logp

Provides log₂(x) partial support.

logp 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 by the logp instruction.

float v = ABSF(src.w);
if (v != 0)
{
    int p = (int)(*(DWORD*)&v >> 23) - 127;
    dest.x = (float)p;    // exponent
    p = (*(DWORD*)&v & 0x7FFFFF) | 0x3f800000;
    dest.y =  *(float*)&p;// mantissa;
    float tmp = (float)(log(v)/log(2));
    DWORD tmpd = *(DWORD*)&tmp & 0xffffff00;
    dest.z = *(float*)&tmpd;
    dest.w = 1;
}
else
{
    dest.x = MINUS_MAX();
    dest.y = 1.0f;
    dest.z = MINUS_MAX();
    dest.w = 1.0f;
}

This instruction provides logarithm base 2 partial precision. It generates an approximate answer in the destination z component (dest.z). It also allows for a more accurate determination of dest.x + function(dest.y), where the function is a user approximation to log₂(dest.y) for dest.y (1.0 <= dest.y < 2.0).

Reduced precision arithmetic is used to evaluate dest.z; however, the approximation error must be less than 1/(2¹¹) in absolute error (10-bit precision), and over the range (1.0 <= t.y < 2.0). A zero source argument generates the result vector (-infinity, 0.0, -infinity, 1.0).

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
logp r0,  r0.w		; take the logp 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 logp yields 
// component values of (0, 1, 0.5) which is the pale green shade 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.