**3.2 Artifacts**

Artifacts are defined as visible differences due to some technical limitation that presents an image. These effects occur in the process of production of video signal, in the phases of capture, compression, transmission, reception and delivery to the final recipient, the displayed picture may differ from the original.

They appear both in analogue and digital systems, but we will focus on the artifacts derived from the digital ones, especially because of video compression in the process of encoding and decoding.

Most common artifacts are caused by three reasons:


The blocking effect, also known as tiling or blockiness, refers to a block pattern in the

compressed sequence. It is due to the independent quantization of individual blocks. It appears especially on compression standards that utilises macroblocks of a certain size when performing a transform. A high impaired image presents artificial horizontal and vertical edges, clearly visible, parallel to the picture frame.

High compression, which reduces bitrate, in DCT-based encoding such as MPEG-2 or H.263 or similar, macroblocks with less information create homogeneous values in block pixels with boundaries in their edges, as a result of truncation of coefficients. Advanced codecs such as H.264 utilizes deblocking filters to reduce the visibility of this artifact. This effect is easy distinguished in next example of "Nasa" sequence and its tiling diagram.

For a given quantization level, block distortion is usually more visible in smoother areas of the picture

Fig. 2. Example of sequence high blocking effect

Blur

Blur artifact is defined as a loss of energy and spatial detail as a reduction of edge sharpness, because of suppression of the high-frequency coefficients by coarse quantization. Blurring is

Artifacts are defined as visible differences due to some technical limitation that presents an image. These effects occur in the process of production of video signal, in the phases of capture, compression, transmission, reception and delivery to the final recipient, the

They appear both in analogue and digital systems, but we will focus on the artifacts derived from the digital ones, especially because of video compression in the process of encoding

Artifacts due to analog and digital formats, its relationship and conversions between

compressed sequence. It is due to the independent quantization of individual blocks. It appears especially on compression standards that utilises macroblocks of a certain size when performing a transform. A high impaired image presents artificial horizontal and

High compression, which reduces bitrate, in DCT-based encoding such as MPEG-2 or H.263 or similar, macroblocks with less information create homogeneous values in block pixels with boundaries in their edges, as a result of truncation of coefficients. Advanced codecs such as H.264 utilizes deblocking filters to reduce the visibility of this artifact. This effect is

For a given quantization level, block distortion is usually more visible in smoother areas of

Blur artifact is defined as a loss of energy and spatial detail as a reduction of edge sharpness, because of suppression of the high-frequency coefficients by coarse quantization. Blurring is

 Artifacts due to coding and compression (block distortion, blurring and ringing). Artifacts due to transmission channel errors (errored blocks by lost packets).

The blocking effect, also known as tiling or blockiness, refers to a block pattern in the

easy distinguished in next example of "Nasa" sequence and its tiling diagram.

**3.2 Artifacts** 

and decoding.

Blocking effect

the picture

Blur

displayed picture may differ from the original.

Most common artifacts are caused by three reasons:

vertical edges, clearly visible, parallel to the picture frame.

Fig. 2. Example of sequence high blocking effect

them (noise and blurring).

also generated when reducing the bitrate of encoding. As this effect occurs in high frequencies, it is more visible in sequences with higher spatial complexity.

Next example of "Nasa" test sequence shows the consequences of reducing the bitrate, from the original one on the left to a high reduction seen on right image.

Fig. 3. Evolution of blurriness when reducing bitrate
