Various approaches have been proposed to reduce image compression effects, but to use standardized compression/decompression techniques and retain the benefits of compression (for instance, lower transmission and storage costs), many of these methods focus on 'post-processing'-that is, processing images when received or viewed. These are most visible in flat areas, where there is little detail to mask the effect. This leads to discontinuities at the block boundaries. In the most extreme case only the DC-coefficient, that is the coefficient which represents the average color of a block, is retained, and the transform block is only a single color after reconstruction.īecause this quantization process is applied individually in each block, neighboring blocks quantize coefficients differently. Statistically, images have more low- frequency than high-frequency content, so it is the low-frequency content that remains after quantization, which results in blurry, low-resolution blocks. The lower the bit rate, the more coarsely the coefficients are represented and the more coefficients are quantized to zero. The transform (for example the discrete cosine transform) is applied to a block of pixels, and to achieve lossy compression, the transform coefficients of each block are quantized. Other names include tiling, mosaicing, pixelating, quilting, and checkerboarding.īlock-artifacts are a result of the very principle of block transform coding. The term macroblocking is commonly used regardless of the artifact's cause. These boundaries can transform block boundaries, prediction block boundaries, or both, and may coincide with macroblock boundaries. Discontinuities at transform block boundaries are visible.Īt low bit rates, any lossy block-based coding scheme introduces visible artifacts in pixel blocks and at block boundaries. Flat blocks are caused by coarse quantization. Block boundary artifacts īlock coding artifacts in a JPEG image. This has been observed to happen with JBIG2 in certain photocopier machines. For example, the numbers '6' and '8' may get replaced. Other lossy algorithms, which use pattern matching to deduplicate similar symbols, are prone to introducing hard to detect errors in printed text. Blockiness in 'busy' regions (block boundary artifacts, sometimes called macroblocking, quilting, or checkerboarding).Staircase noise ( aliasing) along curving edges.When performing block-based discrete cosine transform (DCT) coding for quantization, as in JPEG-compressed images, several types of artifacts can appear. Both frames were then zoomed by a factor of 4 (nearest neighbor interpolation). One frame of the animation was saved as a JPEG (quality 90) and reloaded. Text is a screen capture from a Wikipedia conversation with noise added (intensity 10 in Paint.NET). Illustration of the effect of JPEG compression on a slightly noisy image with a mixture of text and whitespace. Where transform coding is used, it typically assumes the form of one of the basis functions of the coder's transform space. Technically speaking, a compression artifact is a particular class of data error that is usually the consequence of quantization in lossy data compression. However, artifacts are occasionally intentionally produced for artistic purposes, a style known as glitch art or datamoshing. The minimization of perceivable artifacts is a key goal in implementing a lossy compression algorithm. 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. These compression artifacts appear when heavy compression is applied, and occur often in common digital media, such as DVDs, common computer file formats such as JPEG, MP3 and MPEG files, and some alternatives to the compact disc, such as Sony's MiniDisc format. The most common digital compression artifacts are DCT blocks, caused by the discrete cosine transform (DCT) compression algorithm used in many digital media standards, such as JPEG, MP3, and MPEG video file formats.

The compression algorithm may not be intelligent enough to discriminate between distortions of little subjective importance and those objectionable to the user. If the compressor cannot store enough data in the compressed version, the result is a loss of quality, or introduction of artifacts. Lossy data compression involves discarding some of the media's data so that it becomes small enough to be stored within the desired disk space or transmitted ( streamed) within the available bandwidth (known as the data rate or bit rate). Loss of edge clarity and tone 'fuzziness' in heavy JPEG compressionĪ compression artifact (or artefact) is a noticeable distortion of media (including images, audio, and video) caused by the application of lossy compression.

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