VRayPtex node allows the you to load and use Ptex textures with V-Ray. Ptex is a mapping system that applies a separate texture to each face of a subdivision or polygon mesh and therefore requires no UV assignment. A Ptex file can store an arbitrary number of textures along with mipmaps and adjacency data used for filtering across face boundaries.
Ptex File Name - specifies the .ptex file that is going to be used as a texture
Cache Size (MB) - the information in a Ptex file is compressed for storage. In order to be used it has to be uncompressed and stored in the RAM. This parameter controls how much memory is available for the current texture. If the value is large enough for the whole texture the information is extracted only once and is stored during the entire rendering. Otherwise the information is constantly read from the hard drive, extracted and used when necessary
Filter Type - specifies the type of filtering that is going to be applied to the texture.
Width - the width of the filter
Blur - specifies the amount of the blurring.
Sharpness - this option is only available when the Filter Type is set to Bicubic and controls the amount of sharpening applied by the filter
Interpolation between mipmap levels - a ptex texture may have several mipmap levels which are basically versions of the texture at different resolutions. This allows a higher resolution texture to be applied to parts of the object closer to the camera and a lower resolution version to be applied to parts of the object that are far away. When this option is enabled V-Ray will interpolate between different mipmap levels to create a smooth seamless texture in the end
Reverse vertex order - a ptex is generated for a specific mesh. Sometimes the program generating the ptex numbers the vertices of the mesh in the reverse order. When this happens the object is not properly textured and enabling this option fixes the issue.
Automatically select color channels - a ptex may have an arbitrary number of channels and there is no sure way to determine which channel contains which information. When this option is enabled V-Ray tries to select the correct channel for Red Blue and Green. When this option is off the user manually specifies which channel contains which information.
Red/Green/Blue Channel - manually specify the channel numbers for Red Green and Blue
Alpha - specifies the mode in which alpha is applied
Solid - all alpha information is ignored
Alpha from texture - the information for the alpha is taken from the specified Alpha channel in the texture
Alpha from luminance - the alpha information is taken from the Luminance of the RGB channels.
Automatically select alpha channel - when on V-Ray automatically selects the alpha channel from the ptex file
Alpha channel - manually specify which channel from the ptex is the alpha channel