lavfi: add thumbnail video filter.

Changelog

@@ -144,6 +144,7 @@ easier to use. The changes are:
 - Dxtory capture format decoder
 - cellauto source
 - Simple segmenting muxer
+- thumbnail video filter
 
 
 version 0.8:

doc/filters.texi

@@ -2400,6 +2400,26 @@ For example:
 will create two separate outputs from the same input, one cropped and
 one padded.
 
+@section thumbnail
+Select the most representative frame in a given sequence of consecutive frames.
+
+It accepts as argument the frames batch size to analyze (default @var{N}=100);
+in a set of @var{N} frames, the filter will pick one of them, and then handle
+the next batch of @var{N} frames until the end.
+
+Since the filter keeps track of the whole frames sequence, a bigger @var{N}
+value will result in a higher memory usage, so a high value is not recommended.
+
+The following example extract one picture each 50 frames:
+@example
+thumbnail=50
+@end example
+
+Complete example of a thumbnail creation with @command{ffmpeg}:
+@example
+ffmpeg -i in.avi -vf thumbnail,scale=300:200 -frames:v 1 out.png
+@end example
+
 @section transpose
 
 Transpose rows with columns in the input video and optionally flip it.

libavfilter/Makefile

@@ -79,6 +79,7 @@ OBJS-$(CONFIG_SETTB_FILTER)                  += vf_settb.o
 OBJS-$(CONFIG_SHOWINFO_FILTER)               += vf_showinfo.o
 OBJS-$(CONFIG_SLICIFY_FILTER)                += vf_slicify.o
 OBJS-$(CONFIG_SPLIT_FILTER)                  += vf_split.o
+OBJS-$(CONFIG_THUMBNAIL_FILTER)              += vf_thumbnail.o
 OBJS-$(CONFIG_TRANSPOSE_FILTER)              += vf_transpose.o
 OBJS-$(CONFIG_UNSHARP_FILTER)                += vf_unsharp.o
 OBJS-$(CONFIG_VFLIP_FILTER)                  += vf_vflip.o

libavfilter/allfilters.c

@@ -89,6 +89,7 @@ void avfilter_register_all(void)
     REGISTER_FILTER (SHOWINFO,    showinfo,    vf);
     REGISTER_FILTER (SLICIFY,     slicify,     vf);
     REGISTER_FILTER (SPLIT,       split,       vf);
+    REGISTER_FILTER (THUMBNAIL,   thumbnail,   vf);
     REGISTER_FILTER (TRANSPOSE,   transpose,   vf);
     REGISTER_FILTER (UNSHARP,     unsharp,     vf);
     REGISTER_FILTER (VFLIP,       vflip,       vf);

libavfilter/vf_thumbnail.c

+/*
+ * Copyright (c) 2011 Smartjog S.A.S, Clément Bœsch <clement.boesch@smartjog.com>
+ *
+ * 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
+ * Potential thumbnail lookup filter to reduce the risk of an inappropriate
+ * selection (such as a black frame) we could get with an absolute seek.
+ *
+ * Simplified version of algorithm by Vadim Zaliva <lord@crocodile.org>.
+ * @url http://notbrainsurgery.livejournal.com/29773.html
+ */
+
+#include "avfilter.h"
+
+#define HIST_SIZE (3*256)
+
+struct thumb_frame {
+    AVFilterBufferRef *buf;     ///< cached frame
+    int histogram[HIST_SIZE];   ///< RGB color distribution histogram of the frame
+};
+
+typedef struct {
+    int n;                      ///< current frame
+    int n_frames;               ///< number of frames for analysis
+    struct thumb_frame *frames; ///< the n_frames frames
+} ThumbContext;
+
+static av_cold int init(AVFilterContext *ctx, const char *args, void *opaque)
+{
+    ThumbContext *thumb = ctx->priv;
+
+    if (!args) {
+        thumb->n_frames = 100;
+    } else {
+        int n = sscanf(args, "%d", &thumb->n_frames);
+        if (n != 1 || thumb->n_frames < 2) {
+            thumb->n_frames = 0;
+            av_log(ctx, AV_LOG_ERROR,
+                   "Invalid number of frames specified (minimum is 2).\n");
+            return AVERROR(ENOMEM);
+        }
+    }
+    thumb->frames = av_calloc(thumb->n_frames, sizeof(*thumb->frames));
+    if (!thumb->frames) {
+        av_log(ctx, AV_LOG_ERROR,
+               "Allocation failure, try to lower the number of frames\n");
+        return AVERROR(ENOMEM);
+    }
+    av_log(ctx, AV_LOG_INFO, "batch size: %d frames\n", thumb->n_frames);
+    return 0;
+}
+
+static void draw_slice(AVFilterLink *inlink, int y, int h, int slice_dir)
+{
+    int i, j;
+    AVFilterContext *ctx = inlink->dst;
+    ThumbContext *thumb = ctx->priv;
+    int *hist = thumb->frames[thumb->n].histogram;
+    AVFilterBufferRef *picref = inlink->cur_buf;
+    const uint8_t *p = picref->data[0] + y * picref->linesize[0];
+
+    // update current frame RGB histogram
+    for (j = 0; j < h; j++) {
+        for (i = 0; i < inlink->w; i++) {
+            hist[0*256 + p[i*3    ]]++;
+            hist[1*256 + p[i*3 + 1]]++;
+            hist[2*256 + p[i*3 + 2]]++;
+        }
+        p += picref->linesize[0];
+    }
+}
+
+/**
+ * @brief        Compute Sum-square deviation to estimate "closeness".
+ * @param hist   color distribution histogram
+ * @param median average color distribution histogram
+ * @return       sum of squared errors
+ */
+static double frame_sum_square_err(const int *hist, const double *median)
+{
+    int i;
+    double err, sum_sq_err = 0;
+
+    for (i = 0; i < HIST_SIZE; i++) {
+        err = median[i] - (double)hist[i];
+        sum_sq_err += err*err;
+    }
+    return sum_sq_err;
+}
+
+static void end_frame(AVFilterLink *inlink)
+{
+    int i, j, best_frame_idx = 0;
+    double avg_hist[HIST_SIZE] = {0}, sq_err, min_sq_err = -1;
+    AVFilterLink *outlink = inlink->dst->outputs[0];
+    ThumbContext *thumb   = inlink->dst->priv;
+    AVFilterContext *ctx  = inlink->dst;
+    AVFilterBufferRef *picref;
+
+    // keep a reference of each frame
+    thumb->frames[thumb->n].buf = inlink->cur_buf;
+
+    // no selection until the buffer of N frames is filled up
+    if (thumb->n < thumb->n_frames - 1) {
+        thumb->n++;
+        return;
+    }
+
+    // average histogram of the N frames
+    for (j = 0; j < FF_ARRAY_ELEMS(avg_hist); j++) {
+        for (i = 0; i < thumb->n_frames; i++)
+            avg_hist[j] += (double)thumb->frames[i].histogram[j];
+        avg_hist[j] /= thumb->n_frames;
+    }
+
+    // find the frame closer to the average using the sum of squared errors
+    for (i = 0; i < thumb->n_frames; i++) {
+        sq_err = frame_sum_square_err(thumb->frames[i].histogram, avg_hist);
+        if (i == 0 || sq_err < min_sq_err)
+            best_frame_idx = i, min_sq_err = sq_err;
+    }
+
+    // free and reset everything (except the best frame buffer)
+    for (i = 0; i < thumb->n_frames; i++) {
+        memset(thumb->frames[i].histogram, 0, sizeof(thumb->frames[i].histogram));
+        if (i == best_frame_idx)
+            continue;
+        avfilter_unref_buffer(thumb->frames[i].buf);
+        thumb->frames[i].buf = NULL;
+    }
+    thumb->n = 0;
+
+    // raise the chosen one
+    picref = thumb->frames[best_frame_idx].buf;
+    av_log(ctx, AV_LOG_INFO, "frame id #%d (pts_time=%f) selected\n",
+           best_frame_idx, picref->pts * av_q2d(inlink->time_base));
+    avfilter_start_frame(outlink, picref);
+    thumb->frames[best_frame_idx].buf = NULL;
+    avfilter_draw_slice(outlink, 0, inlink->h, 1);
+    avfilter_end_frame(outlink);
+}
+
+static av_cold void uninit(AVFilterContext *ctx)
+{
+    int i;
+    ThumbContext *thumb = ctx->priv;
+    for (i = 0; i < thumb->n_frames && thumb->frames[i].buf; i++) {
+        avfilter_unref_buffer(thumb->frames[i].buf);
+        thumb->frames[i].buf = NULL;
+    }
+    av_freep(&thumb->frames);
+}
+
+static void null_start_frame(AVFilterLink *link, AVFilterBufferRef *picref) { }
+
+static int request_frame(AVFilterLink *link)
+{
+    ThumbContext *thumb = link->src->priv;
+
+    /* loop until a frame thumbnail is available (when a frame is queued,
+     * thumb->n is reset to zero) */
+    while (thumb->n) {
+        int ret = avfilter_request_frame(link->src->inputs[0]);
+        if (ret < 0)
+            return ret;
+    }
+    return 0;
+}
+
+static int poll_frame(AVFilterLink *link)
+{
+    ThumbContext *thumb  = link->src->priv;
+    AVFilterLink *inlink = link->src->inputs[0];
+    int ret, available_frames = avfilter_poll_frame(inlink);
+
+    /* If the input link is not able to provide any frame, we can't do anything
+     * at the moment and thus have zero thumbnail available. */
+    if (!available_frames)
+        return 0;
+
+    /* Since at least one frame is available and the next frame will allow us
+     * to compute a thumbnail, we can return 1 frame. */
+    if (thumb->n == thumb->n_frames - 1)
+        return 1;
+
+    /* we have some frame(s) available in the input link, but not yet enough to
+     * output a thumbnail, so we request more */
+    ret = avfilter_request_frame(inlink);
+    return ret < 0 ? ret : 0;
+}
+
+static int query_formats(AVFilterContext *ctx)
+{
+    static const enum PixelFormat pix_fmts[] = {
+        PIX_FMT_RGB24, PIX_FMT_BGR24,
+        PIX_FMT_NONE
+    };
+    avfilter_set_common_pixel_formats(ctx, avfilter_make_format_list(pix_fmts));
+    return 0;
+}
+
+AVFilter avfilter_vf_thumbnail = {
+    .name          = "thumbnail",
+    .description   = NULL_IF_CONFIG_SMALL("Select the most representative frame in a given sequence of consecutive frames."),
+    .priv_size     = sizeof(ThumbContext),
+    .init          = init,
+    .uninit        = uninit,
+    .query_formats = query_formats,
+    .inputs        = (const AVFilterPad[]) {
+        {   .name             = "default",
+            .type             = AVMEDIA_TYPE_VIDEO,
+            .get_video_buffer = avfilter_null_get_video_buffer,
+            .start_frame      = null_start_frame,
+            .draw_slice       = draw_slice,
+            .end_frame        = end_frame,
+        },{ .name = NULL }
+    },
+    .outputs       = (const AVFilterPad[]) {
+        {   .name             = "default",
+            .type             = AVMEDIA_TYPE_VIDEO,
+            .request_frame    = request_frame,
+            .poll_frame       = poll_frame,
+            .rej_perms        = AV_PERM_REUSE2,
+        },{ .name = NULL }
+    },
+};