qb64-include/grayscale.bm

$if grayscale = undefined then
$let grayscale = defined
Sub grayscale (image As Long)
    Dim mimg As _MEM
    Dim As _Offset start, length
    Dim As _Unsigned _Byte inputRed, inputGreen, inputBlue, nc
    mimg   = _MemImage(image)
    start  = mimg.OFFSET
    length = start + _Width(image) * _Height(image) * 4
    Do
        inputBlue  = _MemGet(mimg, start,     _Unsigned _Byte)
        inputGreen = _MemGet(mimg, start + 1, _Unsigned _Byte)
        inputRed   = _MemGet(mimg, start + 2, _Unsigned _Byte)
        nc = (inputRed * 0.21) + (inputGreen * 0.72) + (inputBlue * 0.07)
        _MemPut mimg, start,     nc As _Unsigned _Byte
        _MemPut mimg, start + 1, nc As _Unsigned _Byte
        _MemPut mimg, start + 2, nc As _Unsigned _Byte
        start = start + 4
    Loop Until start = length
    _MemFree mimg
End Sub
$end if