Bake on Mesh (BoM)

Under this code name resides a simple design that many still don't understand. Let's see what it is about, how to use it and how to create for it.

A bit of history

System avatar

Before the era of mesh avatars, was the system avatar. When you create an account on a grid, unless the grid provided a special default avatar on registration, you know the most basic system avatar : Ruth. Let's examine it, that will later help understanding what BoM is made of.

A system avatar is made of basic wearables: shape, hair, eyes and skin. Other parts can be put on on top of those: shoes, alpha, tattoo, and various clothes (underwear panties and bra, clothing pants and shirt, jacket and skirt).

The avatar on the side picture wears only 4 components, all system wearables : a shape, a skin, a hair and eyes. Each component uses one or more textures through a technical term named channel (we will talk more about channels later).

Shape

It's not a tutorial about shapes, even if there is a lot to say about them. Just know that the shape is the only wearable used the same way since the beginning of SL, to define how your avatar is shaped, whatever you wear to display it (system avatar, rigged mesh or others).

Skin

Skin wearable

Let's see the skin closely. The system skin has two modes of display.

The first, historic, is created with sliders, and really, but really, not used since the beginning of SecondLife (except maybe for some ugly avatars).

The second mode that oldest in SL know, is made of 3 textures : one for the face, one for the upper part of the body, and one for the lower part. When you set those textures in the skin properties, they are used to display on the system avatar (which is basicaly a predefined mesh).

Eyes

Eyes work the same than skin, but always need a texture.

Hair

Hair wearable

Hair is also known are basehair since prim an mesh hairs appeared. They are mostly used to define the scalp, or hair without volume.

The default texture coming with the default hair is used to set hair properties with sliders, like for the skin. That's clearly not used since a very long time, like system skins, but the wearable has other interesting properties:

You can set any texture, and that can be used with BoM.

The Eyebrows tab has some control on the shape, it permits to refine the eyebrows shape by moving the underlying shape.

Other wearables

Among the 4 basic components, avatars can also wear additionnal layers. At the lowest level is the tattoo layer, baking just on top of the skin. For olders who used to be in SecondLife or OpenSim with the system avatar remember it was possible to use those tattoos not only for tattoos, but also for makeup and marks on skins, a total of 6 or 8 (I don't remember exactly) was possible in SL.

Mesh avatars era

Due to limitations of the system avatars and the introduction of rigged mesh into SecondLife, mesh avatars have mostly replaced the system ones. The principle is to wear rigged objects (body, head, etc..) and to hide the system avatar with a wearable alpha (hiding all body, head, eyes and hair).

The main advantage is to have well defined bodies and heads, reactive to evolutions like Bento.

The main disavantage is to be forced to use appliers, meaning a hud sending the correct textures to the mesh. That also reduces the possiblities to combine different layers as available for the system avatar by wearing several tattoos.

A tattoo or anything to display on top of the skin has to be set on additionnal mesh faces, a technique known as onion layers (think of how you peel an onion to have an idea of the concept). That limits a lot the number of layered textures, as several very close mesh faces need to be added to make the onion.

Another problem is about rendering transparency, and the need to play with alpha masking instead of alpha blending for faces on the lower layers (and OpenSim handles setting the alpha mode by script only since 0.9.1, which makes it rather complicated to use for grids still on older versions of OpenSim).

BoM fixes almost all of these issues and extends the possibilities.

Bake on Mesh

Yes, that's what the BoM code says. It's just baking, meaning applying textures from the system channels on worn mesh. Here come those system famous channels I explained at the beginning of the article!

Bake on Prim :)

Actually, the name is not exact, as you can bake on prims too. Let's create 3 cube prims, link them, and wear them. Then let's define faces to be BoM.

As you can see cubes took the textures of the current skin, because I choose Head, Upper and Lower system channels. Also, the system avatar disapeared on parts using BoM. On the picture, the system body and head disapeared, but system eyes and hair are still visible.

Note that if you rez the object on the ground, you wont see the baked textures, but flat colors. It's because the process of baking appears only on the avatar, and then only on worn objects. Linden Labs was cleaver enough to define a specific color to each channel to be able to identify them on rezzed objects.

