//on SET_TEXTURE link message, set texture on face 0 to received UUID
//on FADE_IN link message, gradually change from 0 alpha on face 0 to 1 alpha
//on FADE_OUT link message, gradually change from 1 alpha on face 0 to 0 alpha

float alpha = 1.0;
integer SET_TEXTURE = 99;
integer FADE_IN = 101;
integer FADE_OUT = 100;
float increment = 0.005;

FadeIn()
{
    while(alpha < 1.0)
    {
        alpha += increment;
        if (alpha > 1.0)
        {
            alpha = 1.0;
        }
        llSetAlpha(alpha, 0);
    }
}

FadeOut()
{
    while(alpha > 0)
    {
        alpha -= increment;
        if (alpha < 0.0)
        {
            alpha = 0.0;
        }
        llSetAlpha(alpha, 0);
    }    
}

default
{    
    link_message(integer sender, integer num, string str, key id)
    {
        if (num == SET_TEXTURE)
        {
            llSetTexture(id, 0);
        }
        else if (num == FADE_IN)
        {
            FadeIn();
        }
        else if (num == FADE_OUT)
        {
            FadeOut();
        }
    }
}