camera2捕获的图片-从YUV_420_888到NV21的转换

Question

private static byte[] YUV_420_888toNV21(Image image) {
    byte[] nv21;
    ByteBuffer yBuffer = image.getPlanes()[0].getBuffer();
    ByteBuffer uBuffer = image.getPlanes()[1].getBuffer();
    ByteBuffer vBuffer = image.getPlanes()[2].getBuffer();

    int ySize = yBuffer.remaining();
    int uSize = uBuffer.remaining();
    int vSize = vBuffer.remaining();

    nv21 = new byte[ySize + uSize + vSize];

    //U and V are swapped
    yBuffer.get(nv21, 0, ySize);
    vBuffer.get(nv21, ySize, vSize);
    uBuffer.get(nv21, ySize + vSize, uSize);

    return nv21;
}

为什么两个函数调用的结果是不同的，而请求的类型是相同的？

更新：正如注释中提到的，我得到这种行为是因为不同的图像大小（捕获请求的尺寸要大得多）。但是我们在JNI端的进一步处理操作对于两个请求是相同的，并且不依赖于图像尺寸（只依赖于纵横比，这在两种情况下是相同的）。

Answer 1

如果根本没有填充，并且U和V平面图重叠，实际上表示交错的VU值，代码将只返回正确的NV21。这种情况在预览中经常发生，但在这种情况下，您需要为数组分配额外的W*h/4字节（这可能不是问题）。也许对于捕获的图像，你需要一个更健壮的隐写，例如。

private static byte[] YUV_420_888toNV21(Image image) {

    int width = image.getWidth();
    int height = image.getHeight(); 
    int ySize = width*height;
    int uvSize = width*height/4;

    byte[] nv21 = new byte[ySize + uvSize*2];

    ByteBuffer yBuffer = image.getPlanes()[0].getBuffer(); // Y
    ByteBuffer uBuffer = image.getPlanes()[1].getBuffer(); // U
    ByteBuffer vBuffer = image.getPlanes()[2].getBuffer(); // V

    int rowStride = image.getPlanes()[0].getRowStride();
    assert(image.getPlanes()[0].getPixelStride() == 1);

    int pos = 0;

    if (rowStride == width) { // likely
        yBuffer.get(nv21, 0, ySize);
        pos += ySize;
    }
    else {
        long yBufferPos = -rowStride; // not an actual position
        for (; pos<ySize; pos+=width) {
            yBufferPos += rowStride;
            yBuffer.position(yBufferPos);
            yBuffer.get(nv21, pos, width);
        }
    }

    rowStride = image.getPlanes()[2].getRowStride();
    int pixelStride = image.getPlanes()[2].getPixelStride();

    assert(rowStride == image.getPlanes()[1].getRowStride());
    assert(pixelStride == image.getPlanes()[1].getPixelStride());
    
    if (pixelStride == 2 && rowStride == width && uBuffer.get(0) == vBuffer.get(1)) {
        // maybe V an U planes overlap as per NV21, which means vBuffer[1] is alias of uBuffer[0]
        byte savePixel = vBuffer.get(1);
        try {
            vBuffer.put(1, (byte)~savePixel);
            if (uBuffer.get(0) == (byte)~savePixel) {
                vBuffer.put(1, savePixel);
                vBuffer.position(0);
                uBuffer.position(0);
                vBuffer.get(nv21, ySize, 1);
                uBuffer.get(nv21, ySize + 1, uBuffer.remaining());

                return nv21; // shortcut
            }
        }
        catch (ReadOnlyBufferException ex) {
            // unfortunately, we cannot check if vBuffer and uBuffer overlap
        }

        // unfortunately, the check failed. We must save U and V pixel by pixel
        vBuffer.put(1, savePixel);
    }

    // other optimizations could check if (pixelStride == 1) or (pixelStride == 2), 
    // but performance gain would be less significant

    for (int row=0; row<height/2; row++) {
        for (int col=0; col<width/2; col++) {
            int vuPos = col*pixelStride + row*rowStride;
            nv21[pos++] = vBuffer.get(vuPos);
            nv21[pos++] = uBuffer.get(vuPos);
        }
    }

    return nv21;
}

如果您打算将结果数组传递给C++，您可以利用以下事实

返回的缓冲区将始终具有isDirect返回true，因此底层数据可以映射为JNI中的指针，而无需使用GetDirectBufferAddress进行任何复制。