:ref:`colorspaces` for conversion examples. YUV was chosen because
early television would only transmit brightness information. To add
color in a way compatible with existing receivers a new signal carrier
-was added to transmit the color difference signals. Secondary in the YUV
-format the U and V components usually have lower resolution than the Y
-component. This is an analog video compression technique taking
-advantage of a property of the human visual system, being more sensitive
-to brightness information.
+was added to transmit the color difference signals.
+
+
+Subsampling
+===========
+
+YUV formats commonly encode images with a lower resolution for the chroma
+components than for the luma component. This compression technique, taking
+advantage of the human eye being more sensitive to luminance than color
+differences, is called chroma subsampling.
+
+While many combinations of subsampling factors in the horizontal and vertical
+direction are possible, common factors are 1 (no subsampling), 2 and 4, with
+horizontal subsampling always larger than or equal to vertical subsampling.
+Common combinations are named as follows.
+
+- `4:4:4`: No subsampling
+- `4:2:2`: Horizontal subsampling by 2, no vertical subsampling
+- `4:2:0`: Horizontal subsampling by 2, vertical subsampling by 2
+- `4:1:1`: Horizontal subsampling by 4, no vertical subsampling
+- `4:1:0`: Horizontal subsampling by 4, vertical subsampling by 4
+
+Subsampling the chroma component effectively creates chroma values that can be
+located in different spatial locations:
+
+- .. _yuv-chroma-centered:
+
+ The subsampled chroma value may be calculated by simply averaging the chroma
+ value of two consecutive pixels. It effectively models the chroma of a pixel
+ sited between the two original pixels. This is referred to as centered or
+ interstitially sited chroma.
+
+- .. _yuv-chroma-cosited:
+
+ The other option is to subsample chroma values in a way that place them in
+ the same spatial sites as the pixels. This may be performed by skipping every
+ other chroma sample (creating aliasing artifacts), or with filters using an
+ odd number of taps. This is referred to as co-sited chroma.
+
+The following examples show different combination of chroma siting in a 4x4
+image.
+
+.. flat-table:: 4:2:2 subsampling, interstitially sited
+ :header-rows: 1
+ :stub-columns: 1
+
+ * -
+ - 0
+ -
+ - 1
+ -
+ - 2
+ -
+ - 3
+ * - 0
+ - Y
+ - C
+ - Y
+ -
+ - Y
+ - C
+ - Y
+ * - 1
+ - Y
+ - C
+ - Y
+ -
+ - Y
+ - C
+ - Y
+ * - 2
+ - Y
+ - C
+ - Y
+ -
+ - Y
+ - C
+ - Y
+ * - 3
+ - Y
+ - C
+ - Y
+ -
+ - Y
+ - C
+ - Y
+
+.. flat-table:: 4:2:2 subsampling, co-sited
+ :header-rows: 1
+ :stub-columns: 1
+
+ * -
+ - 0
+ -
+ - 1
+ -
+ - 2
+ -
+ - 3
+ * - 0
+ - Y/C
+ -
+ - Y
+ -
+ - Y/C
+ -
+ - Y
+ * - 1
+ - Y/C
+ -
+ - Y
+ -
+ - Y/C
+ -
+ - Y
+ * - 2
+ - Y/C
+ -
+ - Y
+ -
+ - Y/C
+ -
+ - Y
+ * - 3
+ - Y/C
+ -
+ - Y
+ -
+ - Y/C
+ -
+ - Y
+
+.. flat-table:: 4:2:0 subsampling, horizontally interstitially sited, vertically co-sited
+ :header-rows: 1
+ :stub-columns: 1
+
+ * -
+ - 0
+ -
+ - 1
+ -
+ - 2
+ -
+ - 3
+ * - 0
+ - Y
+ - C
+ - Y
+ -
+ - Y
+ - C
+ - Y
+ * - 1
+ - Y
+ -
+ - Y
+ -
+ - Y
+ -
+ - Y
+ * - 2
+ - Y
+ - C
+ - Y
+ -
+ - Y
+ - C
+ - Y
+ * - 3
+ - Y
+ -
+ - Y
+ -
+ - Y
+ -
+ - Y
+
+.. flat-table:: 4:1:0 subsampling, horizontally and vertically interstitially sited
+ :header-rows: 1
+ :stub-columns: 1
+
+ * -
+ - 0
+ -
+ - 1
+ -
+ - 2
+ -
+ - 3
+ * - 0
+ - Y
+ -
+ - Y
+ -
+ - Y
+ -
+ - Y
+ * -
+ -
+ -
+ -
+ -
+ -
+ -
+ -
+ * - 1
+ - Y
+ -
+ - Y
+ -
+ - Y
+ -
+ - Y
+ * -
+ -
+ -
+ -
+ - C
+ -
+ -
+ -
+ * - 2
+ - Y
+ -
+ - Y
+ -
+ - Y
+ -
+ - Y
+ * -
+ -
+ -
+ -
+ -
+ -
+ -
+ -
+ * - 3
+ - Y
+ -
+ - Y
+ -
+ - Y
+ -
+ - Y
.. toctree::