enabled expressions on x, y, w, h and t parameters for drawgrid and drawbox, added examples

doc/filters.texi

@@ -2886,11 +2886,11 @@ This filter accepts the following options:
 @table @option
 @item x
 @item y
-Specify the top left corner coordinates of the box. Default to 0.
+The expressions which specify the top left corner coordinates of the box. Default to 0.
 
 @item width, w
 @item height, h
-Specify the width and height of the box, if 0 they are interpreted as
+The expressions which specify the width and height of the box, if 0 they are interpreted as
 the input width and height. Default to 0.
 
 @item color, c
@@ -2900,7 +2900,44 @@ value @code{invert} is used, the box edge color is the same as the
 video with inverted luma.
 
 @item thickness, t
-Set the thickness of the box edge. Default value is @code{3}.
+The expression which sets the thickness of the box edge. Default value is @code{3}.
+
+See below for the list of accepted constants.
+@end table
+
+The parameters for @var{x}, @var{y}, @var{w} and @var{h} and @var{t} are expressions containing the
+following constants:
+
+@table @option
+@item dar
+The input display aspect ratio, it is the same as (@var{w} / @var{h}) * @var{sar}.
+
+@item hsub
+@item vsub
+horizontal and vertical chroma subsample values. For example for the
+pixel format "yuv422p" @var{hsub} is 2 and @var{vsub} is 1.
+
+@item in_h, ih
+@item in_w, iw
+The input width and height.
+
+@item sar
+The input sample aspect ratio.
+
+@item x
+@item y
+The x and y offset coordinates where the box is drawn.
+
+@item w
+@item h
+The width and height of the drawn box.
+
+@item t
+The thickness of the drawn box.
+
+These constants allow the @var{x}, @var{y}, @var{w}, @var{h} and @var{t} expressions to refer to 
+each other, so you may for example specify @code(y=x/dar} or @code(h=w/dar).
+
 @end table
 
 @subsection Examples
@@ -2928,6 +2965,12 @@ Fill the box with pink color:
 @example
 drawbox=x=10:y=10:w=100:h=100:color=pink@@0.5:t=max
 @end example
+
+@item
+Draw a 2-pixel red 2.40:1 mask:
+@example
+drawbox=x=-t:y=0.5*(ih-iw/2.4)-t:w=iw+t*2:h=iw/2.4+t*2:t=2:c=red
+@end example
 @end itemize
 
 @section drawgrid
@@ -2939,11 +2982,11 @@ This filter accepts the following options:
 @table @option
 @item x
 @item y
-Specify the coordinates of some point of grid intersection (meant to configure offset). Both default to 0.
+The expressions which specify the coordinates of some point of grid intersection (meant to configure offset). Both default to 0.
 
 @item width, w
 @item height, h
-Specify the width and height of the grid cell, if 0 they are interpreted as the
+The expressions which specify the width and height of the grid cell, if 0 they are interpreted as the
 input width and height, respectively, minus @code{thickness}, so image gets
 framed. Default to 0.
 
@@ -2956,7 +2999,44 @@ Note that you can append opacity value (in range of 0.0 - 1.0)
 to color name after @@ sign.
 
 @item thickness, t
-Set the thickness of the grid line. Default value is @code{1}.
+The expression which sets the thickness of the grid line. Default value is @code{1}.
+
+See below for the list of accepted constants.
+@end table
+
+The parameters for @var{x}, @var{y}, @var{w} and @var{h} and @var{t} are expressions containing the
+following constants:
+
+@table @option
+@item dar
+The input display aspect ratio, it is the same as (@var{w} / @var{h}) * @var{sar}.
+
+@item hsub
+@item vsub
+horizontal and vertical chroma subsample values. For example for the
+pixel format "yuv422p" @var{hsub} is 2 and @var{vsub} is 1.
+
+@item in_h, ih
+@item in_w, iw
+The input grid cell width and height.
+
+@item sar
+The input sample aspect ratio.
+
+@item x
+@item y
+The x and y coordinates of some point of grid intersection (meant to configure offset).
+
+@item w
+@item h
+The width and height of the drawn cell.
+
+@item t
+The thickness of the drawn cell.
+
+These constants allow the @var{x}, @var{y}, @var{w}, @var{h} and @var{t} expressions to refer to 
+each other, so you may for example specify @code(y=x/dar} or @code(h=w/dar).
+
 @end table
 
 @subsection Examples
@@ -2967,6 +3047,12 @@ Draw a grid with cell 100x100 pixels, thickness 2 pixels, with color red and an
 @example
 drawgrid=width=100:height=100:thickness=2:color=red@@0.5
 @end example
+
+@item
+Draw a white 3x3 grid with an opacity of 50%:
+@example
+drawgrid=w=iw/3:h=ih/3:t=2:c=white@@0.5
+@end example
 @end itemize
 
 @anchor{drawtext}