[FFmpeg-devel,V6] examples/hw_decode: Add a HWAccel decoding example.

diff --git a/doc/examples/hw_decode.c b/doc/examples/hw_decode.c
new file mode 100644
index 0000000000..06cc7462fa
--- /dev/null
+++ b/doc/examples/hw_decode.c
@@ -0,0 +1,266 @@ 
+/*
+ * Copyright (c) 2017 Jun Zhao
+ * Copyright (c) 2017 Kaixuan Liu
+ *
+ * HW Acceleration API (video decoding) decode sample
+ *
+ * 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
+ * HW-Accelerated decoding example.
+ *
+ * @example hw_decode.c
+ * This example shows how to do HW-accelerated decoding with output
+ * frames from the HW video surfaces.
+ */
+
+#include <stdio.h>
+
+#include <libavcodec/avcodec.h>
+#include <libavformat/avformat.h>
+#include <libavutil/pixdesc.h>
+#include <libavutil/hwcontext.h>
+#include <libavutil/opt.h>
+#include <libavutil/avassert.h>
+#include <libavutil/imgutils.h>
+
+static AVBufferRef *hw_device_ctx = NULL;
+static enum AVPixelFormat hw_pix_fmt;
+FILE *output_file = NULL;
+
+static enum AVPixelFormat find_fmt_by_hw_type(const enum AVHWDeviceType type)
+{
+    enum AVPixelFormat fmt;
+
+    switch (type) {
+    case AV_HWDEVICE_TYPE_VAAPI:
+        fmt = AV_PIX_FMT_VAAPI;
+        break;
+    case AV_HWDEVICE_TYPE_DXVA2:
+        fmt = AV_PIX_FMT_DXVA2_VLD;
+        break;
+    case AV_HWDEVICE_TYPE_D3D11VA:
+        fmt = AV_PIX_FMT_D3D11;
+        break;
+    case AV_HWDEVICE_TYPE_VDPAU:
+        fmt = AV_PIX_FMT_VDPAU;
+        break;
+    case AV_HWDEVICE_TYPE_VIDEOTOOLBOX:
+        fmt = AV_PIX_FMT_VIDEOTOOLBOX;
+        break;
+    default:
+        fmt = AV_PIX_FMT_NONE;
+        break;
+    }
+
+    return fmt;
+}
+
+static int hw_decoder_init(AVCodecContext *ctx, const enum AVHWDeviceType type)
+{
+    int err = 0;
+
+    if ((err = av_hwdevice_ctx_create(&hw_device_ctx, type,
+                                      NULL, NULL, 0)) < 0) {
+        fprintf(stderr, "Failed to create specified HW device.\n");
+        return err;
+    }
+    ctx->hw_device_ctx = av_buffer_ref(hw_device_ctx);
+
+    return err;
+}
+
+static enum AVPixelFormat get_hw_format(AVCodecContext *ctx,
+                                        const enum AVPixelFormat *pix_fmts)
+{
+    const enum AVPixelFormat *p;
+
+    for (p = pix_fmts; *p != -1; p++) {
+        if (*p == hw_pix_fmt)
+            return *p;
+    }
+
+    fprintf(stderr, "Failed to get HW surface format.\n");
+    return AV_PIX_FMT_NONE;
+}
+
+static int decode_write(AVCodecContext *avctx, AVPacket *packet)
+{
+    AVFrame *frame = NULL, *sw_frame = NULL;
+    AVFrame *tmp_frame = NULL;
+    uint8_t *buffer = NULL;
+    int size;
+    int ret = 0;
+
+    ret = avcodec_send_packet(avctx, packet);
+    if (ret < 0) {
+        fprintf(stderr, "Error during decoding\n");
+        return ret;
+    }
+
+    while (ret >= 0) {
+        if (!(frame = av_frame_alloc()) || !(sw_frame = av_frame_alloc())) {
+            fprintf(stderr, "Can not alloc frame\n");
+            ret = AVERROR(ENOMEM);
+            goto fail;
+        }
+
+        ret = avcodec_receive_frame(avctx, frame);
+        if (ret == AVERROR(EAGAIN) || ret == AVERROR_EOF) {
+            av_frame_free(&frame);
+            av_frame_free(&sw_frame);
+            return 0;
+        } else if (ret < 0) {
+            fprintf(stderr, "Error while decoding\n");
+            goto fail;
+        }
+
+        if (frame->format == hw_pix_fmt) {
+            /* retrieve data from GPU to CPU */
+            if ((ret = av_hwframe_transfer_data(sw_frame, frame, 0)) < 0) {
+                fprintf(stderr, "Error transferring the data to system memory\n");
+                goto fail;
+            }
+            tmp_frame = sw_frame;
+        } else
+            tmp_frame = frame;
+
