如何解码具有透明背景的WebM / VP9？ 我已经与此战斗了几天。我有一个视频，显示浏览器中具有透明背景的视频，但我无法在本机Android设备上模仿这种行为。它也给了我一个

Question

ffprobe output_with_alpha_fixed.webm
  libavutil      59. 39.100 / 59. 39.100
  libavcodec     61. 19.100 / 61. 19.100
  libavformat    61.  7.100 / 61.  7.100
  libavdevice    61.  3.100 / 61.  3.100
  libavfilter    10.  4.100 / 10.  4.100
  libswscale      8.  3.100 /  8.  3.100
  libswresample   5.  3.100 /  5.  3.100
  libpostproc    58.  3.100 / 58.  3.100
Input #0, matroska,webm, from 'output_with_alpha_fixed.webm':
  Metadata:
    ENCODER         : Lavf61.7.100
  Duration: 00:00:02.00, start: 0.000000, bitrate: 833 kb/s
  Stream #0:0: Video: vp9 (Profile 0), yuv420p(tv, unknown/smpte170m/bt709, progressive), 512x512, SAR 1:1 DAR 1:1, 25 fps, 25 tbr, 1k tbn
      Metadata:
        alpha_mode      : 1
        ENCODER         : Lavc61.19.100 libvpx-vp9
        DURATION        : 00:00:02.000000000

the ffprobe的notes：

单声流（无单独的alpha流）

alpha_mode

=1

• yuv420p

• 我使用

  libvpx
手动解码流。我将在下面放置代码，但TLDR是我确切地看到了FFProbe上面指示的内容。 WebM容器只有一个流。 libvpx将流解释为YUV，而不是Yuva。
• 框架呈正确渲染，但具有黑色背景。 我不确定该怎么办。如何使用透明背景（例如Web浏览器）和FFMPEG来解码？
在解码时是我的日志输出：

Tracks: 1
Frame received: 512x512 (bit-depth: 8)
Image format: 0x102
Alpha detected: No
Strides - Y: 576, U: 288, V: 288, Alpha: 576
VPX decoder destroyed

我用来解码的C代码是：

    vpx_codec_ctx_t codec;
    vpx_codec_dec_cfg_t cfg = {0};
    vpx_codec_iface_t* iface = vpx_codec_vp9_dx();

    cfg.threads = 4;  // Enable multi-threading

    if (vpx_codec_dec_init(&codec, iface, &cfg, 0)) {
        LOGE("Failed to initialize VPX decoder");
        return nullptr;
    }

    unsigned int bufferLength = static_cast<unsigned int>(length);
    if (bufferLength == 0) {
        LOGE("Buffer is empty");
        vpx_codec_destroy(&codec);
        return nullptr;
    }

    vpx_codec_err_t decodeStatus = vpx_codec_decode(&codec, buffer, bufferLength, nullptr, 0);
    if (decodeStatus != VPX_CODEC_OK) {
        LOGE("Failed to decode VP9 video. Error: %s", vpx_codec_error(&codec));
        vpx_codec_destroy(&codec);
        return nullptr;
    }

    vpx_codec_iter_t iter = nullptr;
    vpx_image_t* img = vpx_codec_get_frame(&codec, &iter);
    if (img == nullptr) {
        LOGE("No frame received from decoder");
        vpx_codec_destroy(&codec);
        return nullptr;
    }

    LOGI("Frame received: %dx%d (bit-depth: %d)", img->d_w, img->d_h, img->bit_depth);
    LOGI("Image format: 0x%x", img->fmt);

    bool has_alpha = (img->fmt & VPX_IMG_FMT_HAS_ALPHA) != 0;
    LOGI("Alpha detected: %s", has_alpha ? "Yes" : "No");

    int width = img->d_w;
    int height = img->d_h;
    int frameSize = width * height * 4;  // RGBA buffer
    uint8_t* rgba = new uint8_t[frameSize];

    uint8_t* y = img->planes[VPX_PLANE_Y];
    uint8_t* u = img->planes[VPX_PLANE_U];
    uint8_t* v = img->planes[VPX_PLANE_V];
    uint8_t* alpha = img->planes[VPX_PLANE_ALPHA];  // Extract alpha plane

    int y_stride = img->stride[VPX_PLANE_Y];
    int u_stride = img->stride[VPX_PLANE_U];
    int v_stride = img->stride[VPX_PLANE_V];
    int a_stride = img->stride[VPX_PLANE_ALPHA];

    LOGI("Strides - Y: %d, U: %d, V: %d, Alpha: %d", y_stride, u_stride, v_stride, a_stride);

    // YUV to RGBA conversion with alpha handling
    for (int j = 0; j < height; ++j) {
        for (int i = 0; i < width; ++i) {
            int yIndex = j * y_stride + i;
            int uvIndex = (j / 2) * u_stride + (i / 2);

            int Y = y[yIndex];
            int U = u[uvIndex] - 128;
            int V = v[uvIndex] - 128;

            int R = Y + 1.402 * V;
            int G = Y - 0.344 * U - 0.714 * V;
            int B = Y + 1.772 * U;

            R = (R > 255) ? 255 : (R < 0) ? 0 : R;
            G = (G > 255) ? 255 : (G < 0) ? 0 : G;
            B = (B > 255) ? 255 : (B < 0) ? 0 : B;

            int rgbaIndex = (j * width + i) * 4;
            rgba[rgbaIndex + 0] = static_cast<uint8_t>(R);
            rgba[rgbaIndex + 1] = static_cast<uint8_t>(G);
            rgba[rgbaIndex + 2] = static_cast<uint8_t>(B);
            rgba[rgbaIndex + 3] = (alpha) ? alpha[j * a_stride + i] : 255;  // Alpha or fully opaque

        }
    }

    vpx_codec_destroy(&codec);
    LOGI("VPX decoder destroyed");

    jbyteArray result = env->NewByteArray(frameSize);
    env->SetByteArrayRegion(result, 0, frameSize, reinterpret_cast<jbyte*>(rgba));
    delete[] rgba;

是的，我有同样的错误...也许您可以尝试MPV。