Resize (downsize) YUV420sp image

Question

I am trying to resize (scale down) an image which comes in YUV420sp format. Is it possible to do such image resizing without converting it into RGB, so directly manipulating

Answer 1

YUV 4:2:0 planar looks like this:

----------------------
|     Y      | Cb|Cr |
----------------------

where:

Y = width x height pixels
Cb = Y / 4 pixels
Cr = Y / 4 pixels

Total num pixels (bytes) = width * height * 3 / 2

And the subsamling used like this:

Which means that each chroma-pixel-value is shared between 4 luma-pixels.

One approach is just to remove pixels, making sure that corresponding Y-Cb-Cr relationship are kept/recalculated.

Something close to the Nearest-neighbor interpolation but reversed.

Another approach is to first convert the 4:2:0 subsampling to 4:4:4

Here you have a 1 to 1 mapping between luma and chroma data.

This is the correct way to interpolate chroma between 4:2:0 and 4:2:2 (luma is already at correct resolution) Code in python, follow html-link for c-dito. Code is not very pythonic, just a direct translation of the c-version.

def __conv420to422(self, src, dst):
    """
    420 to 422 - vertical 1:2 interpolation filter

    Bit-exact with
    http://www.mpeg.org/MPEG/video/mssg-free-mpeg-software.html
    """
    w = self.width >> 1
    h = self.height >> 1

    for i in xrange(w):
        for j in xrange(h):
            j2 = j << 1
            jm3 = 0 if (j<3) else j-3
            jm2 = 0 if (j<2) else j-2
            jm1 = 0 if (j<1) else j-1
            jp1 = j+1 if (j>8

            dst[i+w*j2] = pel if pel > 0 else 0
            dst[i+w*j2] = pel if pel < 255 else 255

            pel = (3*src[i+w*jp3]
                 -16*src[i+w*jp2]
                 +67*src[i+w*jp1]
                +227*src[i+w*j]
                 -32*src[i+w*jm1]
                 +7*src[i+w*jm2]+128)>>8

            dst[i+w*(j2+1)] = pel if pel > 0 else 0
            dst[i+w*(j2+1)] = pel if pel < 255 else 255
    return dst

Run this twice to get 4:4:4. Then it's just a matter of removing rows and columns.

Or you can just quadruple the chroma-pixels to go from 4:2:0 to 4:4:4, remove rows and columns and then average 4 Cb/Cr values into 1 to get back to 4:2:0 again, it all depends on how strict you need to be :-)