BlendModeのVividLight | Codetter（こーどったー）

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    screen 1, 256, 256
    title "base color : bottom layer"
    repeat 256
        y = cnt
        repeat 256
            x = cnt
            i = x
            color i, i, i
            pset x, y
        loop
    loop
    screen 2, 256, 256
    title "blend color : top layer"
    repeat 256
        y = cnt
        repeat 256
            x = cnt
            i = 255 -y
            color i, i, i
            pset x, y
        loop
    loop
    screen 0, 256, 256
    title "final output"
    repeat 256
        y = cnt
        repeat 256
            x = cnt
            base_i = double(x) /255.0
            blend_i = double( 255 -y ) /255.0
        #if 0
                // まちがい
            if ( blend_i < 0.5 ) {
                final_i = base_i / ( 1.0 - 2.0*blend_i )
            } else {
                final_i = 1.0 - ( 1.0-base_i ) / ( 2.0 * ( blend_i - 0.5 ) )
            }
        #else
                // ただしくはこっち
                    // 0除算扱いになってしまうのでガードを適当に
            if ( blend_i < 0.5 ) {
                div = ( 2.0 * blend_i )
                if ( div > 0.0 ) {
                    final_i = 1.0 - ( 1.0-base_i ) / div
                } else {
                    final_i = 0.0
                }
            } else {
                div = ( 1.0 - 2.0*(blend_i-0.5) )
                if ( div > 0.0 ) {
                    final_i = base_i / div
                } else {
                    final_i = 1.0
                }
            }
        #endif
                // clamp
            final_i = limitf( final_i, 0.0 , 1.0 )
            i = int( 255.0 * final_i )
            color i, i, i
            pset x, y
        loop
    loop
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