//  Convert RGB image to grayscale (8 bit/pixel)
//    #10 = buffer that contains image data
//  On return:
//    #20 = buffer for the new grayscale image

:RGB_TO_GRAYSCALE:
File_Open("|(VEDIT_TEMP)\gray.data", OVERWRITE+NOEVENT+NOMSG)
#20 = Buf_Num
BOF
Del_Char(ALL)
Buf_Switch(#10)
Repeat(File_Size/3) {
    #9 =  Cur_Char() * 2126
    #9 += Cur_Char(1) * 7152
    #9 += Cur_Char(2) * 722
    Char(3)
    Buf_Switch(#20)
    Ins_Char(#9 / 10000)
    Buf_Switch(#10)
}
Return