When using Arnold it is best to use a tiled mip-mapped texture format such as .exr or .tx that has been created using maketx.
.tx textures are:
- Tiled (usually the tiles are 64×64 pixels).
If you already have tiled and mip-mapped exr's that have been created by another renderer, you won't need to convert those files to .tx
Due to (1), Arnold’s texture system can load one tile at a time, as needed, rather than having to wastefully load the entire texture map in memory. This can result in faster texture load times, as texels that will never be seen in the rendered image will not even be loaded. In addition to the speed improvement, only the most recently used tiles are kept in memory, in a texture cache of default size 512 mb (can be tuned via options.texture_max_memory_MB). Tiles that have not been used in a long time are simply discarded from memory, to make space for new tiles. Arnold will never use more than 512 mb, even if you use hundreds, or thousands of 4k and 8k images. But then, if you only use a handful of 1k textures, this will not matter.
Due to (2), the textures are anti-aliased, even at low AA sample settings. Neither of these are possible with JPEG or other untiled/unmipped formats (unless you tell Arnold to auto-tile and auto-mip the textures for you, but this is very inefficient because it has to be done per rendered frame, rather than a one-time pre-process with maketx).
It should be worth pointing out that .tx files are basically .exr or .tif files that have been renamed to .tx. This means .tx files can be read by image editors, although you may need to rename the file to .exr or .tif so that the image editor will load it. However, .tx files have a few extra custom attributes set that are not normally included in .exr/.tif files which can make rendering even faster. For instance, they will include a hash so that if you try to load two different files that contain the same data, Arnold only needs to load this data once.
The example below shows a speed increase of seven times when using .tx files compared to .jpg files:
The process is quite simple. You will need the maketx.exe utility, the texture that needs to be converted and a dos shell. Below is an example of how you would convert a .tif file to a .tx file.
A tutorial that covers this process can be found here.
If an object has multiple UVW tags, then the first one is exported as the main UV set (uvidxs and uvlist). The other tags become indexed user data. The name of the user parameter is the name of the UVW tag. Tags with an existing name are skipped and a warning message is added to the log in this case.