Convert YUV4:4:4 to YUV4:2:2 images

Question

There is a lot of information on the internet about the differences between YUV4:4:4 to YUV4:2:2 formats, however, I can not find anything that tells how to convert the YUV4:4:4 to YUV4:2:2. Since such conversion is performed using software, I was hoping that there should be some developers that have done it and could direct me to the sources that describe the conversion algorithm. Of course, the software code would be nice to have, but having the access to the theory would be sufficient enough to write my own software. Specifically, I would like to know pixel structure and how the bytes are managed during conversion.

I found several similar questions like this and this, however, could not get my question answered. Also, I posted this question on the Photography forum, and they considered it as a software question.

Answer 1

The reason why you can't find specific description, is that there are many ways to do it.
Lets start from Wikipedia: https://en.wikipedia.org/wiki/Chroma_subsampling#4:2:2

4:4:4:
Each of the three Y'CbCr components have the same sample rate, thus there is no chroma subsampling. This scheme is sometimes used in high-end film scanners and cinematic post production.

and

4:2:2:
The two chroma components are sampled at half the sample rate of luma: the horizontal chroma resolution is halved. This reduces the bandwidth of an uncompressed video signal by one-third with little to no visual difference.

Note: Terms YCbCr and YUV are used interchangeably.
https://en.wikipedia.org/wiki/YCbCr

Y′CbCr is often confused with the YUV color space, and typically the terms YCbCr and YUV are used interchangeably, leading to some confusion; when referring to signals in video or digital form, the term "YUV" mostly means "Y′CbCr".

Data memory ordering:
Again there is more than one format.
Intel IPP documentation defines two main categories: "Pixel-Order Image Formats" and "Planar Image Formats".
There is a nice documentation here: https://software.intel.com/en-us/node/503876
Refer here: http://www.fourcc.org/yuv.php#NV12 for YUV pixel arrangement formats.
Refer here: http://scc.ustc.edu.cn/zlsc/sugon/intel/ipp/ipp_manual/IPPI/ippi_ch6/ch6_image_downsampling.htm#ch6_image_downsampling for downsampling description.

Let's assume "Pixel-Order" format:

YUV 4:4:4 data order: Y0 U0 V0  Y1 U1 V1  Y2 U2 V2  Y3 U3 V3  
YUV 4:2:2 data order: Y0  U0    Y1  V0    Y2  U1    Y3  V1  

Each element is a single byte, and Y0 is the lower byte in memory.
The 4:2:2 data order described above is named UYVY or YUY2 pixel format.

Conversion algorithms:

1. "Naive sub-sampling":
   "Throw" every second U/V component:
   Take U0, and throw U1, take V0 and throw V1...
   Source: Y0 U0 V0 Y1 U1 V1 Y2 U2 V2
   Destination: Y0 U0 Y1 V0 Y2 U2 Y3 V2
   I can't recommend it, since it causes aliasing artifacts.

2. Average each U/V pair:
   Take Destination U0 equals source (U0+U1)/2, same for V0...
   Source: Y0 U0 V0 Y1 U1 V1 Y2 U2 V2
   Destination: Y0 (U0+U1)/2 Y1 (V0+V1)/2 Y2 (U2+U3)/2 Y3 (V2+V3)/2

3. Use other interpolation method for down-sampling U and V (cubic interpolation for example).
   Usually you will not be able to see any differences compared to simple average.

---

C implementation:

The question is not tagged as C, but I think the following C implementation may be helpful.
The following code converts pixel-ordered YUV 4:4:4 to pixel-ordered YUV 4:2:2 by averaging each U/V pair:

//Convert single row I0 from pixel-ordered YUV 4:4:4 to pixel-ordered YUV 4:2:2.
//Save the result in J0.
//I0 size in bytes is image_width*3
//J0 size in bytes is image_width*2
static void ConvertRowYUV444ToYUV422(const unsigned char I0[],
                                     const int image_width,
                                     unsigned char J0[])
{
    int x;

    //Process two Y,U,V triples per iteration:
    for (x = 0; x < image_width; x += 2)
    {
        //Load source elements
        unsigned char y0    = I0[x*3];                  //Load source Y element
        unsigned int u0     = (unsigned int)I0[x*3+1];  //Load source U element (and convert from uint8 to uint32).
        unsigned int v0     = (unsigned int)I0[x*3+2];  //Load source V element (and convert from uint8 to uint32).

