adds documentation for -force and -noalpha parameters

issue ampas#27

Author: scottdyer

ctlrender/usage.cc

@@ -83,6 +83,10 @@ void usage(const char *section) {
 "                          option must be used.\n"
 "                          See below for details on the '-format' option.\n"
 "\n"
+"                          If the destination file already exists, ctlrender \n"
+"                          will refuse to overwrite the file unless the\n"
+"                          -force parameter is supplied.\n"
+"\n"
 "    -input_scale <value>  Specifies a scaling value for the input.\n"
 "                          Details on this are provided with '-help scale'.\n"
 "\n"
@@ -108,6 +112,12 @@ void usage(const char *section) {
 "    -param2 ...           Details on this and similar options are provided\n"
 "    -param3 ...           with '-help param'\n"
 "\n"
+"    -noalpha              Specifies that <source_file> does not contain an \n"
+"                          alpha channel.\n"
+"\n"
+"    -force                Allows overwriting of the destination file, if\n"
+"                          the destination file already exists.\n"
+"\n"
 "    -verbose              Increases the level of output verbosity.\n"
 "    -quiet                Decreases the level of output verbosity.\n"
 "\n"