Texture Format

Tiled Textures

The OpenImageIO utility maketx or oiiotool should be used to convert common file formats to the optimal TX format. For working with some publicly available USD scenes that do not come with tiled/mipmapped textures, see our usd_mipmap_images tool.

Gamma Correction

MoonRay assumes scene linear color spaces for all textures except for 8-bit RGB, on which MoonRay applies an inverse gamma-2.2 curve.

Texture Format

Texture loading during rendering should use the best trade-off between renderer memory usage, disk space, network traffic, and reading performance.

The central aspect is ensuring textures use the least memory once loaded in the renderer’s in-memory texture cache. Memory usage is a premium in a ray tracer, especially in larger scenes.

• 8-bit textures use half the memory compared to 16-bit half-float EXR textures once loaded in the renderer’s in-memory texture cache. You would have to double the renderer’s texture cache size to get the same performance, so we recommend leveraging 8-bit textures as much as possible.
• EXR textures should use 16-bit floats instead of 32-bit floats per channel, which can save another 2x in memory usage. 32-bit float precision is never needed for texture maps.
• Grayscale textures should use single-channel files and never RGB or RGBA files. Using single-channel files can save 3x to 4x in memory usage, respectively. As a contrived but not unheard-of example, compounding the three bullet points, we can make grayscale textures use 16x less renderer memory when using 8-bit single-channel textures, compared to 32-bit float RGBA textures.

To lower the disk space and network traffic used by textures, use the file format choice and compression options judiciously. The issues are:

• The EXR format doesn’t support 8-bit per channel images, but the TX format does.
• Both EXR and TX formats can be either lossily or losslessly compressed. The “zip” compression is the best to-date lossless compression for both formats. The “dwa-med” or “dwa-hi” can be used to do lossy-compression of EXR files, and the “jpeg” compression (with high-quality settings equal to or above 90) can be used to do effective and artifact-free lossy-compression of TX files.

General tests across various formats show that texture load / decoding time is generally insignificant compared to total render time across these options.

Texture Format Recommendation

Surfacing artists can author their textures in a fully-linear color pipeline and save them to 16-bit half-float EXR (or another preferred format). However, before rendering, the painted textures should be converted to mipmapped and tiled “render-ready” textures as follows:

• Surfacing color textures: using 8-bit gamma 2.2 is sufficient visually, so using TX files, either zip or jpeg-compressed (-q >= 90), is advised.
• High-dynamic-range lighting color textures: use 16-bit EXR with dwa-med, dwa-hi, or zip compression.
• Normal-Displacement, bump, and normal maps: use 16-bit half-float EXR render-ready textures to avoid precision / visual artifacts. We should use single-channel for normal-displacement maps. Compression should be zip.
• Vector Displacement maps should be 32-bit float RGB zip-compressed.
• Other grayscale masks or control maps (e.g., roughness or radius) are best as 8-bit single-channel TX, either zip or jpeg-compressed (-q >= 90).

Texture Conversion Examples

Creating a render-ready texture with zip compression:

maketx input.exr -d half --oiio --compression zip -o output.tx

Creating a render-ready texture with dwa-hi compression:

maketx input.exr -d half --oiio --compression dwaa:45 -o output.tx

Creating a render-ready texture with dwa-med compression:

maketx input.exr -d half --oiio --compression dwaa:85 -o output.tx

Creating an 8-bit render-ready texture from a linear EXR¹:

maketx input.exr --colorconvert linear sRGB -d uint8 --oiio --compression zip -o output.tx

Creating a dithered 8-bit render-ready texture from a linear EXR:

oiiotool input.exr --powc 1.0/2.2 --dither -d uint8 -o transitional.tif
maketx transitional.tif -d uint8 --oiio --compression zip -o output.tx

Forcing a single channel for grayscale textures:

maketx input.exr --nchannels 1 -d uint8 --oiio --compression zip -o output.tx