Class Name: Texture Coordinate

Superclass - <Texture Coordinate Entry>

Subclasses

Definition

An {s, t} tuple in image space (also known as texture space) used to specify how a texel from the image space is to be mapped to a location in "object" space.

Each MIP level of a 2D <Image> defines an image that has a width and height specified in texels. Regardless of the actual size of the image, the image space is defined by treating the two-dimensional image as a square, defined by the lower-left coordinate of (0.0, 0.0) and the upper-right coordinate of (1.0, 1.0). A < Texture Coordinate> is a coordinate within this image space.

A <Texture Coordinate> specifies an exact location within a given image space, and this location is mapped to the "object space" location associated with the <Vertex>, <Point>, or <Tack Point> of which the <Texture Coordinate> is a component. A textured geometric object generally has a <Texture Coordinate> for each vertex of the object, and the surface of the geometric object is 'painted' or 'covered' with the given <Image>, interpolating what part of the <Image> should be displayed where based on the <Texture Coordinates> of the object's vertices.

Whether a specified < Texture Coordinate> corresponds to exactly one texel in the texture definition, or to a blending of many texel values from the definition, is a decision made by the texture interpolation algorithm used to display the texture. The methods for calculating the interpolated texture values and for blending the texture onto the object are determined by an <Image Mapping Function>. (Note that by the Image Mapping Functions and Texture Coordinates rule, <Texture Coordinates> will only appear within the scope of some < Image Mapping Function>.)

Primary Page in DRM Diagram:

• 18

Secondary Pages in DRM Diagram:

• 5

Example

1. The s, t coordinates to map to the lower left corner of a <Polygon>.

FAQs

Given a <Texture Coordinate>, the s value of which is greater than 1.0, how should it be interpreted?

The interpretation of the < Texture Coordinate>'s s value depends on the value of the image_wrap_s field of the relevant < Image Mapping Function>.

• If image_wrap_s specifies that the <Image> is to be clamped, then any s values less than 0.0 are to be clamped to 0.0, and any s values greater than 1.0 are to be clamped to 1.0. That is, s values less than 0.0 are to be treated as though they had been 0.0, and those greater than 1.0 are treated as though they had been 1.0
• If image_wrap_s specifies that the <Image> is to be repeated, then any s less than 0.0 or greater than 1.0 are to be treated as though the <Image> tiles the image space out to infinity.

  In practical terms, during repeating, the integer portion of the <Texture Coordinate> is ignored, and copies of the texture map tile the surface.

Given a <Texture Coordinate>, the t value of which is greater than 1.0, how should it be interpreted?

The interpretation of the < Texture Coordinate>'s t value depends on the value of the image_wrap_t field of the relevant < Image Mapping Function>.

• If image_wrap_t specifies that the <Image> is to be clamped, then any t values less than 0.0 are to be clamped to 0.0, and any t values greater than 1.0 are to be clamped to 1.0. That is, t values less than 0.0 are to be treated as though they had been 0.0, and those greater than 1.0 are treated as though they had been 1.0
• If image_wrap_t specifies that the <Image> is to be repeated, then any t less than 0.0 or greater than 1.0 are to be treated as though the <Image> tiles the image space out to infinity.

  In practical terms, during repeating, the integer portion of the <Texture Coordinate> is ignored, and copies of the texture map tile the surface.

Reference: OpenGL Programming Guide, 3rd edition for more information on clamping and repeating textures.

Constraints

• <<Image Mapping Functions and Texture Coordinates>>

Composed of (two-way)

• zero or one <Texture Coordinate Control Link> instance

Component of (two-way)(inherited)

• zero or one <Texture Coordinate Table> instance

Component of (two-way)

• zero or more <Base Vertex> instances
• zero or more <Point> instances
• zero or more <Tack Point> instances
• zero or more <Texture Coordinate Set> instances

Inherited Field Elements

Field Elements

Notes

Fields Notes

s

 The s value of the (s,t) coordinate.

t

 The t value of the (s,t) coordinate.