Question

Draw block diagram of the JPEG default encoder for encoding still colour image.

Describe progressive and hierachical mode of JPEG compression standard, respectively

Describe how image artifacts occur in decoded still images

Answer 1

JPEG Compression Modes:

The JPEG standard defined four compression modes: Hierarchical, Progressive, Sequential and lossless. Below figure shows the relationship of major JPEG compression modes and encoding processes.

1. Sequential: Sequential-mode images are encoded from top to bottom. Sequential mode supports sample data with 8 and 12 bits of precision. In the sequential JPEG, each color component is completely encoded in single scan. Within sequential mode, two alternate entropy encoding processes are defined by the JPEG standard: one uses Huffman encoding; the other uses arithmetic coding.
2. Progressive: In progressive JPEG images, components are encoded in multiple scans. The compressed data for each component is placed in a minimum of 2 and as many as 896 scans. The initial scans create a rough version of the image, while subsequent scans refine it.
3. Lossless: preserves exact, original image, small compression ration, less use
4. Hierarchical: JPEG is a super-progressive mode in which the image Is broken down into a number of subimages called frames. A frame is a collection of one or more scans. In hierarchical mode, the first frame creates a low-resolution version of image. The remaining frames refine the image by increasing the solution.

A compression artifact (or artefact) is a noticeable distortion of media (including images, audio, and video) caused by the application of lossy compression.

Lossy data compression involves discarding some of the media's data so that it becomes simplified enough to be stored within the desired disk space or be transmitted (or streamed) within the bandwidth limitations (known as a data rate or bit rate for media that is streamed). If the compressor could not reproduce enough data in the compressed version to reproduce the original, the result is a diminishing of quality, or introduction of artifacts. Alternatively, the compression algorithm may not be intelligent enough to discriminate between distortions of little subjective importance and those objectionable to the viewer.

Compression artifacts occur in many common media such as DVDs, common computer file formats such as JPEG, MP3, or MPEG files, and some alternatives to the compact disc, such as Sony's MiniDisc format. Uncompressed media (such as on Laserdiscs, Audio CDs, and WAV files) or losslessly compressed media (such as FLAC or PNG) do not suffer from compression artifacts.

The minimization of perceivable artifacts is a key goal in implementing a lossy compression algorithm. However, artifacts are occasionally intentionally produced for artistic purposes, a style known as glitch art^[ or datamoshing.

Technically speaking, a compression artifact is a particular class of data error that is usually the consequence of quantization in lossy data compression. Where transform coding is used, they typically assume the form of one of the basis functions of the coder's transform space.