llSetLinkAlpha(integer link, float alpha, integer face)

An integer value defining the link number of the prim to be read.

The following special link specification constants can be used:-

A float value between 0.0 and 1.0 being the degree of opacity. 1.0 being fully opaque.

An integer value defining the face of the prim to be affected. ALL_SIDES affects all faces.

• Note that the viewer shows transparency, not opacity, and it shows it in percentage. To convert from viewer transparency to an alpha value suitable for use with this function, use this formula: alpha=(100-transparency)/100. For example, a transparency of 35% is an alpha value of 0.65.
• The function silently fails if 'face' indicates a face that does not exist, or if 'link' indicates a non-existent prim.

llSetLinkAlpha(3, 0.5, 6); // Will set face 6 of link 3 to 50% transparent
llSetLinkAlpha(2, 0.0, ALL_SIDES);  // Will make link 3 or this object disappear

llSetLinkAlpha-example.lsl

// This will cause all other prims of the current object to blink on and
// off continuously (appear and disappear)
integer  flip;
default
{
    state_entry()
    {
        llSetTimerEvent(0.5);
    }
    timer()
    {
        flip = !flip;
        llSetLinkAlpha(LINK_ALL_OTHERS, flip, ALL_SIDES);
    }
}

• llSetAlpha to affect only the current prim.
• llSetColor to set the color of a face in the current prim, or llSetLinkColor to set it for any chosen prim.
• The PrimitiveParams family of functions, with PRIM_COLOR, allows changing color and alpha at the same time.