log

Provides log₂(x) support with full float precision of at least 1/2²⁰.

log dest, src

Registers

dest

Destination register.

src

Input source register.

To learn more about registers, see Registers.

Remarks

The following code fragment shows the operations performed.

float v = ABSF(src.w);
if (v != 0)
{
    dest.x = dest.y = dest.z = dest.w = 
        (float)(log(v)/log(2));  
}
else
{
    dest.x = dest.y = dest.z = dest.w = MINUS_MAX();
}

This instruction accepts a scalar source (src.w) of which the sign bit is ignored. The result is replicated to all four channels.

The input exponent must be in the range �128 to 128. The approximation error must be less than 1/2²⁰ in absolute error, and over the range (1.0 <= dest.y < 2.0).

A zero source generates (-infinity, -infinity, -infinity, -infinity).

Example

// This example changes the vertex color.

// shader file

vs.1.0
m4x4 oPos, v0, c0	; transform vertices by view/projection matrix

mov r0.w, c4.w		; load constant color
log r0,  r0.w		; take the log and copy it to all four components
mov oD0, r0 		; output color

// The constant color loaded in r0 is (0, 0, 0, 1.5). The log of 1.5 is 
// approx. 0.4 which yields a shade of gray since it is copied to all 
// four components(x, y, z, w). The resulting color is shown below.

// Additional code is used to initialize the constant registers.

D3DXMATRIX mat, matView, matProj;
D3DXMatrixMultiply(&mat, &matView, &matProj);
D3DXMatrixTranspose(&mat, &mat);
// set C0 with the view and projection matrix
m_pd3dDevice->SetVertexShaderConstant(0, &mat, 4);

// set register c4
float log[] = {0, 0, 0, 1.5f};
m_pd3dDevice->SetVertexShaderConstant(4, &log, 1);

// The first SetVertexShaderConstant method binds the C0 register 
// with the first row of the view/projection matrix (rows 2, 3, 4 are 
// bound to registers C2, C3, c4 also). The m4x4 instruction (in the 
// shader file) loads the data.
// The second SetVertexShaderConstant method binds the C4 register 
// with the data in the log array. Notice that only the w component is used.