lavfi/palettegen: Allow setting the background colour.

doc/filters.texi

@@ -11458,6 +11458,9 @@ If not set, the maximum of colors in the palette will be 256. You probably want
 to disable this option for a standalone image.
 Set by default.
 
+@item transparency_color
+Set the color that will be used as background for transparency.
+
 @item stats_mode
 Set statistics mode.
 

libavfilter/version.h

@@ -31,7 +31,7 @@
 
 #define LIBAVFILTER_VERSION_MAJOR   7
 #define LIBAVFILTER_VERSION_MINOR   0
-#define LIBAVFILTER_VERSION_MICRO 100
+#define LIBAVFILTER_VERSION_MICRO 101
 
 #define LIBAVFILTER_VERSION_INT AV_VERSION_INT(LIBAVFILTER_VERSION_MAJOR, \
                                                LIBAVFILTER_VERSION_MINOR, \

libavfilter/vf_palettegen.c

@@ -27,6 +27,7 @@
 #include "libavutil/internal.h"
 #include "libavutil/opt.h"
 #include "libavutil/qsort.h"
+#include "libavutil/intreadwrite.h"
 #include "avfilter.h"
 #include "internal.h"
 
@@ -74,6 +75,7 @@ typedef struct PaletteGenContext {
     struct range_box boxes[256];            // define the segmentation of the colorspace (the final palette)
     int nb_boxes;                           // number of boxes (increase will segmenting them)
     int palette_pushed;                     // if the palette frame is pushed into the outlink or not
+    uint8_t[4] transparency_color;          // background color for transparency
 } PaletteGenContext;
 
 #define OFFSET(x) offsetof(PaletteGenContext, x)
@@ -81,6 +83,7 @@ typedef struct PaletteGenContext {
 static const AVOption palettegen_options[] = {
     { "max_colors", "set the maximum number of colors to use in the palette", OFFSET(max_colors), AV_OPT_TYPE_INT, {.i64=256}, 4, 256, FLAGS },
     { "reserve_transparent", "reserve a palette entry for transparency", OFFSET(reserve_transparent), AV_OPT_TYPE_BOOL, {.i64=1}, 0, 1, FLAGS },
+    { "transparency_color", "set a background color for transparency", OFFSET(transparency_color), AV_OPT_TYPE_COLOR, {.str="lime"}, CHAR_MIN, CHAR_MAX, FLAGS },
     { "stats_mode", "set statistics mode", OFFSET(stats_mode), AV_OPT_TYPE_INT, {.i64=STATS_MODE_ALL_FRAMES}, 0, NB_STATS_MODE-1, FLAGS, "mode" },
         { "full", "compute full frame histograms", 0, AV_OPT_TYPE_CONST, {.i64=STATS_MODE_ALL_FRAMES}, INT_MIN, INT_MAX, FLAGS, "mode" },
         { "diff", "compute histograms only for the part that differs from previous frame", 0, AV_OPT_TYPE_CONST, {.i64=STATS_MODE_DIFF_FRAMES}, INT_MIN, INT_MAX, FLAGS, "mode" },
@@ -250,7 +253,7 @@ static void write_palette(AVFilterContext *ctx, AVFrame *out)
 
     if (s->reserve_transparent) {
         av_assert0(s->nb_boxes < 256);
-        pal[out->width - pal_linesize - 1] = 0x0000ff00; // add a green transparent color
+        pal[out->width - pal_linesize - 1] = AV_RB32(&s->transparency_color) >> 8;
     }
 }