Texture Publishing to mental ray

Nailing down a good texture pipeline can be confusing!  Hopefully this clears up some of the whys and hows for efficiently handling texture of images with mental ray, providing a straightforward solution to a complicated problem.

[You can also find a test example on the ARC forum here: https://forum.nvidia-arc.com/showthread.php?13316-Maya-and-mental-ray-texture-caching ]

Texture Filtering

Non-filtered images may result in artifacts such as moire patterns (click to view full size).

When sampling a textured object, color information is lost between sampling points.  This is because a single sample only calls a single texture pixel (texel).  Information for texels that don’t get sampled is lost – which can be a real problem if that information was important.  For highly detailed textures or for textures with regular patterns this sampling limitation may manifest itself as an artifact such as a moiré pattern.

Aliasing-free texture sampling is restricted by a mathematical limitation known as the Nyquist rate which states that the number of samples required to adequately estimate a signal must be at least twice the noise frequency.  This means there are only two options to remove rendering artifacts caused by noisy textures: sample more or filter the texture.

Filtering a texture removes high frequency noise from images making them easier to sample.  While this does away with many potential aliasing artifacts, it also removes detail leaving your textures looking fuzzy.  This is not a problem for textures that occupy limited screen space because that is detail that would have been lost anyways.  However it is a problem for textures that occupy lots of screen space because that detail might have been visually important.  Luckily for us, texture mipmapping provides a convenient solution for both situations.

Mipmapping

Mipmapped images reduce texture artifacts by filtering (click to view full size).

Mipmapping (from the Latin phrase multum in parvo meaning “much in little”) is the process of pre-filtering one large image into many smaller images of progressively decreasing resolutions. The largest images retain detail that the smallest images lose to filtering.  These mipmap images are generally organized into a multi-resolution file sometimes known as a pyramid image.

Mipmapped “Pyramid Image”

The advantage of multi-resolution images is that they provide mental ray with the ability to read from appropriately sized pre-filtered images based on the amount of screen space that the textured object occupies.  This allows superior anti-aliasing performance while using less samples.

For the above renders, mental ray read from higher and higher mipmap levels as the checkered texture receded into the background.  The highest mipmap levels have lost virtually all detail and appear as a middle grey.  As a result of filtering, mipmapping reduced the total number of eye rays from 5552111 to 5144177!

It is advisable to use mipmapped textures.  Generally, mipmapped textures should be used with the filter declaration (see miFilter attribute below).  The purpose of the filter declaration is two fold:

1. filter tells mental ray to mipmap images on demand if they are not already so.  When computed on demand, full upper pyramid layers are computed and kept in memory, totaling 1/3 of the base (largest resolution) image.
2. You can control how selective mental ray is when choosing mipmap levels (and thus the blur of the textures) by setting filter size (see miFilterSize attribute below).  A filter size above 1.0 increases the texture blur by reading from a higher level (lower resolution) mipmap.  A filter size below 1.0 reduces texture blur by reading from a lower (higher resolution) mipmap.  Generally filter size should correspond to the number of times a texture repeats itself in UV space, i.e. an image that repeats itself 10 times should use a filter size value of 10.0.

Elliptical Lookup

Elliptical lookup has removed practically all texture artifacts (click to view full size).

Texels are square when viewed with photoshop or some other 2D application, but this is not always the case when rendering in a 3D environment.  When textures are rendered at glancing angles, the screen space each texel occupies become distorted – i.e. more texels may fit into a screen pixel in one direction than another.

To avoid artifacts caused by symmetric filtering for textures viewed non-symmetrically, some textures require an additional elliptical lookup on top of mipmapping.  Elliptical lookup works by projecting the circular area surrounding a sample onto the texture image, resulting in an ellipse.  The texels within the ellipse are then averaged resulting in a more accurate return value.

The above render shows the artifact-free result elliptical lookups provide.  Not only is the strange middle grey circle gone, but the number of eye rays has been further reduced to 4705444!

While elliptical lookup is relatively fast, not every textured object requires it.  To avoid unnecessary render time, elliptical lookup should be enabled on a per-texture basis.  The default lookup size of 8.0 is generally a good starting point for most situations.  Reducing this value will speed up render times, but may reintroduce artifacts.

