Understanding Chroma Subsampling

07 Sep 2025 in Blog on Website, Jekyll

Chroma Subsampling animation

Speed 120 ms/pixel

Show boundaries Show calculations

RGB

Difference

Y (orig)

U (orig)

V (orig)

YUV 4:4:4 → RGB

Diff vs Orig

Y (4:2:2)

U (4:2:2)

V (4:2:2)

YUV 4:2:2 → RGB

Diff vs Orig

Y (4:2:0)

U (4:2:0)

V (4:2:0)

YUV 4:2:0 → RGB

Diff vs Orig

How RGB ↔ YUV Conversion Works

Step 1: RGB → YUV

For each pixel (r, g, b):

      Y = 0.299r + 0.587g + 0.114b
      U = -0.169r - 0.331g + 0.500b + 128
      V =  0.500r - 0.419g - 0.081b + 128

So every pixel gets its own Y, U, and V values.

Step 2: YUV 4:4:4

Here, all pixels keep their individual Y, U, V values. When converting back, you get the same RGB values — no quality loss.

Step 3: YUV 4:2:2 (horizontal subsampling)

Instead of storing U and V for every pixel, we share them between two horizontal neighbors.

      U₄₂₂ = (U₀ + U₁) / 2
      V₄₂₂ = (V₀ + V₁) / 2

Each pixel still has its own Y, but both share the same U, V values → horizontal chroma detail is reduced.

Step 4: YUV 4:2:0 (horizontal + vertical subsampling)

Now we average U and V across a 2×2 block of pixels:

      U₄₂₀ = (U₀₀ + U₀₁ + U₁₀ + U₁₁) / 4
      V₄₂₀ = (V₀₀ + V₀₁ + V₁₀ + V₁₁) / 4

Each pixel in the block keeps its own Y, but they all share the same U, V → both horizontal and vertical chroma detail are reduced.

Step 5: YUV → RGB

When reconstructing, we combine each pixel’s Y with the shared U, V:

      R = Y + 1.402 (V - 128)
      G = Y - 0.344136 (U - 128) - 0.714136 (V - 128)
      B = Y + 1.772 (U - 128)

This gives the final RGB value used to draw the pixel.