        //Load next source elements
        unsigned char y1    = I0[x*3+3];                //Load source Y element
        unsigned int u1     = (unsigned int)I0[x*3+4];  //Load source U element (and convert from uint8 to uint32).
        unsigned int v1     = (unsigned int)I0[x*3+5];  //Load source V element (and convert from uint8 to uint32).

        //Calculate destination U, and V elements.
        //Use shift right by 1 for dividing by 2.
        //Use plus 1 before shifting - round operation instead of floor operation.
        unsigned int u01    = (u0 + u1 + 1) >> 1;       //Destination U element equals average of two source U elements.
        unsigned int v01    = (v0 + v1 + 1) >> 1;       //Destination U element equals average of two source U elements.

        J0[x*2]     = y0;   //Store Y element (unmodified).
        J0[x*2+1]   = (unsigned char)u01;   //Store destination U element (and cast uint32 to uint8).
        J0[x*2+2]   = y1;   //Store Y element (unmodified).
        J0[x*2+3]   = (unsigned char)v01;   //Store destination V element (and cast uint32 to uint8).
    }
}


//Convert image I from pixel-ordered YUV 4:4:4 to pixel-ordered YUV 4:2:2.
//I - Input image in pixel-order data YUV 4:4:4 format.
//image_width - Number of columns of image I.
//image_height - Number of rows of image I.
//J - Destination "image" in pixel-order data YUV 4:2:2 format.
//Note: The term "YUV" referees to "Y'CbCr".

//I is pixel ordered YUV 4:4:4 format (size in bytes is image_width*image_height*3):
//YUVYUVYUVYUV
//YUVYUVYUVYUV
//YUVYUVYUVYUV
//YUVYUVYUVYUV
//
//J is pixel ordered YUV 4:2:2 format (size in bytes is image_width*image_height*2):
//YUYVYUYV
//YUYVYUYV
//YUYVYUYV
//YUYVYUYV
//
//Conversion algorithm:
//Each element of destination U is average of 2 original U horizontal elements
//Each element of destination V is average of 2 original V horizontal elements
//
//Limitations:
//1. image_width must be a multiple of 2.
//2. I and J must be two separate arrays (in place computation is not supported). 
static void ConvertYUV444ToYUV422(const unsigned char I[],
                                  const int image_width,
                                  const int image_height,
                                  unsigned char J[])
{
    //I0 points source row.
    const unsigned char *I0;    //I0 -> YUYVYUYV...

    //J0 and points destination row.
    unsigned char *J0;          //J0 -> YUYVYUYV

    int y;  //Row index

    //In each iteration process single row.
    for (y = 0; y < image_height; y++)
    {
        I0 = &I[y*image_width*3];   //Input row width is image_width*3 bytes (each pixel is Y,U,V).

        J0 = &J[y*image_width*2];   //Output row width is image_width*2 bytes (each two pixels are Y,U,Y,V).

        //Process single source row into single destination row
        ConvertRowYUV444ToYUV422(I0, image_width, J0);
    }
}

---

Planar representation of YUV 4:2:2

Planar representation may be more intuitive than "Pixel-Order" format.
In planar representation each color channel is represented as a separate matrix, which can be displayed as an image.

Example:

• Original image in RGB format (before converting to YUV):
  

• Image channels in YUV 4:4:4 format:
  
  (Left YUV triple is represented in gray levels, and right YUV triple is represented using false colors).

• Image channels in YUV 4:2:2 format (after horizontal Chroma subsampling):
  
  (Left YUV triple is represented in gray levels, and right YUV triple is represented using "false colors").

As you can see, in 4:2:2 format, the U an V channels are down-sampled (shrunk) in the horizontal axis.

Remark:
The "false colors" representation of U and V channels is used for emphasizing that Y is the Luma channel and U and V are the Chrominance channels.

---

Higher order interpolation and Anti-Aliasing filter:
Following MATLAB code sample shows how to perform down-sampling with higher order interpolation and Anti-Aliasing filter.
The sample also shows the down-sampling method used by FFMPEG.
Note: you don't need to know MATLAB programming in order to understand the samples.
You do need some knowledge of image filtering by convolution between a Kernel and an image.

%Prepare the input:
%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%
load('mandrill.mat', 'X', 'map'); %Load input image
RGB = im2uint8(ind2rgb(X, map));  %Convert to RGB (the mandrill sample image is an indexed image)
YUV = rgb2ycbcr(RGB);             %Convert from RGB to YUV (MATLAB function rgb2ycbcr uses BT.601 conversion formula)

%Separate YUV to 3 planes (Y plane, U plane and V plane)
Y = YUV(:, :, 1);
U = YUV(:, :, 2);
V = YUV(:, :, 3);

U = double(U); %Work in double precision instead of uint8.

[M, N] = size(Y); %Image size is N columns by M rows.
%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%


%Linear interpolation without Anti-Aliasing filter:
%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%
%Horizontal down-sampling U plane using Linear interpolation (without Anti-Aliasing filter).
%Simple averaging is equivalent to linear interpolation.
U2 = (U(:, 1:2:end) + U(:, 2:2:end))/2;
refU2 = imresize(U, [M, N/2], 'bilinear', 'Antialiasing', false); %Use MATLAB imresize function as reference
disp(['Linear interpolation max diff = ' num2str(max(abs(double(U2(:)) - double(refU2(:)))))]); %Print maximum difference.
%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%


%Cubic interpolation without Anti-Aliasing filter:
%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%
%Horizontal down-sampling U plane using Cubic interpolation (without Anti-Aliasing filter).
%Following operations are equivalent to cubic interpolation:
%1. Convolution with filter kernel [-0.125, 1.25, -0.125]
%2. Averaging pair elements
fU = imfilter(U, [-0.125, 1.25, -0.125], 'symmetric');
U2 = (fU(:, 1:2:end) + fU(:, 2:2:end))/2;
U2 = max(min(U2, 240), 16); %Limit to valid range of U elements (valid range of U elements in uint8 format is [16, 240])
refU2 = imresize(U, [M, N/2], 'cubic', 'Antialiasing', false); %Use MATLAB imresize function as reference
refU2 = max(min(refU2, 240), 16); %Limit to valid range of U elements
disp(['Cubic interpolation max diff = ' num2str(max(abs(double(U2(:)) - double(refU2(:)))))]); %Print maximum difference.
%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%


%Linear interpolation with Anti-Aliasing filter:
%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%
%Horizontal down-sampling U plane using Linear interpolation with Anti-Aliasing filter.
%Remark: The Anti-Aliasing filter is the filter used by MATLAB specific implementation of 'bilinear' imresize.
%Following operations are equivalent to Linear interpolation with Anti-Aliasing filter:
%1. Convolution with filter kernel [0.25, 0.5, 0.25]
%2. Averaging pair elements
fU = imfilter(U, [0.25, 0.5, 0.25], 'symmetric');
U2 = (fU(:, 1:2:end) + fU(:, 2:2:end))/2;
refU2 = imresize(U, [M, N/2], 'bilinear', 'Antialiasing', true); %Use MATLAB imresize function as reference
disp(['Linear interpolation with Anti-Aliasing max diff = ' num2str(max(abs(double(U2(:)) - double(refU2(:)))))]); %Print maximum difference.
%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%