+        size = av_image_get_buffer_size(tmp_frame->format, tmp_frame->width,
+                                        tmp_frame->height, 1);
+        buffer = av_malloc(size);
+        if (!buffer) {
+            fprintf(stderr, "Can not alloc buffer\n");
+            ret = AVERROR(ENOMEM);
+            goto fail;
+        }
+        ret = av_image_copy_to_buffer(buffer, size,
+                                      (const uint8_t * const *)tmp_frame->data,
+                                      (const int *)tmp_frame->linesize, tmp_frame->format,
+                                      tmp_frame->width, tmp_frame->height, 1);
+        if (ret < 0) {
+            fprintf(stderr, "Can not copy image to buffer\n");
+            goto fail;
+        }
+
+        if ((ret = fwrite(buffer, 1, size, output_file)) < 0) {
+            fprintf(stderr, "Failed to dump raw data.\n");
+            goto fail;
+        }
+
+    fail:
+        av_frame_free(&frame);
+        av_frame_free(&sw_frame);
+        if (buffer)
+            av_freep(&buffer);
+        if (ret < 0)
+            return ret;
+    }
+
+    return 0;
+}
+
+int main(int argc, char *argv[])
+{
+    AVFormatContext *input_ctx = NULL;
+    int video_stream, ret;
+    AVStream *video = NULL;
+    AVCodecContext *decoder_ctx = NULL;
+    AVCodec *decoder = NULL;
+    AVPacket packet;
+    enum AVHWDeviceType type;
+
+    if (argc < 4) {
+        fprintf(stderr, "Usage: %s <vaapi|dxva2|d3d11va> <input file> <output file>\n", argv[0]);
+        return -1;
+    }
+
+    av_register_all();
+
+    type = av_hwdevice_find_type_by_name(argv[1]);
+    hw_pix_fmt = find_fmt_by_hw_type(type);
+    if (hw_pix_fmt == -1) {
+        fprintf(stderr, "Cannot support '%s' in this example.\n", argv[1]);
+        return -1;
+    }
+
+    /* open the input file */
+    if (avformat_open_input(&input_ctx, argv[2], NULL, NULL) != 0) {
+        fprintf(stderr, "Cannot open input file '%s'\n", argv[2]);
+        return -1;
+    }
+
+    if (avformat_find_stream_info(input_ctx, NULL) < 0) {
+        fprintf(stderr, "Cannot find input stream information.\n");
+        return -1;
+    }
+
+    /* find the video stream information */
+    ret = av_find_best_stream(input_ctx, AVMEDIA_TYPE_VIDEO, -1, -1, &decoder, 0);
+    if (ret < 0) {
+        fprintf(stderr, "Cannot find a video stream in the input file\n");
+        return -1;
+    }
+    video_stream = ret;
+
+    if (!(decoder_ctx = avcodec_alloc_context3(decoder)))
+        return AVERROR(ENOMEM);
+
+    video = input_ctx->streams[video_stream];
+    if (avcodec_parameters_to_context(decoder_ctx, video->codecpar) < 0)
+        return -1;
+
+    decoder_ctx->get_format  = get_hw_format;
+    av_opt_set_int(decoder_ctx, "refcounted_frames", 1, 0);
+
+    if (hw_decoder_init(decoder_ctx, type) < 0)
+        return -1;
+
+    if ((ret = avcodec_open2(decoder_ctx, decoder, NULL)) < 0) {
+        fprintf(stderr, "Failed to open codec for stream #%u\n", video_stream);
+        return -1;
+    }
+
+    /* open the file to dump raw data */
+    output_file = fopen(argv[3], "w+");
+
+    /* actual decoding and dump the raw data */
+    while (ret >= 0) {
+        if ((ret = av_read_frame(input_ctx, &packet)) < 0)
+            break;
+
+        if (video_stream == packet.stream_index)
+            ret = decode_write(decoder_ctx, &packet);
+
+        av_packet_unref(&packet);
+    }
+
+    /* flush the decoder */
+    packet.data = NULL;
+    packet.size = 0;
+    ret = decode_write(decoder_ctx, &packet);
+    av_packet_unref(&packet);
+
+    if (output_file)
+        fclose(output_file);
+    avcodec_free_context(&decoder_ctx);
+    avformat_close_input(&input_ctx);
+    av_buffer_unref(&hw_device_ctx);
+
+    return 0;
+}