Texel Density VS Trim Sheet

yuvwxyz

Hi. first of all apology, if this have been discuss before, but I have not read any topic where both are discuss at the same time.

These have been bugging me, after understanding the concept of Texel density, I came across a workflow where Trim sheet is used, and it totally ignores everything about TD, UVs being stretch here and there. So is TD not that important? Which is more effective in the current gen development?

Can someone clarify these. Thanks in advance.

Axi5

There's been a lot of posts are trim sheets lately, has there been a popular GDC talk involving them or something? Last I saw was some work from Sunset Overdrive.

I don't know much about Trim Sheets construction but from glances are previous posts I can gather that they are horizontally or vertically tiling textures placed in strips in a texture so that modular assets can make good use of them. Going forth with this post I'll be using this assumption so please correct me if I'm wrong.

On topic:

I think you're mistaken if you're asking this question since you can't really compare these things, they're apples to oranges. One is a fact of working with textures, the other is a particular workflow, I'll clear up some of this below.

Texels

A Texel is a texture sample at a particular space, for example sampling a texture in space 0.1, 0.3 could be a particular colour of an image. A texel is not a pixel, several texels can be combined to create a final pixel colour, likewise multiple pixels can access the same texel. A simple way to demonstrate visually what is happening is to just zoom in and out of images in Photoshop, the data doesn't increase or decrease it is just accessed by more or fewer pixels on your screen.

Texel Density

Texel Density is a term used to describe how many useful pixels of texture data there are in a texture map. A useful piece of texture data is a piece of data that is used by the model, any gaps in the models UVs are waste (but unavoidable in most cases). It is also used to refer to how large a particular texel is in world space. Have you ever wondered how large your textured pixel is in game? It rarely ever works out perfect for unique assets, but for walls or other tileable assets you'll usually have a reference i.e. 512pixels = 1 meter. Remember though, if you get really close to this wall you will start to see the same texel scaled up and the opposite. Texel density should be more or less uniform between most assets in a level to make sure that it all looks coherent, and take what I have just said very loosely, there will be many assets on which that "rule" doesn't make sense.

Putting it all Together

A trim sheet, if designed properly won't have many useless pixels of data since it's mostly horizontally/vertically tiled and lined up tightly. Texel Density doesn't disappear, it is actually still very much at play here, you're just removing the useless pixels from the texture map. What this means for Texel Density is that the balance of getting both the texture and the UVs to have the correct amount of detail for it's scale, now places more of the job on the UVs of to get the most out of the texture.

Where to go from here? Take a closer look at plain old tiled textures since they're pretty much the same thing but used on a per material basis.

poopipe

Any sensible pipeline will make use of both unique maps and tileables/trim sheets. 
They're not mutually exclusive either, tileables and trim sheets can (and should)  still be used at correct texel density.

Generally you'll find unique maps are used on instanced prop objects and Tileables/trims are used on unique / large scale world geometry - largely because that makes the most sense in terms of resources/render performance.

gnoop

Texel density  is not a dogma.   Texel size  variations allows you to  make more focus for key details and lees to another. Let you use texture space more rationally   etc.   Stretched pixels make certain architectural elements looks better   from a scene  dominant  viewing angle .     In a word  common sense is applicable here  to a much extent,  same  as  everywhere.

yuvwxyz

Thanks guys. So long story short, having a consistent texel density is not the norm. I guess I'm just overthinking this, and just need someone to slap it directly to my face.

Anyway I'm planning on using trim sheets on my new project, and having consistent TD as much as possible. And the only way I can see these work, is to have 2 UV (1 for scale-all-you-want NM detail, and other for  texel-consistent UV)

I just read this a while ago, and helps as well
http://polycount.com/discussion/118678/consistent-texel-density-vs-importance-space-efficiency

HAWK12HT

Here you may find some more input on this matter. 

http://polycount.com/discussion/192294/texel-density-and-units-in-depth#latest

yuvwxyz

Thanks @HAWK12HT! great thread, will definitely read it 

EarthQuake

gnoop said:

Texel density  is not a dogma.   Texel size  variations allows you to  make more focus for key details and lees to another. Let you use texture space more rationally   etc.   Stretched pixels make certain architectural elements looks better   from a scene  dominant  viewing angle .     In a word  common sense is applicable here  to a much extent,  same  as  everywhere.

Consistent texel density is very important in most games, from most angles, most of the time. There are specific instances where it is less important, and rules can bend or even be broken, but one should definitely understand the main concepts of consistent texel density first and foremost.

Specifically to this thread/trims: Trims are often combined with the primary material using a secondary UV set, added as additional detail on top of a regular material which has consistent texel density relative to the rest of the environment. Usually a trim texture has a bevel detail or something like that which is specifically designed to be stretched (this means it doesn't have fine noise-like detail that would make stretching evident), or it might be added at a different texel density than the base layer (wildly differing densities can be very apparent so they should generally be avoided). Texel density may be less a concern specifically for the trim component of the material, but for the base content consistency is nearly always important.

Density variance can sometimes be a useful trick, for instance if you're creating a first person view weapon, you'll generally want more texel density given to the areas closer to the camera, especially elements like sights or scopes if the user can look down them. Areas further away or offscreen can be given less density. For this sort of case it's a very logical matter of how large the pixels will appear in 3D space.

On the other hand, applying varying texel density to things that are arbitrarily deemed "more important" is often a very bad idea. For instance, using a 4K texture for the head of a character and a 1K for the body - there have been some games that have done stuff like this, and it always looks terrible. It's generally better to have consistent density than wild variances that give more detail to whatever the artist is most interested in.

yuvwxyz

EarthQuake said:

Specifically to this thread/trims: Trims are often combined with the primary material using a secondary UV set, added as additional detail on top of a regular material which has consistent texel density relative to the rest of the environment. Usually a trim texture has a bevel detail or something like that which is specifically designed to be stretched (this means it doesn't have fine noise-like detail that would make stretching evident), or it might be added at a different texel density than the base layer (wildly differing densities can be very apparent so they should generally be avoided). Texel density may be less a concern specifically for the trim component of the material, but for the base content consistency is nearly always important.

Thanks EarthQuake. This is exactly, what I'm looking for. I'm currently watching Alex Senechal's Advance Tiling Tutorial and I think it's the closest to what I need to learn. 

These stuff really are confusing 

HAWK12HT

@EarthQuake "Specifically to this thread/trims: Trims are often combined with the primary material using a secondary UV set, added as additional detail on top of a regular material which has consistent texel density relative to the rest of the environment. Usually a trim texture has a bevel detail or something like that which is specifically designed to be stretched (this means it doesn't have fine noise-like detail that would make stretching evident), or it might be added at a different texel density than the base layer (wildly differing densities can be very apparent so they should generally be avoided). Texel density may be less a concern specifically for the trim component of the material, but for the base content consistency is nearly always important."

Uh so I can have a plane with its uv0 with correct TD base metal (tileable) for example covering entire 0-1 space and second uv1 covering may be a small part in 0-1 space based on trim sheet, correct? but what about when its unique asset that you must fit in 0-1 space with different parts and unique painted details? 

poopipe

If you need unique UVs at correct density then you need a texture big enough to support that. You won't need to worry about the trims in that case because you'll be able to put the information into your unique maps. 

Earthquake suggested A way to handle this, not THE way. You have to assess lots of factors and make a call as to the best option for your situation.