log---vs.md

title	description	ms.assetid	ms.topic	ms.date	topic_type	api_name	api_type	api_location
log - vs	Full precision log₂(x). | log - vs	53c91825-ec54-4b04-bf08-52d4b1c5122d	reference	05/31/2018	apiref

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.

Related topics

Vertex Shader Instructions