log - vs

Full precision log₂(x).

Syntax

log dst, src

where

dst is the destination register.
src is a source register. Source register requires explicit use of replicate swizzle, that is, exactly one of the .x, .y, .z, .w swizzle components (or the .r, .g, .b, .a equivalents) must be specified.

Remarks

Vertex shader versions	1_1	2_0	2_x	2_sw	3_0	3_sw
log	x	x	x	x	x	x

The following code fragment shows the operations performed.

float v = abs(src);
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 = -FLT_MAX;
}

This instruction accepts a scalar source whose sign bit is ignored. The result is replicated to all four channels.

This instruction provides 21 bits of precision.