avfilter/vf_lensfun: add scale parameter

The lensfun filter wraps the lensfun library which performs
transformations on videos to correct for lens distortion. Often this
results in areas in the input being mapped to areas that fall outside
the boundaries of the output. The library has a parameter called scale
which is a scale factor applied to the output video. By decreasing it it
is possible to regain the areas of the video which would otherwise have
been lost. There is a special value of 0 which indicates that the
library should automatically determine a scale factor that results in
the output frame being filled (i.e. little or no black/unmapped areas).

This patch adds a corresponding scale option to the lensfun filter which
is passed through to the library. The existing behaviour of using the
automatic value of 0 is retained as the default behaviour, while other
values will be passed through to the library.
Signed-off-by: Daniel Playfair Cal <daniel.playfair.cal@gmail.com>

doc/filters.texi

@@ -11417,6 +11417,15 @@ focus distance is only used for vignetting and only slightly affects the
 vignetting correction process. If unknown, leave it at the default value (which
 is 1000).
 
+@item scale
+The scale factor which is applied after transformation. After correction the
+video is no longer necessarily rectangular. This parameter controls how much of
+the resulting image is visible. The value 0 means that a value will be chosen
+automatically such that there is little or no unmapped area in the output
+image. 1.0 means that no additional scaling is done. Lower values may result
+in more of the corrected image being visible, while higher values may avoid
+unmapped areas in the output.
+
 @item target_geometry
 The target geometry of the output image/video. The following values are valid
 options:

libavfilter/vf_lensfun.c

@@ -79,6 +79,7 @@ typedef struct LensfunContext {
     float focal_length;
     float aperture;
     float focus_distance;
+    float scale;
     int target_geometry;
     int reverse;
     int interpolation_type;
@@ -108,6 +109,7 @@ static const AVOption lensfun_options[] = {
     { "focal_length", "focal length of video (zoom; constant for the duration of the use of this filter)", OFFSET(focal_length), AV_OPT_TYPE_FLOAT, {.dbl=18}, 0.0, DBL_MAX, FLAGS },
     { "aperture", "aperture (constant for the duration of the use of this filter)", OFFSET(aperture), AV_OPT_TYPE_FLOAT, {.dbl=3.5}, 0.0, DBL_MAX, FLAGS },
     { "focus_distance", "focus distance (constant for the duration of the use of this filter)", OFFSET(focus_distance), AV_OPT_TYPE_FLOAT, {.dbl=1000.0f}, 0.0, DBL_MAX, FLAGS },
+    { "scale", "scale factor applied after corrections (0.0 means automatic scaling)", OFFSET(scale), AV_OPT_TYPE_FLOAT, {.dbl=0.0}, 0.0, DBL_MAX, FLAGS },
     { "target_geometry", "target geometry of the lens correction (only when geometry correction is enabled)", OFFSET(target_geometry), AV_OPT_TYPE_INT, {.i64=LF_RECTILINEAR}, 0, INT_MAX, FLAGS, "lens_geometry" },
         { "rectilinear", "rectilinear lens (default)", 0, AV_OPT_TYPE_CONST, {.i64=LF_RECTILINEAR}, 0, 0, FLAGS, "lens_geometry" },
         { "fisheye", "fisheye lens", 0, AV_OPT_TYPE_CONST, {.i64=LF_FISHEYE}, 0, 0, FLAGS, "lens_geometry" },
@@ -228,7 +230,7 @@ static int config_props(AVFilterLink *inlink)
                                    lensfun->focal_length,
                                    lensfun->aperture,
                                    lensfun->focus_distance,
-                                   0.0,
+                                   lensfun->scale,
                                    lensfun->target_geometry,
                                    lensfun_mode,
                                    lensfun->reverse);