The basic processing unit of the design is called a
macroblock, and H.261 was the first standard in which the macroblock concept appeared. Each macroblock consists of a 16×16 array of
luma samples and two corresponding 8×8 arrays of
chroma samples, using
4:2:0 sampling and a
YCbCr color space. The coding algorithm uses a hybrid of
motion-compensated inter-picture prediction and spatial
transform coding with
scalar quantization, zig-zag scanning and
entropy encoding. The inter-picture prediction reduces temporal redundancy, with
motion vectors used to compensate for motion. Whilst only integer-valued motion vectors are supported in H.261, a blurring filter can be applied to the prediction signal – partially mitigating the lack of fractional-sample motion vector precision. Transform coding using an 8×8
discrete cosine transform (DCT) reduces the spatial redundancy. The DCT that is widely used in this regard was introduced by
N. Ahmed, T. Natarajan and
K. R. Rao in 1974. Scalar quantization is then applied to round the transform coefficients to the appropriate precision determined by a step size control parameter, and the quantized transform coefficients are zig-zag scanned and entropy-coded (using a "
run-level"
variable-length code) to remove statistical redundancy. The H.261 standard actually only specifies how to decode the video. Encoder designers were left free to design their own encoding algorithms (such as their own
motion estimation algorithms), as long as their output was constrained properly to allow it to be decoded by any decoder made according to the standard. Encoders are also left free to perform any pre-processing they want to their input video, and decoders are allowed to perform any post-processing they want to their decoded video prior to display. One effective post-processing technique that became a key element of the best H.261-based systems is called
deblocking filtering. This reduces the appearance of block-shaped artifacts caused by the block-based
motion compensation and spatial transform parts of the design. Indeed, blocking artifacts are probably a familiar phenomenon to almost everyone who has watched digital video. Deblocking filtering has since become an integral part of the more recent standards
H.264 and
HEVC (although even when using these newer standards, additional post-processing is still allowed and can enhance visual quality if performed well). Design refinements introduced in later standardization efforts have resulted in significant improvements in compression capability relative to the H.261 design. This has resulted in H.261 becoming essentially obsolete, although it is still used as a backward-compatibility mode in some video-conferencing systems (such as
H.323) and for some types of internet video. However, H.261 remains a major historical milestone in the field of video coding development. ==Software implementations==