YUV and image-format examples

A raw frame's size depends only on resolution, sampling and bit depth. For an 8-bit 1920×1080 frame:

frame_bytes = width × height × (bytes_per_pixel)

Notice that 10-bit 4:2:0 needs the same space as 8-bit 4:4:4: each sample now occupies two bytes, but there are still only 1.5 samples per pixel.

YUY2 stores two horizontally adjacent pixels in four bytes: Y0 U Y1 V. To read pixel (x, y) in a frame of width W:

stride = W × 2 row = stride × y pair = (x - (x mod 2)) / 2 base = row + pair × 4 Y = base + (x mod 2) × 2 U = base + 1 V = base + 3

Worked example — pixel (100, 50) in a 1920-wide YUY2 frame:

stride = 1920 × 2 = 3840 row = 3840 × 50 = 192000 pair = 100 / 2 = 50 base = 192000 + 50 × 4 = 192200 Y at 192200 · U at 192201 · V at 192203

Pixel 100 is even, so its Y sits at base + 0; pixel 101 would use base + 2 and share the same U and V.

Take a 4-pixel-wide, 2-row block. Each cell below is one sample. Y is always full resolution; the difference is how many U and V samples remain.

The dots are samples that were never stored; the decoder fills them in by copying from a neighbour. That is why 4:2:0 saves half the bytes of 4:4:4.

Given a tiny 2×2 reference and a distorted version, PSNR is computed in four steps.

reference = [100 110 | 120 130] distorted = [102 108 | 119 134] errors = [ +2 -2 | -1 +4] squared = [ 4 4 | 1 16] MSE = (4 + 4 + 1 + 16) / 4 = 6.25 PSNR = 10·log10(255² / MSE) = 10·log10(65025 / 6.25) = 10·log10(10404) ≈ 40.17 dB

A result around 40 dB for natural content usually means the difference is hard to see; below about 30 dB it is often visible. See PSNR, SSIM and your eyes for the caveats.

For a 4×2 frame, here is the order in which the first samples are written to memory in two common layouts.

I420 (planar, 4:2:0): Y0 Y1 Y2 Y3 | Y4 Y5 Y6 Y7 U0 U1 | V0 V1 YUY2 (packed, 4:2:2): Y0 U0 Y1 V0 | Y2 U1 Y3 V1 Y4 U2 Y5 V2 | Y6 U3 Y7 V3

Same picture, different order. A decoder that expects one layout and receives the other produces colour-swapped or sheared output. More in Planar vs. packed YUV.