Note: mental ray will mipmap on demand if elliptical filtering is specified for non-pyramidal images.

Tiled and Cached Textures

Keeping all texture information in memory can become quite expensive for large scene, especially when using pyramid images.  mental ray supports texture caching for certain tiled texture formats.  When texture caching, only certain tiles of a texture are loaded into memory.  These tiles are automatically removed or replaced by recently accessed ones, dramatically reducing the memory consumption during rendering.

Starting with mental ray 3.9, texture caching can be specified globally using the “{_MI_REG_TEXTURE_CACHE}” registry variable set to “on” / “off” / “local”.  The defualt “local” means that only local textures are cached (see miLocal attribute below).  Native .map files are always considered local.  It is important that local textures be saved locally or on a caching file server where images can be shared across multiple machines (on a render farm) to avoid network overhead.

Another registry variable, “{_MI_REG_TEXTURE_CACHE_SIZE}”, exists to set a maximum size for the texture cache in megabytes (e.g. “512” corresponds to 512MB).  mental ray will dynamically determine the cache size for the default value of “0”.

It is advisable to use tiled/cacheable textures to reduce memory consumption.

Image Formats and Conversion

• OpenEXR (.exr)
  • Can be saved as mipmap pyramid
  • Can be tiled/cached
  • Personally recommended by me!
• mental ray’s native memory-mapped image (.map)
  • Can be saved as mipmap pyramid
  • Can be tiled/cached
• TIFF (.tif)
  • Can be saved as mipmap pyramid
  • Can be tiled/cached
  • note: Photoshop has been known to do some odd things to tifs (like layering)
• Bitmap Image File (.bmp, .dib)
  • Can be tiled/cached
• Maya IFF (.iff)
  • Can be tiled/cached

imf_copy (which ships with mental ray and Maya) is a convenient command-line tool that can be used to create mipmapped-tiled images. For example, I could run this command if I wanted to publish my working TIFF texture to an mipmapped-tiled zip-compressed OpenEXR texture:

imf_copy -p -r -k zip working_texture.tif published_texture.exr

Alternatively, exrmaketiled and/or the openimageIO toolkit can provide increased flexibility for publishing textures.

Setting up the Maya file node for mental ray

Maya file node set up for elliptical lookup

If using a pre-mipmapped image (which you should be):

1. File Attributes > Filter Type → Mipmap (This enables the miFilter option)
2. File Attributes > Pre Filter → off (we are using mipmapping with the option of elliptical lookup instead)
3. mental ray > Override Global Auto-Conversion Settings → on (I don’t trust any global Maya auto settings)
4. mental ray > Convert File To Optimized Format → off (texture has been already mipmapped!)
5. mental ray > Advanced Elliptical Filtering → generally off, on if this texture is producing rendering artefacts
6. Extra Attributes > Mi Local → preferably on for texture caching, otherwise off.
7. Extra Attributes > Mi Filter Size → generally 1.0, higher for repeated textures.

Note: The last two are dynamic attributes so they might not be present on all file nodes.  See below on how to add them.

Python script to add texture attributes to all Maya file nodes in a scene:

import pymel.core as pm

# Add Extension Attributes to all file nodes.
pm.addExtension(nt="file", ln="miLocal", at="bool")
pm.addExtension(nt="file", ln="miFilterSize", at="float", dv=1.0)

# Delete Extension Attributes from all file nodes.
pm.deleteExtension(nt="file", at="miLocal", fd=True)
pm.deleteExtension(nt="file", at="miFilterSize", fd=True)

Last Notes

As a rule-of-thumb, I recommend using mipmapped-tiled OpenEXRs for all textures and to declare them local.  The one exception is the environment map which will only benefit from tiling, not mipmapping.  To pre-blur the environment, try using the mia_envblur shader combined with “Single Env Sample” on material shaders.

A formalized texture publishing step between texture creation and rendering provides an excellent opportunity to both optimize image formats and linearize data.  For more information on colorspaces and linear color workflow, see Linear Color Workflow(s) in Maya.