将alpha信道写入解码ffmpeg帧

Question

有没有任何优化的函数，也许是在libswscale或libswresample中，比仅仅通过像素迭代更好地做这样的事情？

基本上我希望能够编写这样一个函数，如果我有这样一个函数，如sws_scale_and_add_alpha:

void* FFmpegLib_nextFrame_withAlpha(void* _handle, uint8_t* my_alpha_channel)
{
    FFmpegLibHandle* handle = (FFmpegLibHandle*)_handle;
    AVPacket        packet;
    int             frameFinished;

    while(av_read_frame(handle->pFormatCtx, &packet) >= 0) {
        // Is this a packet from the video stream?
        if(packet.stream_index==handle->videoStream) {
            // Decode video frame
            avcodec_decode_video2(handle->pCodecCtx, handle->pFrame, &frameFinished, &packet);
            // Did we get a video frame?
            if(frameFinished) {
                sws_scale_and_add_alpha
                (
                    handle->sws_ctx,
                    (uint8_t const * const *)handle->pFrame->data,
                    handle->pFrame->linesize,
                    0,
                    handle->pCodecCtx->height,
                    handle->pFrameARGB->data,
                    handle->pFrameARGB->linesize,
                    my_alpha_channel
                );

                return handle->pFrameARGB->data;
            }
        }
    }

    return NULL;
}

Answer 1

我想了两种方法来做这件事。通常，如果我想在命令行中合并一个alpha通道，ffmpeg为此提供一个alphamerge筛选器。而且我非常肯定你可以在C中做同样的事情，尽管编程可能很困难，（甚至在ffmpeg源代码中有一个视频滤波器的例子。）。

第二种是针对AVFrame结构我们自己进行编码。AVFrame的数据字段保存像素信息。我们需要把我们的阿尔法通道装进去。

首先将压缩后的图像帧像往常一样转换成打包的ARGB

// pFrameARGB should have been allocated and of pix_fmt `AV_PIX_FMT_ARGB`
sws_scale(sws_ctx, pFrame->data, pFrame->linesize, 0, height, pFrameARGB->data, pFrameARGB->linesize);

// cpp example, easy to convert to pure C
auto p = pFrameARGB->data[0];
for (auto i = 0; i < width * height; i++) {
    auto num = i * sizeof(uint8_t) * 4;
    auto div_result = std::div(num, width * sizeof(uint8_t) * 4);

    auto offset = pFrameARGB->linesize * div_result.quot + div_result.rem;
    p[offset] = my_alpha_channel[i];
}