optional automatically upstretching of the video-output-resolution to fit aspect-ratio of desktop-resolution depending on a maximum allowed stretching of individual choice.

The common desktop-resolution-aspept-ratios are: 4:3, 5:4, 16:9 and 16:10.
A broadcasting-signal can have any aspect-ratio from 1:1 to 221:100.

Strength-divider tuning/tryouts:

bc-ar: dr-ar: stretch: divider for 15% / 20% / 25% / 30% / 35% / 40% / 45% (full)
1:1 -> 16:9 = X:43,75% = 3 2,2 1,8 1,5 1,3 1,1 1
1:1 -> 16:10 = X:37,5% = 2,5 1,9 1,5 1,3 1,1 1 1
5:4 -> 16:9 = X:29,69% = 2 1,5 1,2 1 1 1 1
1:1 -> 4:3 = X:25% = 1,7 1,3 1 1 1 1 1
4:3 -> 16:9 = X:25% = 1,7 1,3 1 1 1 1 1
5:4 -> 16:10 = X:21,875% = 1,5 1,1 1 1 1 1 1
1:1 -> 5:4 = X:20% = 1,4 1 1 1 1 1 1
4:3 -> 16:10 = X:16,66% = 1,1 1 1 1 1 1 1
16:10 -> 4:3 = Y:16,66% = 1,1 1 1 1 1 1 1
221:100 -> 16:9 = Y:19,56% = 1,4 1 1 1 1 1 1
16:10 -> 5:4 = Y:21,875% = 1,5 1,1 1 1 1 1 1
16:9 -> 4:3 = Y:25% = 1,7 1,3 1 1 1 1 1
221:100 -> 16:10 = Y:27,6% = 1,9 1,4 1 1 1 1 1
16:9 -> 5:4 = Y:29,69% = 2 1,5 1,2 1 1 1 1
221:100 -> 4:3 = Y:39,67% = 2,7 2 1,6 1,4 1,2 1 1
221:100 -> 5:4 = Y:43,44% = 2,9 2,2 1,8 1,5 1,3 1,1 1

You can easily see that even full-stretching-mode in any case will do no more stretching than 43,75% and that such high percentaged are not the regular case. Anyway, if you find stretching to distractive at those rare aspect-ratio-traversals, than you can change settings to a lower maximum-allowed-stretching, so it's not or minimal affecting the stretching of the regular-screen-aspect-ratios. It gets you almost absolut highest efficiency on using your full screen for watching movies.

Since it is not to much complicated I try some coding.

Note: this is PHP-Notation, it may not work exactly that way in C++

$_GET[something] means calling information either from main function or from settings-file.

##begin

//At first set output resolution X,Y to broadcast-resolution X,Y for safe:
$or-x = $bc-sr-x;
$or-y = $bc-sr-y;

//Function for determining the aspect-ratios of resolutions through the greatest common dividers:
ar ( $ar-a , ar-b ) {
    if ( $ar-b > 0) {
       $ar-a / ($ar-a / $ar-b) .':'.$ar-b / ( $ar-a / $ar-b );
    }
}

//Broadcast-signal-resolution-X,Y and aspect-ratio:
$bc-sr-x = $_GET[$bc-signal-resolution-x_width];
$bc-sr-y = $_GET[$bc-signal-resolution-y_heigth];
$bc-sr-ar = ar ( $bc-sr-x , $bc-sr-y );

//Desktop-resolution-X,Y and aspect-ratio, too:
$dr-x = $_GET[$desktop-resolution-x_width];
$dr-y = $_GET[$desktop-resolution-y_heigth];
$dr-ar = ar ( $dr-x , $dr-y );

//An array thats stores information about aspect-meta-modes:
$aspect-meta-mode-array = array (
          array ( '1:1' , '16:9' ) ,
          array ( '1:1' , '16:10' ) ,
          array ( '5:4' , '16:9' ) ,
          array ( '1:1' , '4:3' ) ,
          array ( '4:3' , '16:9' ) ,
          array ( '5:4' , '16:10' ) ,
          array ( '1:1' , '5:4' ) ,
          array ( '4:3' , '16:10' ) ,
          array ( '16:10' , '4:3' ) ,
          array ( '221:100' , '16:9' ),
          array ( '16:10' , '5:4' ),
          array ( '16:9' , '4:3' ),
          array ( '221:100' , '16:10' ),
          array ( '16:9' , '5:4' ),
          array ( '221:100' , '4:3' ),
          array ( '221:100' , '5:4' ));

