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Question: What are Channels, and how do I use them in Rhino?

Using the new texture-mapping tool in Rhino 4, an object may be given multiple sets of UV mapping coordinates. A set of UV

coordinates (UV Set in Maxwell) may be one of several types, and is used to determine how a texture map is 'projected' onto the object

it is associated with. The projector-types available in Rhino include:

 Surface: This is technically not a projection. The texture-map rectangle is basically stretched out across the object's surface

 Planar: A theoretical plane is created, sized to the bounding-box dimension of the surface which it is applied to. The texture-map

rectangle is scaled to match the size of the plane, and the image pixels are projected through the plane in a direction perpendicular

to it

 Box: similar to Planar, but here there are 6 planes surrounding the object, all 6 of which are mapped as with the Planar projection.

An individual mesh-face's orientation determines which side of the mapping box it takes gets its coordinates from

 Spherical: The texture-map image is stretched/squished onto a theoretical sphere, that is, pixels at the center (figuring top-to-

bottom) are mapped around the sphere's equator, and moving toward the top or bottom of the image, each row of pixels is

compressed to fit the radius of the sphere at that cross-section height. At the top and bottom of the sphere, the last line of pixels is

theoretically squished to infinity. Now, each pixel is projected onto the mapped object, using the theoretical surface normal at its

location on the sphere

 Cylindrical: The texture-map is stretched to fit a theoretical cylinder, sized to enclose the object to which it is applied. Each pixel is

projected onto to the mapped object using the theoretical surface normal at its location on the cylinder

 Capped Cylindrical: Identical to Cylindrical, but here the ends of the cylinder are capped and mapped using a Planar projection

A Texture's Channel number is used to determine which UV Set the Texture uses, and should be set to correspond with the index of the

desired texture-mapping in Rhino's texture-mapping list:

This allows the various Textures that any material may use to be mapped individually onto the objects which the material is applied to,

with visual adjustments possible using either the global Tile/Offset parameters in the Maxwell Texture Editor, or the per-object UV

mapping controls provided by the Rhino texture-mapping tool.

Note that Rhino starts numbering mappings at 1, and prohibits the re-numbering of a mapping to 0. This is because channel 0 is

reserved for the 'current' mapping shown in the viewport. The Maxwell plugin follows the same convention, so to use a Texture with a

certain Rhino mapping, simply set the Texture's Channel number to the same channel number used by the desired Rhino texture-

mapping. Also, because it is possible to 'skip' projector numbers (e.g. a material with one Texture, for which the Channel is set to 4),

Maxwell will give objects the channel 0 (viewport) UV coordinates for any channels which have no explicit Rhino texture-mapping

channel.

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