9、ffplay音频的波谱图及频谱图源码分析

ffplay音频的波谱图及频谱图源码分析

当ffplay指定启动参数-showmode [0/1/2]时，值为0，正常播放（默认）；值为1，显示波形；值为2，显示频谱图。

在ffplay中由show_mode变量控制显示方式取值分别为:SHOW_MODE_VIDEO/SHOW_MODE_WAVES/SHOW_MODE_RDFT

在函数video_display(…)中判断是否有音频流并且show_mode不等于SHOW_MODE_VIDEO，如果成立调用函数video_audio_display(…)
进行波谱或频谱显示

下面详细分析video_audio_display(…)

static void video_audio_display(VideoState *s)
{
    int i, i_start, x, y1, y, ys, delay, n, nb_display_channels;
    int ch, channels, h, h2;
    int64_t time_diff;
    int rdft_bits, nb_freq;
    //计算离散傅里叶变换输入数组的长度log2的值,绘制频谱时使用双通道，并且填充显示区域高度
    for (rdft_bits = 1; (1 << rdft_bits) < 2 * s->height; rdft_bits++)
        ;
    nb_freq = 1 << (rdft_bits - 1);
    /* compute display index : center on currently output samples */
    channels = s->audio_tgt.channels;
    nb_display_channels = channels;
    if (!s->paused) {
        //该语句块计算当前波/频谱图显示音频数据的开始索引
        int data_used= s->show_mode == SHOW_MODE_WAVES ? s->width : (2*nb_freq);
        n = 2 * channels;
        delay = s->audio_write_buf_size;
        delay /= n;

        /* to be more precise, we take into account the time spent since
           the last buffer computation */
        if (audio_callback_time) {
            time_diff = av_gettime_relative() - audio_callback_time;
            delay -= (time_diff * s->audio_tgt.freq) / 1000000;
        }

        delay += 2 * data_used;
        if (delay < data_used)
            delay = data_used;

        i_start= x = compute_mod(s->sample_array_index - delay * channels, SAMPLE_ARRAY_SIZE);
        if (s->show_mode == SHOW_MODE_WAVES) {
            //如果为波谱图查找振幅变化最大的音频部分数据的索引
            h = INT_MIN;
            for (i = 0; i < 1000; i += channels) {
                int idx = (SAMPLE_ARRAY_SIZE + x - i) % SAMPLE_ARRAY_SIZE;
                int a = s->sample_array[idx];
                int b = s->sample_array[(idx + 4 * channels) % SAMPLE_ARRAY_SIZE];
                int c = s->sample_array[(idx + 5 * channels) % SAMPLE_ARRAY_SIZE];
                int d = s->sample_array[(idx + 9 * channels) % SAMPLE_ARRAY_SIZE];
                int score = a - d;
                if (h < score && (b ^ c) < 0) {
                    h = score;
                    i_start = idx;
                }
            }
        }

        s->last_i_start = i_start;
    } else {
        i_start = s->last_i_start;
    }

    if (s->show_mode == SHOW_MODE_WAVES) {
        //设置颜色为白色
        SDL_SetRenderDrawColor(renderer, 255, 255, 255, 255);

        /* total height for one channel */
        h = s->height / nb_display_channels;//每一个通道波谱显示区域占有的高度
        /* graph height / 2 */
        h2 = (h * 9) / 20;
        for (ch = 0; ch < nb_display_channels; ch++) {
            i = i_start + ch;
            y1 = s->ytop + ch * h + (h / 2); /* position of center line */
            for (x = 0; x < s->width; x++) {
                y = (s->sample_array[i] * h2) >> 15;//y的绝对值一定小于h
                if (y < 0) {
                    y = -y;
                    ys = y1 - y;
                } else {
                    ys = y1;
                }
                fill_rectangle(s->xleft + x, ys, 1, y);//绘制高度为1的矩形
                i += channels;
                if (i >= SAMPLE_ARRAY_SIZE)
                    i -= SAMPLE_ARRAY_SIZE;
            }
        }

        SDL_SetRenderDrawColor(renderer, 0, 0, 255, 255);
        //绘制不同声道波谱图的分割线
        for (ch = 1; ch < nb_display_channels; ch++) {
            y = s->ytop + ch * h;
            fill_rectangle(s->xleft, y, s->width, 1);
        }
    } else {
        if (realloc_texture(&s->vis_texture, SDL_PIXELFORMAT_ARGB8888, s->width, s->height, SDL_BLENDMODE_NONE, 1) < 0)
            return;

        nb_display_channels= FFMIN(nb_display_channels, 2);//显示最多2声道的频谱图
        if (rdft_bits != s->rdft_bits) {
            av_rdft_end(s->rdft);
            av_free(s->rdft_data);
            s->rdft = av_rdft_init(rdft_bits, DFT_R2C);//初始化离散傅里叶变换的Context
            s->rdft_bits = rdft_bits;
            s->rdft_data = av_malloc_array(nb_freq, 4 *sizeof(*s->rdft_data));//双声道，每一通道的输入数组大小为2*nb_freq
        }
        if (!s->rdft || !s->rdft_data){
            av_log(NULL, AV_LOG_ERROR, "Failed to allocate buffers for RDFT, switching to waves display\n");
            s->show_mode = SHOW_MODE_WAVES;
        } else {
            FFTSample *data[2];
            SDL_Rect rect = {.x = s->xpos, .y = 0, .w = 1, .h = s->height};
            uint32_t *pixels;
            int pitch;
            for (ch = 0; ch < nb_display_channels; ch++) {
                data[ch] = s->rdft_data + 2 * nb_freq * ch;
                i = i_start + ch;
                for (x = 0; x < 2 * nb_freq; x++) {
                    double w = (x-nb_freq) * (1.0 / nb_freq);
                    data[ch][x] = s->sample_array[i] * (1.0 - w * w);//减小了音频值，为何这样处理??
                    i += channels;
                    if (i >= SAMPLE_ARRAY_SIZE)
                        i -= SAMPLE_ARRAY_SIZE;
                }
                av_rdft_calc(s->rdft, data[ch]);//进行离散傅里叶变换
            }
            /* Least efficient way to do this, we should of course
             * directly access it but it is more than fast enough. */
            if (!SDL_LockTexture(s->vis_texture, &rect, (void **)&pixels, &pitch)) {
                pitch >>= 2;//每一个像素4字节,pixels为uint32_t指针
                pixels += pitch * s->height;
                //for循环填充每一个像素
                for (y = 0; y < s->height; y++) {
                    double w = 1 / sqrt(nb_freq);
                    //声道1频率值转换为颜色值，显示为红色
                    int a = sqrt(w * sqrt(data[0][2 * y + 0] * data[0][2 * y + 0] + data[0][2 * y + 1] * data[0][2 * y + 1]));
                     //声道2频率值转换为颜色值，显示为绿色
                    int b = (nb_display_channels == 2 ) ? sqrt(w * hypot(data[1][2 * y + 0], data[1][2 * y + 1]))
                                                        : a;
                    a = FFMIN(a, 255);
                    b = FFMIN(b, 255);
                    pixels -= pitch;//上一行
                    *pixels = (a << 16) + (b << 8) + ((a+b) >> 1);
                }
                SDL_UnlockTexture(s->vis_texture);
            }
            SDL_RenderCopy(renderer, s->vis_texture, NULL, NULL);
        }
        if (!s->paused)
            s->xpos++;
        if (s->xpos >= s->width)
            s->xpos= s->xleft;
    }
}