Evaluates at the pixel centroid with an offset.
numeric EvaluateAttributeSnapped( in attrib numeric value, in int2 offset );
Type: attrib numeric
The input value.
A 2D offset from the pixel center using a 16x16 grid.
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)
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 Model 5 and higher shader models||yes|
This function is supported in the following types of shaders: