lavfi: add framestep filter

This filter is inspired upon libmpcodecs/vf_framestep.c, by Daniele
Forghieri.

Only-keyframe output is not supported, since that feature can be achieved
through the more versatile select filter.

Changelog

@@ -48,6 +48,7 @@ version next:
 - ICO muxer
 - SubRip encoder and decoder without embedded timing
 - edge detection filter
+- framestep filter
 
 
 version 0.11:

doc/filters.texi

@@ -2084,6 +2084,13 @@ Desired output framerate.
 
 @end table
 
+@section framestep
+
+Select one frame every N.
+
+This filter accepts in input a string representing a positive
+integer. Default argument is @code{1}.
+
 @anchor{frei0r}
 @section frei0r
 

libavfilter/Makefile

@@ -95,6 +95,7 @@ OBJS-$(CONFIG_FADE_FILTER)                   += vf_fade.o
 OBJS-$(CONFIG_FIELDORDER_FILTER)             += vf_fieldorder.o
 OBJS-$(CONFIG_FIFO_FILTER)                   += fifo.o
 OBJS-$(CONFIG_FORMAT_FILTER)                 += vf_format.o
+OBJS-$(CONFIG_FRAMESTEP_FILTER)              += vf_framestep.o
 OBJS-$(CONFIG_FPS_FILTER)                    += vf_fps.o
 OBJS-$(CONFIG_FREI0R_FILTER)                 += vf_frei0r.o
 OBJS-$(CONFIG_GRADFUN_FILTER)                += vf_gradfun.o

libavfilter/allfilters.c

@@ -87,6 +87,7 @@ void avfilter_register_all(void)
     REGISTER_FILTER (FIFO,        fifo,        vf);
     REGISTER_FILTER (FORMAT,      format,      vf);
     REGISTER_FILTER (FPS,         fps,         vf);
+    REGISTER_FILTER (FRAMESTEP,   framestep,   vf);
     REGISTER_FILTER (FREI0R,      frei0r,      vf);
     REGISTER_FILTER (GRADFUN,     gradfun,     vf);
     REGISTER_FILTER (HFLIP,       hflip,       vf);

libavfilter/version.h

@@ -29,8 +29,8 @@
 #include "libavutil/avutil.h"
 
 #define LIBAVFILTER_VERSION_MAJOR  3
-#define LIBAVFILTER_VERSION_MINOR  10
+#define LIBAVFILTER_VERSION_MINOR  11
-#define LIBAVFILTER_VERSION_MICRO 101
+#define LIBAVFILTER_VERSION_MICRO 100
 
 #define LIBAVFILTER_VERSION_INT AV_VERSION_INT(LIBAVFILTER_VERSION_MAJOR, \
                                                LIBAVFILTER_VERSION_MINOR, \

libavfilter/vf_framestep.c

@@ -0,0 +1,142 @@
+/*
+ * Copyright (c) 2012 Stefano Sabatini
+ *
+ * This file is part of FFmpeg.
+ *
+ * FFmpeg is free software; you can redistribute it and/or
+ * modify it under the terms of the GNU Lesser General Public
+ * License as published by the Free Software Foundation; either
+ * version 2.1 of the License, or (at your option) any later version.
+ *
+ * FFmpeg is distributed in the hope that it will be useful,
+ * but WITHOUT ANY WARRANTY; without even the implied warranty of
+ * MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the GNU
+ * Lesser General Public License for more details.
+ *
+ * You should have received a copy of the GNU Lesser General Public
+ * License along with FFmpeg; if not, write to the Free Software
+ * Foundation, Inc., 51 Franklin Street, Fifth Floor, Boston, MA 02110-1301 USA
+ */
+
+/**
+ * @file framestep filter, inspired on libmpcodecs/vf_framestep.c by
+ * Daniele Fornighieri <guru AT digitalfantasy it>.
+ */
+
+#include "avfilter.h"
+#include "internal.h"
+#include "video.h"
+
+typedef struct {
+    int frame_step, frame_count, frame_selected;
+} FrameStepContext;
+
+static av_cold int init(AVFilterContext *ctx, const char *args)
+{
+    FrameStepContext *framestep = ctx->priv;
+    char *tailptr;
+    long int n;
+
+    framestep->frame_step = 1;
+
+    if (args) {
+        n = strtol(args, &tailptr, 10);
+        if (*tailptr || n <= 0 || n >= INT_MAX) {
+            av_log(ctx, AV_LOG_ERROR,
+                   "Invalid argument '%s', must be a positive integer <= INT_MAX\n", args);
+            return AVERROR(EINVAL);
+        }
+    }
+
+    framestep->frame_step = n;
+    return 0;
+}
+
+static int config_output_props(AVFilterLink *outlink)
+{
+    AVFilterContext *ctx = outlink->src;
+    FrameStepContext *framestep = ctx->priv;
+    AVFilterLink *inlink = ctx->inputs[0];
+
+    outlink->frame_rate =
+        av_div_q(inlink->frame_rate, (AVRational){framestep->frame_step, 1});
+
+    av_log(ctx, AV_LOG_VERBOSE, "step:%d frame_rate:%d/%d(%f) -> frame_rate:%d/%d(%f)\n",
+           framestep->frame_step,
+           inlink->frame_rate.num, inlink->frame_rate.den, av_q2d(inlink->frame_rate),
+           outlink->frame_rate.num, outlink->frame_rate.den, av_q2d(outlink->frame_rate));
+    return 0;
+}
+
+static int start_frame(AVFilterLink *inlink, AVFilterBufferRef *ref)
+{
+    FrameStepContext *framestep = inlink->dst->priv;
+
+    framestep->frame_selected = 0;
+    if (!(framestep->frame_count++ % framestep->frame_step)) {
+        inlink->cur_buf = NULL;
+        framestep->frame_selected = 1;
+        return ff_start_frame(inlink->dst->outputs[0], ref);
+    }
+    return 0;
+}
+
+static int draw_slice(AVFilterLink *inlink, int y, int h, int slice_dir)
+{
+    FrameStepContext *framestep = inlink->dst->priv;
+
+    if (framestep->frame_selected)
+        return ff_draw_slice(inlink->dst->outputs[0], y, h, slice_dir);
+    return 0;
+}
+
+static int end_frame(AVFilterLink *inlink)
+{
+    FrameStepContext *framestep = inlink->dst->priv;
+
+    if (framestep->frame_selected)
+        return ff_end_frame(inlink->dst->outputs[0]);
+    return 0;
+}
+
+static int request_frame(AVFilterLink *outlink)
+{
+    FrameStepContext *framestep = outlink->src->priv;
+    AVFilterLink *inlink = outlink->src->inputs[0];
+    int ret;
+
+    framestep->frame_selected = 0;
+    do {
+        ret = ff_request_frame(inlink);
+    } while (!framestep->frame_selected && ret >= 0);
+
+    return ret;
+}
+
+AVFilter avfilter_vf_framestep = {
+    .name      = "framestep",
+    .description = NULL_IF_CONFIG_SMALL("Select one frame every N frames."),
+    .init      = init,
+    .priv_size = sizeof(FrameStepContext),
+
+    .inputs = (const AVFilterPad[]) {
+        {
+            .name             = "default",
+            .type             = AVMEDIA_TYPE_VIDEO,
+            .get_video_buffer = ff_null_get_video_buffer,
+            .start_frame      = start_frame,
+            .draw_slice       = draw_slice,
+            .end_frame        = end_frame,
+        },
+        { .name = NULL }
+    },
+    .outputs = (const AVFilterPad[]) {
+        {
+            .name             = "default",
+            .type             = AVMEDIA_TYPE_VIDEO,
+            .config_props     = config_output_props,
+            .request_frame    = request_frame,
+        },
+        { .name = NULL }
+    },
+};