%Cubic interpolation with Anti-Aliasing filter:
%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%
%Horizontal down-sampling U plane using Cubic interpolation with Anti-Aliasing filter.
%Remark: The Anti-Aliasing filter is the filter used by MATLAB specific implementation of 'cubic' imresize.
%Following operations are equivalent to Linear interpolation with Anti-Aliasing filter:
%1. Convolution with filter kernel [-0.0234375, -0.046875, 0.2734375, 0.59375, 0.2734375, -0.046875, -0.0234375]
%2. Averaging pair elements
h = [-0.0234375, -0.046875, 0.2734375, 0.59375, 0.2734375, -0.046875, -0.0234375];
fU = imfilter(U, h, 'symmetric');
U2 = (fU(:, 1:2:end) + fU(:, 2:2:end))/2;
U2 = max(min(U2, 240), 16); %Limit to valid range of U elements
refU2 = imresize(U, [M, N/2], 'cubic', 'Antialiasing', true); %Use MATLAB imresize function as reference
refU2 = max(min(refU2, 240), 16); %Limit to valid range of U elements
disp(['Cubic interpolation with Anti-Aliasing max diff = ' num2str(max(abs(double(U2(:)) - double(refU2(:)))))]); %Print maximum difference.
%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%


%FFMPEG implementation of horizontal down-sampling U plane.
%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%
%FFMPEG uses cubic interpolation with Anti-Aliasing filter (different filter kernel):
%Remark: I didn't check the source code of FFMPEG to verify the values of the filter kernel.
%I can't tell how FFMPEG actually implements the conversion.
%Following operations are equivalent to FFMPEG implementation (with minor differences):
%1. Convolution with filter kernel [-115, -231, 1217, 2354, 1217, -231, -115]/4096
%2. Averaging pair elements
h = [-115, -231, 1217, 2354, 1217, -231, -115]/4096;
fU = imfilter(U, h, 'symmetric');
U2 = (fU(:, 1:2:end) + fU(:, 2:2:end))/2;
U2 = max(min(U2, 240), 16); %Limit to valid range of U elements (FFMPEG actually doesn't limit the result)

%Save Y,U,V planes to file in format supported by FFMPEG
f = fopen('yuv444.yuv', 'w');
fwrite(f, Y', 'uint8');
fwrite(f, U', 'uint8');
fwrite(f, V', 'uint8');
fclose(f);

%For executing FFMPEG within MATLAB, download FFMPEG and place the executable in working directory (ffmpeg.exe for Windows)
%FFMPEG converts source file in YUV444 format to destination file in YUV422 format.
if isunix
    [status, cmdout] = system(['./ffmpeg -y -s ', num2str(N), 'x', num2str(M), ' -pix_fmt yuv444p -i yuv444.yuv -pix_fmt yuv422p yuv422.yuv']);
else
    [status, cmdout] = system(['ffmpeg.exe -y -s ', num2str(N), 'x', num2str(M), ' -pix_fmt yuv444p -i yuv444.yuv -pix_fmt yuv422p yuv422.yuv']);
end
f = fopen('yuv422.yuv', 'r');
refY = (fread(f, [N, M], '*uint8'))';
refU2 = (fread(f, [N/2, M], '*uint8'))'; %Read down-sampled U plane (FFMPEG result from file).
refV2 = (fread(f, [N/2, M], '*uint8'))';
fclose(f);

%Limit to valid range of U elements.
%In FFMPEG down-sampled U and V may exceed valid range (there is probably a way to tell FFMPEG to limit the result).
refU2 = max(min(refU2, 240), 16);

%Difference exclude first column and last column (FFMPEG treats the margins different than MATLAB)
%Remark: There are minor differences due to rounding (I guess).
disp(['FFMPEG Cubic interpolation with Anti-Aliasing max diff = ' num2str(max(max(abs(double(U2(:, 2:end-1)) - double(refU2(:, 2:end-1))))))]);
%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%

---

Examples for different kind of down-sampling methods.
Linear interpolation versus Cubic interpolation with Anti-Aliasing filter:
In the first example (mandrill) there are no visible differences.
In the second example (circle and rectangle) there are minor visible differences.
The third example (lines) demonstrates aliasing artifacts.
Remark: displayed images where up-sampled from YUV422 to YUV444 using Cubic interpolation and converted from YUV444 to RGB.

• Linear interpolation versus Cubic with Anti-Aliasing (mandrill):
  

• Linear interpolation versus Cubic with Anti-Aliasing (circle and rectangle):
  

• Linear interpolation versus Cubic with Anti-Aliasing (demonstrates Aliasing artifacts):