//Strength-mode setting in percentage and divider:
$sm_p = $_GET[$strength-mode_setting];
$sm_d = $sm_d_array [ $aspect-meta-mode-array [ $bc-sr-ar ] [ $dr-ar ] ] [ $sm_p ];

//An array that stores information about our dividers:
$sm_d-array = array (
                      array ('15'=>'3','20'=>'2,2','25'=>'1,8','30'=>'1,5','35'=>'1,3','40'=>'1,1','45'=>'1'),
                      array ('15'=>'2,5','20'=>'1,9','25'=>'1,5','30'=>'1,3','35'=>'1,1','40'=>'1','45'=>'1'),
                      array ('15'=>'2','20'=>'1,5','25'=>'1,2','30'=>'1','35'=>'1','40'=>'1','45'=>'1'),
                      array ('15'=>'1,7','20'=>'1,3','25'=>'1','30'=>'1','35'=>'1','40'=>'1','45'=>'1'),
                      array ('15'=>'1,7','20'=>'1,3','25'=>'1','30'=>'1','35'=>'1','40'=>'1','45'=>'1'),
                      array ('15'=>'1,5','20'=>'1,1','25'=>'1','30'=>'1','35'=>'1','40'=>'1','45'=>'1'),
                      array ('15'=>'1,4','20'=>'1','25'=>'1','30'=>'1','35'=>'1','40'=>'1','45'=>'1'),
                      array ('15'=>'1,1','20'=>'1','25'=>'1','30'=>'1','35'=>'1','40'=>'1','45'=>'1'),
                      array ('15'=>'1,1','20'=>'1','25'=>'1','30'=>'1','35'=>'1','40'=>'1','45'=>'1'),
                      array ('15'=>'1,4','20'=>'1','25'=>'1','30'=>'1','35'=>'1','40'=>'1','45'=>'1'),
                      array ('15'=>'1,5','20'=>'1,1','25'=>'1','30'=>'1','35'=>'1','40'=>'1','45'=>'1'),
                      array ('15'=>'1,7','20'=>'1,3','25'=>'1','30'=>'1','35'=>'1','40'=>'1','45'=>'1'),
                      array ('15'=>'1,9','20'=>'1,4','25'=>'1','30'=>'1','35'=>'1','40'=>'1','45'=>'1'),
                      array ('15'=>'2','20'=>'1,5','25'=>'1,2','30'=>'1','35'=>'1','40'=>'1','45'=>'1'),
                      array ('15'=>'2,7','20'=>'2','25'=>'1,6','30'=>'1,4','35'=>'1,2','40'=>'1','45'=>'1'),
                      array ('15'=>'2,9','20'=>'2,2','25'=>'1,8','30'=>'1,5','35'=>'1,3','40'=>'1,1','45'=>'1'));

//Another variables for calculating fully stretched video-output-resolutions:
$fsor-x= $bc-sr-y / $dr-y * dr-x;
$fsor-y= $bc-sr-x / $dr-x * dr-y;

//Function that checks either X or Y need stretching and finally stretches our output:
stretch ($sm_d)
if ( ( $bc-sr-x / $bc-sr-y ) < ( $dr-x / $dr-y ) ) {
   if ( $sm_d = 1 ) {
      $or-x = $fsor-x;
      $or-y = $bc-sr-y;
   } elseif ( $sm_d > 1 ) {
      $or-x = ( ( $fsor-x - ( ( $fsor-x - $bc-sr-x ) / $sm_d ) ;
      $or-y = $bc-sr-y;
   }
} elseif ( ( $bc-sr-x / $bc-sr-y ) > ( $dr-x / $dr-y ) ) {
   if ( $sm-d = 1 ) {
      $or-y = $fsor-y ;
      $or-x = $bc-sr-x;
   } elseif ( $sm_d > 1 ) {
      $or-y = ( ( $fsor-y - ( ( $fsor-y - $bc-sr-y ) / $sm_d ) ;
      $or-x = $bc-sr-x;
   }
}

Thats it at all. Im pretty tired now. Please accept and push this. Feel free to make C++ and implement it for me-tv, i would like to beta-test my own feature :)

Michael: You should not put important content here. Anyone can delete this.

You are still lacking the detail of what needs to be done. Developer needed.