Re: 3D Lut in software

On Feb 22, 2015, at 3:35 PM, Gonzalo Garramuno <[email protected]> wrote:

> My application uses a 3D OpenGL Texture in log space for its table lookups, with code borrowed from playexr.  However, now I need to do the 3d lut lookup in software so I can display the color picker correctly showing the transformed colors.
> I ask here for some help in seeing what I am doing wrong.  Sorry for the long code.

...

> And here's my software evaluation routine which tries to mimic the shader code:
> 
> void GLLut3d::evaluate( const Imath::V3f& rgb, Imath::V3f& out ) const
> {
>    using namespace Imath;
> 
>    V3f pMin( lutMin, lutMin, lutMin );
>    V3f pMax( lutMax, lutMax, lutMax );
>    V3i size( _lutN, _lutN, _lutN );
> 
>    out.x = lutT + lutM * log( Imath::clamp( rgb.x, lutMin, lutMax ) );
>    out.y = lutT + lutM * log( Imath::clamp( rgb.y, lutMin, lutMax ) );
>    out.z = lutT + lutM * log( Imath::clamp( rgb.z, lutMin, lutMax ) );
> 
>    out = Ctl::lookup3D( (V3f*)&lut[0], size, pMin, pMax, out );
>    out.x = exp( out.x );
>    out.y = exp( out.y );
>    out.z = exp( out.z );
> }
> 
> 
> The problem seems to be the Ctl::lookup3D function, which returns all negative numbers no matter what.

They should be negative numbers:

    0.1 == exp(-2.30259)
    0.2 == exp(-1.60944)
    0.3 == exp(-1.20397)
    0.4 == exp(-0.91629)
    0.5 == exp(-0.69315)
    0.6 == exp(-0.51083)
    0.7 == exp(-0.35667)
    0.8 == exp(-0.22314)
    0.9 == exp(-0.10536)
    1.0 == exp( 0.00000)

Unless you expect out values greater than 1.0, the values feeding into exp() will all be negative.

- Jim