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EvaluateAttributeSnapped function

Evaluates at the pixel centroid with an offset.

Syntax

numeric EvaluateAttributeSnapped(
  in  attrib numeric value,
  in  
            int2 offset
);

Parameters

value [in]

Type: attrib numeric

The input value.

offset [in]

Type: int2

A 2D offset from the pixel center using a 16x16 grid.

Remarks

The range for the offset parameter must be defined by the following byte code.

Only the least significant 4 bits of the first two components (U, V) of the pixel offset are used. The conversion from the 4-bit fixed point to float is as follows (MSB...LSB), where the MSB is both a part of the fraction and determines the sign:

  • 1000 = -0.5f (-8 / 16)
  • 1001 = -0.4375f (-7 / 16)
  • 1010 = -0.375f (-6 / 16)
  • 1011 = -0.3125f (-5 / 16)
  • 1100 = -0.25f (-4 / 16)
  • 1101 = -0.1875f (-3 / 16)
  • 1110 = -0.125f (-2 / 16)
  • 1111 = -0.0625f (-1 / 16)
  • 0000 = 0.0f ( 0 / 16)
  • 0001 = 0.0625f ( 1 / 16)
  • 0010 = 0.125f ( 2 / 16)
  • 0011 = 0.1875f ( 3 / 16)
  • 0100 = 0.25f ( 4 / 16)
  • 0101 = 0.3125f ( 5 / 16)
  • 0110 = 0.375f ( 6 / 16)
  • 0111 = 0.4375f ( 7 / 16)

Note  The left and top edges of a pixel are included in the offset; however, the bottom and right edges are not included. All other bits in the 32-bit integer U and V offset values are ignored.

An implementation can take the offset provided by the shader and obtain a full 32-bit fixed point value (28.4), which spans the valid range, by performing the following calculation:


iU = (iU<<28)>>28  // keep lowest 4 bits and sign extend, which yields [-8..7]

If an implementation must map the offset to a floating-point offset, it performs the following calculation:


fU = ((float)iU)/16

Minimum Shader Model

This function is supported in the following shader models.

Shader ModelSupported
Shader Model 5 and higher shader modelsyes

 

This function is supported in the following types of shaders:

VertexHullDomainGeometryPixelCompute
x

 

See also

Shader Model 5

 

 

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