The rectangular area of the picture, usually 8 x 8 pixels in size, which are individually subjected to DCT coding as part of a digital picture compression process.
When performing block-based discrete cosine transform (DCT) coding for quantization, as in JPEG-compressed images, several types of artifacts can appear.
Blockiness in "busy" regions (block boundary artifacts, sometimes called macro blocking, quilting, or checkerboarding)
Other lossy algorithms, which use pattern matching to deduplicate similar symbols, are prone to introducing hard to detect errors in the printed text. For example, the numbers "6" and "8" may get replaced. This has been observed to happen with JBIG2 in certain photocopier machines.
Block coding artifacts in a JPEG image. Flat blocks are caused by coarse quantization. Discontinuities at transform block boundaries are visible.
At low bit rates, any lossy block-based coding scheme introduces visible artifacts in pixel blocks and at block boundaries. These boundaries can transform block boundaries, prediction block boundaries, or both, and may coincide with macroblock boundaries. The term macro blocking is commonly used regardless of the artifact's cause. Other names include tiling, mosaicing, pixelating, quilting, and checkerboarding.
Block-artifacts are a result of the very principle of block transform coding. 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. The lower the bit rate, the more coarsely the coefficients are represented and the more coefficients are quantized to zero. 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. 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.
Because this quantization process is applied individually in each block, neighboring blocks quantize coefficients differently. This leads to discontinuities at the block boundaries. These are most visible in flat areas, where there is little detail to mask the effect.