Terminator 2: Judgment Day when you see through the T-1000's chest after gunfire, a round invisiprim can be used to provide that illusion.

How do I see everything I'm wearing?

What other items are more effective as attachments than meshes?

Attachments are useful whenever something can't be accomplished with the Appearance sliders and needs to extrude from your body, even in subtle ways. For example, prim eyelashes for women are quite the fashion sensation, as are painted fingernail designs.

Another notable example are prim skirts, because they've seen a resurgence in recent years. Mesh skirts used to be typical, but ever since the advent of flexible prims in 2005, many content creators have made skirts far more elaborate than what the Skirt mesh clothing type (in the Appearance Editor's Skirt tab) can do.

Prim eyes aren't as plentiful, but some exist for special purposes, like the bug-eyed cartoon or anime look, each eye having a different color (since the mesh doesn't allow for this), or animated cyber-eyes with optical circuitry.

Trend-wise, realistic humanoid avatars make their mesh skin, shape, and eyes visible, but wear prim hair. They're also clothed in a combination of mesh clothing that clings tightly, but also has prim extensions. For example, baggy pants that are fundamentally mesh, but have saggy bottoms that are actually sculpted prims. Also, accessories like watches, necklaces, and so on are often prims. There's no end to creative ingenuity!

For many real examples of the above made by fellow Residents that you can buy and wear on your own avatar, check out the Xstreet virtual marketplace.

Are meshes or prims more laggy?

This question is too general to be useful. We must look at what's specifically involved. Broadly put, an excess amount of complex prims can add to Viewer-side lag. For example, many tori, found in some curved hair, have a greater relative amount of polygons. This increases your Avatar Rendering Cost.

At the same time, we're hoping to improve the avatar mesh in the future, as there are rendering optimizations that could be done to boost performance.

General lag caused by avatar graphics can be decreased by enabling Avatar Impostors, which draws distant avatars more "cheaply".