1. RGB and YUV

First understand why video encoding uses YUV420 instead of rgb. Then you need to understand the principles of the two, look at the picture and talk

Therefore, from the memory point of view, the yuv unit pixel uses lower memory, but the effect of the two representations is the same. It can also be considered that rgb is overdrawn, and the area that the human eye cannot distinguish is also drawn.

Let’s take a look at the temporary memory usage comparison between the two. Here we use the ·rgb24 comparison

RGB24 uses 24 bits to represent a pixel, and RGB components are represented by 8 bits, with a value range of 0-255. In a 2*2 pixel area, the number of bytes temporarily used by RRG is 2*2*3=12 bytes. Then expressed by yuv, the number of bytes occupied is 4(Y)+1(u)+1(v)=6 bytes, where Y occupies 4 bytes, U and V each occupy 1 byte, the ratio is 4 :1:1

So on a device with a width and height of w*h,Use rgb to indicate that the number of bytes occupied by the encoding is w*h*3, and use yuv to indicate that the temporarily used memory is w*h*+w*h/4+w*h/4 = w*h*3/2.

2. YUV and NV21

The first thing to understand is that yuv has many encoding formats, of which yuv420 is one type, and nv21 is another type of yuv420. And nv21 is a video encoding for android devices.

nv21 encoding format: For example, a 1920*1280 picture, after nv21 encoding, will become 1920*1280 bytes in front of it are all Y, starting from 1920*1280 bytes in length, U and V will be arranged alternately, their words The section length is 1920*1280/4 respectively. Please see the picture below

As shown in the color box, 4 Ys will correspond to one UV.

Three, NV21 and I420

I420 is also a kind of YUV420 encoding format. Due to the reasons of android mobile phone manufacturers, the data collected by the camera is always NV21 encoded data, but for this kind of data cannot be displayed on the Apple or Windows platform, then this encoding format is required The data needs to be re-encoded, and all manufacturers are adapted to the I420 encoding format.

I420 encoding format: For example, a 1920*1280 picture, after I420 encoding, will become 1920*1280 bytes in front of it are all Y, starting from 1920*1280 bytes, U will be arranged first, and the total byte length is 1920*1280/4, arrange V starting from 1920*1280+1920*1280/4, and the byte length is 1920*1280/4, so the total byte length is the same as NV21, but the encoding order of UV is different. Please see the picture below: