These textures are pretty much the color of the mesh. The game would look pretty dull without them if you ask me.
Some _di textures have an alpha channel, which dictate how transparent the texture will be.
One thing to note is that Old Engine _di textures are diffuse maps, and that Dragon Engine _di textures are albedo maps. The difference being that diffuse maps contain baked lighting, while albedo maps do not.
The suffix "_d" is used for assets.
The difference between _mts between each engine will be covered in their own dedicated page. Only their general purpose will be covered here.
These textures are responsible for affecting how shiny a material is and also affects the ambient occlusion. In the Dragon Engine, the alpha channel of these textures mask the overlay textures.
Assets also utilize this texture however they are formatted differently and are named differently depending on the engine.
These textures do not exist in Ryu ga Gotoku: Kenzan!.
These textures are normal maps. They are used to fake details on otherwise low-poly models. Yakuza mainly uses two types of normal maps: yellow normal maps and green normal maps.
Yellow normal maps are the most common in Dragon Engine games, and green normal maps are the most common in Old Engine. Both engines support both types of normal maps. It also supports the standard purple normal maps.
Green normal maps store the X+ axis in the alpha channel, and store the Y+ axis in the green channel. Yellow normal maps store the X+ axis in the red channel, and store the Y+ axis in the green channel. Ishin Kiwami's green channel is in the Y- axis. The Z+ axis is calculated in the shader with this shader code:
//Remap normal map to -1 to 1
normalMap.x = 1 - (2 * tn.x);
normalMap.y = 1 - (2 * tn.y);
// Dot product of the vector (normalMap.x,normalMap.y) and itself, multiplied by -1 and then added by 1. Then, pick the largest number between the result of that and 0 (so that the result never becomes a negative number), and square root it.
tmp.z = 1 + -(dot(normalMap.xy, normalMap.xy));
normalMap.z = sqrt(max(0, tmp.z));
The engine can differentiate between green and yellow normal maps by reading the header of the .DDS file. If the .DDS file's header is "DxT5xGxR", it reads it as a green normal map.
For assets they usually end with the prefix "_n".
These textures are used as repeating color detail. The repetition scale is determined by material parameters in Old Engine while Dragon Engine uses a separate UV on the model. The RD is multiplied onto the _di texture.
In Old Engine, the scale of the _rd textures is determined by 2 values. These 2 values can be found in Unk12 in the Yakuza Properties tab when importing a .GMD with this Blender add-on. The first value is the X scale value and the second is the Y scale value.
The value on the 1st column, 3rd row is the X scale value. The one next to it is the Y scale value.
These textures are similar to _tn textures, except they're also scaled like _rd textures.
In Old Engine, the scale values can be found in the Attribute Set Floats tab.
More or less the same as _tn textures, except they're used to control the wrinkles of faces. In-game, they are combined with the _tn. They're blended in-game using the mesh's vertex colors.