YUV color spacing

Question 1

According to this site, YV12 looks the same as NV12 except that NV12 is interleaved and YV12 is not. I think that YV12 correspond to YVU420p and NV12 is YVU420sp, but I'm not 100% sure about that.

What this means however, is that instead of having triplets [yuv][yuv][yuv][...] repeated all over your buffer to draw the image, one triplet per pixel, you have a large buffer of [y], then a buffer 1/4 of the [y] size containing all the [u], then another one 1/4 of [y] containing the [v]. For an image of 320x240 (76800 pixels), you will have 76800 Ys at the start of your buffer, then 19200 Us, then 19200 Vs.

In the NV12 case, you have a large buffer of [y], then a buffer 1/2 of the [y] size containing [uv] pairs. When drawing the image, or converting to another format such as RGB, you need a least 2 pointers, one that will read Ys and another one that will read the Us and Vs

When reading such as compressed format, the trick is that you have one Y per pixel, but the UV components change only on odd rows and odd columns and your eye will not see the difference. You reuse the same UV values for 4 pixels at a time while changing the Y for every pixel.

Question 2

YUV is widely used in video processing, what you are talking about is chroma sub-sampling. The idea behind this, is that human eye is less sensitive to change in color, than to change in brightness.

You can find very thorough explanation of the process on wiki: https://en.wikipedia.org/wiki/Chroma_subsampling

Here is image of various chroma sub-sampling schemes used: http://commons.wikimedia.org/wiki/File:Common_chroma_subsampling_ratios.svg