android – camera2捕获图片 – 从YUV_420_888转换为NV21

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;
}

虽然这个函数适用于cameraCaptureSessions.setRepeatingRequest,但在调用cameraCaptureSessions.capture时,我们在进一步处理(在JNI端)时会出现分段错误.两者都通过ImageReader请求YUV_420_888格式.

为什么两个函数调用的结果不同,而请求的类型是相同的？

更新：正如评论中所提到的,由于图像大小不同(捕获请求的维度大得多),我得到了这种行为.但是我们在JNI方面的进一步处理操作对于两个请求都是相同的,并且不依赖于图像尺寸(仅在纵横比上,在两种情况下都是相同的).

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 = width - rowStride; // not an actual position
        for (; pos<ySize; pos+=width) {
            yBufferPos += rowStride - width;
            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);
        vBuffer.put(1, (byte)0);
        if (uBuffer.get(0) == 0) {
            vBuffer.put(1, (byte)255);
            if (uBuffer.get(0) == 255) {
                vBuffer.put(1, savePixel);
                vBuffer.get(nv21, ySize, uvSize);

                return nv21; // shortcut
            }
        }

        // 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;
}

如果你打算将结果数组传递给C,你可以利用fact那个

the buffer returned will always have isDirect return true, so the underlying data could be mapped as a pointer in JNI without doing any copies with GetDirectBufferAddress.

这意味着可以在C中以最小的开销完成相同的转换.在C中,你甚至可能发现实际的像素排列已经是NV21了！

PS实际上,这可以用Java完成,开销可以忽略不计,请参阅if行(pixelStride == 2&& … above.