YUV_420_888三星Galaxy S7(Camera2)解读

Question

我编写了一个从YUV_420_888到位图的转换，考虑了以下逻辑（根据我的理解）：

总结一下这个方法：内核的坐标x和y与y平面(2D-分配）的非填充部分的x和y以及输出位图的x和y是一致的。然而，U平面和V平面具有不同于Y平面的结构，因为它们使用1字节来覆盖4个像素，此外，它们可能具有超过一个像素步幅，此外，它们还可能具有与Y平面不同的填充。因此，为了让内核有效地访问U和V，我将它们放入一维分配中，并创建了一个索引“uvindex”，它给出了在一维分配中对应的U和V的位置，对于（非填充的）y平面（以及输出位图）中给定的(x，y)坐标。

为了保持RS内核的精简，我通过LaunchOptions限制X范围，从而排除了yPlane中的填充区域（这反映了Y平面的延伸，因此在内核中可以忽略）。因此，我们只需要考虑uvIndex中的uvPixelStride和uvRowStride，即用于访问U值和V值的索引。

Renderscript内核，名为yuv420888.rs

  #pragma version(1)
  #pragma rs java_package_name(com.xxxyyy.testcamera2);
  #pragma rs_fp_relaxed

  int32_t width;
  int32_t height;

  uint picWidth, uvPixelStride, uvRowStride ;
  rs_allocation ypsIn,uIn,vIn;

 // The LaunchOptions ensure that the Kernel does not enter the padding  zone of Y, so yRowStride can be ignored WITHIN the Kernel.
 uchar4 __attribute__((kernel)) doConvert(uint32_t x, uint32_t y) {

 // index for accessing the uIn's and vIn's
uint uvIndex=  uvPixelStride * (x/2) + uvRowStride*(y/2);

// get the y,u,v values
uchar yps= rsGetElementAt_uchar(ypsIn, x, y);
uchar u= rsGetElementAt_uchar(uIn, uvIndex);
uchar v= rsGetElementAt_uchar(vIn, uvIndex);

// calc argb
int4 argb;
    argb.r = yps + v * 1436 / 1024 - 179;
    argb.g =  yps -u * 46549 / 131072 + 44 -v * 93604 / 131072 + 91;
    argb.b = yps +u * 1814 / 1024 - 227;
    argb.a = 255;

uchar4 out = convert_uchar4(clamp(argb, 0, 255));
return out;
}

Java端：

    private Bitmap YUV_420_888_toRGB(Image image, int width, int height){
    // Get the three image planes
    Image.Plane[] planes = image.getPlanes();
    ByteBuffer buffer = planes[0].getBuffer();
    byte[] y = new byte[buffer.remaining()];
    buffer.get(y);

    buffer = planes[1].getBuffer();
    byte[] u = new byte[buffer.remaining()];
    buffer.get(u);

    buffer = planes[2].getBuffer();
    byte[] v = new byte[buffer.remaining()];
    buffer.get(v);

    // get the relevant RowStrides and PixelStrides
    // (we know from documentation that PixelStride is 1 for y)
    int yRowStride= planes[0].getRowStride();
    int uvRowStride= planes[1].getRowStride();  // we know from   documentation that RowStride is the same for u and v.
    int uvPixelStride= planes[1].getPixelStride();  // we know from   documentation that PixelStride is the same for u and v.


    // rs creation just for demo. Create rs just once in onCreate and use it again.
    RenderScript rs = RenderScript.create(this);
    //RenderScript rs = MainActivity.rs;
    ScriptC_yuv420888 mYuv420=new ScriptC_yuv420888 (rs);

    // Y,U,V are defined as global allocations, the out-Allocation is the Bitmap.
    // Note also that uAlloc and vAlloc are 1-dimensional while yAlloc is 2-dimensional.
    Type.Builder typeUcharY = new Type.Builder(rs, Element.U8(rs));

    //using safe height
    typeUcharY.setX(yRowStride).setY(y.length / yRowStride);

    Allocation yAlloc = Allocation.createTyped(rs, typeUcharY.create());
    yAlloc.copyFrom(y);
    mYuv420.set_ypsIn(yAlloc);

    Type.Builder typeUcharUV = new Type.Builder(rs, Element.U8(rs));
    // note that the size of the u's and v's are as follows:
    //      (  (width/2)*PixelStride + padding  ) * (height/2)
    // =    (RowStride                          ) * (height/2)
    // but I noted that on the S7 it is 1 less...
    typeUcharUV.setX(u.length);
    Allocation uAlloc = Allocation.createTyped(rs, typeUcharUV.create());
    uAlloc.copyFrom(u);
    mYuv420.set_uIn(uAlloc);

    Allocation vAlloc = Allocation.createTyped(rs, typeUcharUV.create());
    vAlloc.copyFrom(v);
    mYuv420.set_vIn(vAlloc);

    // handover parameters
    mYuv420.set_picWidth(width);
    mYuv420.set_uvRowStride (uvRowStride);
    mYuv420.set_uvPixelStride (uvPixelStride);

    Bitmap outBitmap = Bitmap.createBitmap(width, height, Bitmap.Config.ARGB_8888);
    Allocation outAlloc = Allocation.createFromBitmap(rs, outBitmap, Allocation.MipmapControl.MIPMAP_NONE, Allocation.USAGE_SCRIPT);

    Script.LaunchOptions lo = new Script.LaunchOptions();
    lo.setX(0, width);  // by this we ignore the y’s padding zone, i.e. the right side of x between width and yRowStride
    //using safe height
    lo.setY(0, y.length / yRowStride);

    mYuv420.forEach_doConvert(outAlloc,lo);
    outAlloc.copyTo(outBitmap);

    return outBitmap;
}

在Nexus7(API22)上测试，这将返回漂亮的彩色位图。然而，该设备具有微不足道的pixelstrides(=1)，并且没有填充（即rowstride=width）。在brandnew Samsung S7(API23)上测试，我得到的图片颜色不正确--除了绿色的。但是这张图片并没有显示出对绿色的普遍偏见，只是看起来非绿色的颜色没有被正确地复制。注意，S7应用的U/V像素步幅为2，并且没有填充。

Answer 1

看

floor((float) uvPixelStride*(x)/2)

它从Y x坐标计算U,V行偏移量(uv_row_offset）。

如果uvPixelStride=2，则随着x的增加：

x = 0, uv_row_offset = 0
x = 1, uv_row_offset = 1
x = 2, uv_row_offset = 2
x = 3, uv_row_offset = 3

uvPixelStride * floor(x/2)

uvPixelStride * (x/2)

x = 0, uv_row_offset = 0
x = 1, uv_row_offset = 0
x = 2, uv_row_offset = 2
x = 3, uv_row_offset = 2

看看这是否修复了颜色错误。实际上，这里不正确的寻址将意味着每一个其他颜色样本都来自错误的颜色平面，因为潜在的YUV数据很可能是半平面的（因此U平面从V平面+1字节开始，两个平面交错）