This chapter is a continuation of the previous chapter on materials, but is focused on texture maps. Karma CPU supports UV and Ptex texture maps; XPU does not yet support Ptex. Each family of materials has its own shader for reading maps.

• Copernicus networks can create or modify texture maps.

• Materials can read COP textures by passing op: prefixed paths to COP nodes, rather than file paths.

  • COP nodes can have multiple inputs; you can specify an input by name or index:

    Index	op:/stage/copnet1/null3[2]
    Name	op:/stage/copnet1/null3{my_color_input}

• When the USD ROP writes to disk, these textures get written out automatically along with the USD files. These files are written as optimizedtexture, meaning they are tiled and mip-mapped EXRs.

• Quick Surface Material has shortcuts to setup COP textures.

Karma supports every image format supported by Houdini, along with many supported via OpenImageIO. The texture format affects performance and color space assumptions.

Using mip-mapped and tiled texture maps will significantly improve performance, particularly time-to-first pixel. Generally it’s best to stick with .exr or .rat textures. When Karma encounters non-tiled formas, it automatically converts them to .rat (see Auto-Conversion for details).

You can use the imaketx command-line utility to generate mip-mapped .exr and .rat files from other image formats. Note that imaketx only mip-maps 1 AOV for .exr images, while .rat supports mip-mapping all channels.

[09:02:04] Texture Disk Cache: Converting image /jobs/gramophone.usdz[Gramophone_Normal.png] to texture /tmp/houdini_temp/hfsT7MYFL
[09:02:05] Texture Disk Cache: Finished writing output file /tmp/houdini_temp/hfsT7MYFL
[09:04:11] Texture Disk Cache: Converting image /jobs/maps/mandril.jpg to texture /jobs/aces/maps/mandril.jpg.rat
[09:04:11] Texture Disk Cache: Finished writing output file /jobs/maps/mandril.jpg.rat    

Filtering controls ¶

Normally Karma automatically handles choosing mip-levels and texture map filtering. Karma offers some additional controls Additional inputs can be added to mtlximage, that offer explicit control over mip levels and texture map quality.

• The Karma Texture Controls recipe is the easiest way to add filtering options to MtlX Image nodes.

• For editing already-existing materials, create an Edit Properties node and point it to a mtlximage shader prim. Click Edit Properties button will shows the available controls

• Some controls are also available on the Karma Material Properties.

Karma renders internally in scene linear colorspace, so texture maps must be transformed into linear space. Because Hydra doesn’t send through parameter metadata, Karma determines the source color space in a few ways:

1. If a color space input is “raw”, or the type is a vector, Karma does not transform the image values.

2. If a reader node is set to “Auto”, Karma goes through a process to detect the source color space:

   1. Check for role strings embedded in the file name, such as diffuse_map.rec709.exr.

   2. Check metadata in the image file for a colorspace specification.

   3. Assume sRGB for 8-bit images, linear other formats/depths.

Because Hydra doesn’t pass through parameter metadata, MtlX Image nodes with a color3 or color4 signature are always in “auto” mode. To force MtlX Image nodes to read their images in linear format (without transformation), set the parameter’s signature to a non-Color value (such as float, vector3, or vector4).

The normal map node takes the input normal and transforms it by the basis that’s formed by the shading normal, tangent and bitangents. However, when dealing with MikkT tangent space normal maps, you have to consider several things. There’s a recipe to add the karma_mikkt shader property. You can execute the recipe when you right-click the node and choose Recipes ▸ Karma MikkT Normal Map.

This property indicates that the tangent space normal map input should be treated as a MikkT tangent space normal map. In this case, the tangentu and signs attributes/primvars will be used to properly define the basis. If Karma MikkT Normal Map is turned off (or not present), or the tangentu primvar doesn’t exist, then the map is interpreted as a Karma-specific tangent space normal map (if the signs attribute/primvar isn’t present, it will default to -1).

• For a Karma-specific tangent space normal map, Karma will use it’s default tangent shading frame to apply the normal map operation.

• To make Karma-specific tangent space normal maps work in Houdini VK, the tangentu primvar must exist. You can create the primvar through the Poly Frame SOP. There, choose the Tangent UV Gradient style. Then use a Geom Prop Value to read that primvar, and wire it into the tangent or geometry_tangent inputs on your surface shader.

For more information on building shaders and materials for Karma, see Using MaterialX in Solaris.