Bake on Mesh

Now let's use BoM on a real mesh avatar part, like the face. I am wearing a modified Athena head (Catya). Modified because it does not exist with BoM enabled, and I had to totally hide unused faces (layers for makeup for example).

The picture shows I am using the same texture properties than my simple cube, but on a face of the mesh head. Of course, I had to set this on several faces, but you get the idea.

I also have defined my system hair to contain my base hair (or scalp as some prefer to say). To be able to change it, I have assigned the hair system channel to faces of the head on top of the skin ones (the famous onion).

Bake hair

I could have used a system tatoo using only the head channel and containing the base hair texture, but this does not go well with materials you could set on the skin: base hair would then have the same material rendering than the skin, what I dont want. So if I want to change my base hair, I define a new system hair and set the texture to it.

How to use BoM in practice?

That's where things become more complicated. As BoM resumes in assigning system texture channels to mesh faces, each mesh avatar provider can define their own way to do it!

That's how Athena team decided to use a tattoo for base hair, assigning the hair channel to eyelashes, and skirt to lipstick. I wont judge those decisions, you just need to be aware of them if using Athena BoM avatars.

For my own project, Femme, it will be different (it's not released yet, I am still working on it), and thus will make wearables incompatibles between Athena and Femme. I think the same can occur with Ruth2.

Add wearables

Tattoo wearable

When you want to add things on top of your basic BoM avatar, you simply wear a system wearable (or rather add, wearing could detach another wearable of the same kind). Let's picture it with a tattoo (it works with any wearable like underwear, clothing, etc..)

I have created a tattoo wearable from two textures (upper and lower channels). Just Adding it applies the tattoo to the avatar.

When looking into the mesh faces properties, we can see the texture (which is set to Bake the upper channel) is computed. It's the result of the skin plus the tattoo on top of it.

You can theorically add 64 levels of layers this way! I have not tried personnally, but we can think the old limit of 8 would be already enough. If you create a clothing wearable (top or bottom, underwear or cloth layer), you can also add on top of them. It will work like good old system avatar, but on your lovely mesh avatar.

Limits on OpenSim

While BoM is available on any OpenSim supporting baked textures (all currently used versions have it!), it has some limits on versions before 0.9.1.

Changing alpha mode

You are surely used to use your body HUD to hide some parts of your body when wearing some clothes that are not rigged exactly so that they fit perfectly.

Using mesh alpha hiding forces the face property Alpha Mode to Blend. It's important because 0.8.2 does not play well with BoM and blending, and it's not possible to reset the alpha mode to None by script (with the HUD). So your mesh body will keep blending once you have hidden parts of it. This will result in parts not responding correctly to some ambiant lights, or worse hide those parts !

One solution is to manually reset the alpha mode (tricky). Another is to use different copies of the body for each alpha set.

Auxilliary channels

BoM has introduced new extra channels, that mesh designers can use to map to specific parts of the avatar. The problem is that only OpenSim 0.9.1 and onwards support those extra channels. That means parts using those channels wont display correctly on older versions, showing a flat colored texture.

Universal Wearables

Another nice addition of BoM is a new wearable, named Universal. It works like a tattoo wearable, but can accept any BoM channel (old system ones plus the new auxilliary channels).

You can login or teleport on a region running on Opensim before 0.9.1 if you wear such wearable. The good news is the message is explicit in the viewer and you can simply detach it. The drawack is on your look, depending on how you use it.

Conclusion

I think BoM is a really great addition, just like rigged mesh avatar were, like Project Bento was. The issue for OpenSim is about compatibility and some glitches with alpha blending.

BoM permits to play with makeup, tattoos and various skin marks like it was possible before mesh avatars. You can put moles or freckles depending on your mood, combine them, even combine tattoos splitted in parts, plus many other possibilities I didn't think of.

Femme project

I didn't want to focus on my project for this tutorial, because as I said, each mesh avatar provider has their own usage of BoM. Mine will be partial, to stay compatible a maximum with older OpenSim versions (my main account is on Sacrarium, not on 0.9.1 yet). I will surely make a specific article about it when it's ready :)