https://sims4studio.com/thread/1728/information-on-cas-specular-rles

The Studio team has been working on .rles for quite awhile and have discovered some things about how it works. This isn't a tutorial but it is meant to give some very basic information about the specular to get people started making their own.

To begin with the CAS specular is an image resource with 5 channels. These channels are represented in Studio with two separate images. What we have called the specular overlay has red, green, blue, and alpha channels. What we have called the specular mask has an alpha channel. You may ask why did EA make the specular so complicated and why does Studio retain all this complexity for editing instead of just ignoring or autogenerating the data represented by the specular mask? A few examples will demonstrate the power that the EA .rles has to make a wide variety of effects that would not be possible using a simple 4 channel image like all the other images in Sims 4.

The EA lingerie is a good example to start with because the effect of all 5 channels is seen clearly here in the area of the garment that covers the skin. The first 4 channels are used to create different levels of shininess ranging from almost matte fabric portions to very shiny metal portions. The 5th channel is used to apply that shininess in different ways to accentuate the illusion of translucent fabric overlying skin. Here is the garment:

And here is the 5th channel for it (the specular mask in Studio) where different alpha levels are used to apply the data from the other 4 channels to create the effect:

Without the ability to edit this 5th channel it would not be possible for creators to create a specular like the one seen in EA's corset item. A second, more complex, example of a similar use is the alien suit, shown here in Studio's model viewer (where the specular isn't being composited yet):

The first 4 channels are used to generate a variety of shine effects on the suit:

And the 5th is used to apply them differently depending on location:

EA also uses the specular 5th channel to create gradient effects like that seen in the gradient sunglasses.

So does this mean it's difficult to make a specular using Studio? Absolutely not! For a demonstration of that please see

the tutorial posted by Untraditionalnerd

which shows how easy the process can be using two separate projects and two different 2D editors.

But what do the various channels do? A technical description of how they work isn't yet available. But what we've done is create some various combinations of channels so that you can see what the result is if that specular is applied to a garment in the game. This shows the original EA item on the top left. On the top right is the same garment with a solid light grey specular overlay set at approximately 60% transparency. On the bottom left is the same garment with only the red and alpha channels of the overlay in use...in the game it looks a lot like vinyl. The bottom right shows just blue and alpha channels in use and this setting makes the garment look like hard plastic in the game.