avfilter/vf_libplacebo: add flexible crop exprs

Motivated by a desire to use vf_libplacebo as a GPU-accelerated
cropping/padding/zooming filter. This commit adds support for setting
the `input/target.crop` fields as dynamic expressions.

Re-use the same generic variables available to other scale and crop type
filters, and also add some more that we can afford as a result of being
able to set these properties dynamically.

It's worth pointing out that `out_t/ot` is currently redundant with
`in_t/t` since it will always contain the same PTS values, but I plan on
changing this in the near future.

I decided to also expose `crop_w/crop_h` and `pos_w/pos_h` as variables
in the expression parser itself, since this enables the fairly common
use case of determining dimensions first and then placing the image
appropriately, such as is done in the default behavior (which centers
the cropped/placed region by default).

1 files changed, 50 insertions, 5 deletions

diff --git a/doc/filters.texi b/doc/filters.texi
index 34212b513d..839a1c6a9f 100644
--- a/doc/filters.texi
+++ b/doc/filters.texi
@@ -15997,10 +15997,31 @@ in source frames.
 @table @option
 @item w
 @item h
-Set the output video dimension expression. Default value is the input dimension.
+Set the output video dimension expression. Default values are @code{iw} and
+@code{ih}.
 
 Allows for the same expressions as the @ref{scale} filter.
 
+@item crop_x
+@item crop_y
+Set the input crop x/y expressions, default values are @code{(iw-cw)/2} and
+@code{(ih-ch)/2}.
+
+@item crop_w
+@item crop_h
+Set the input crop width/height expressions, default values are @code{iw} and
+@code{ih}.
+
+@item pos_x
+@item pos_y
+Set the output placement x/y expressions, default values are @code{(ow-pw)/2}
+and @code{(oh-ph)/2}.
+
+@item pos_w
+@item pos_h
+Set the output placement width/height expressions, default values are @code{ow}
+and @code{oh}.
+
 @item format
 Set the output format override. If unset (the default), frames will be output
 in the same format as the respective input frames. Otherwise, format conversion
@@ -16012,9 +16033,9 @@ Work the same as the identical @ref{scale} filter options.
 
 @item normalize_sar
 If enabled, output frames will always have a pixel aspect ratio of 1:1. This
-will introduce padding/cropping as necessary. If disabled (the default), any
-aspect ratio mismatches, including those from e.g. anamorphic video sources,
-are forwarded to the output pixel aspect ratio.
+will introduce additional padding/cropping as necessary. If disabled (the
+default), any aspect ratio mismatches, including those from e.g. anamorphic
+video sources, are forwarded to the output pixel aspect ratio.
 
 @item pad_crop_ratio
 Specifies a ratio (between @code{0.0} and @code{1.0}) between padding and
@@ -16026,7 +16047,7 @@ approaches.
 
 @item fillcolor
 Set the color used to fill the output area not covered by the output image, for
-example as a result of @ref{normalize_sar}. For the general syntax of this
+example as a result of @option{normalize_sar}. For the general syntax of this
 option, check the @ref{color syntax,,"Color" section in the ffmpeg-utils
 manual,ffmpeg-utils}. Defaults to @code{black}.
 
@@ -16051,6 +16072,30 @@ BT.2020+PQ, overriding the usual input frame metadata. These will also be
 picked as the values of @code{auto} for the respective frame output options.
 @end table
 
+In addition to the expression constants documented for the @ref{scale} filter,
+the @option{crop_w}, @option{crop_h}, @option{crop_x}, @option{crop_y},
+@option{pos_w}, @option{pos_h}, @option{pos_x} and @option{pos_y} options can
+also contain the following constants:
+
+@table @option
+@item crop_w, cw
+@item crop_h, ch
+The computed values of @option{crop_w} and @option{crop_h}.
+
+@item pos_w, pw
+@item pos_h, ph
+The computed values of @option{pos_w} and @option{pos_h}.
+
+@item in_t, t
+The input frame timestamp, in seconds. NAN if input timestamp is unknown.
+
+@item out_t, ot
+The input frame timestamp, in seconds. NAN if input timestamp is unknown.
+
+@item n
+The input frame number, starting with 0.
+@end table
+
 @subsubsection Scaling
 The options in this section control how libplacebo performs upscaling and (if
 necessary) downscaling. Note that libplacebo will always internally operate on