Editing JPEG (section)

==Effects of JPEG compression==
JPEG compression artifacts blend well into photographs with detailed non-uniform textures, allowing higher compression ratios. Notice how a higher compression ratio first affects the high-frequency textures in the upper-left corner of the image, and how the contrasting lines become more fuzzy. The very high compression ratio severely affects the quality of the image, although the overall colors and image form are still recognizable. However, the precision of colors suffer less (for a human eye) than the precision of contours (based on luminance). This justifies the fact that images should be first transformed in a color model separating the luminance from the chromatic information, before subsampling the chromatic planes (which may also use lower quality quantization) in order to preserve the precision of the luminance plane with more information bits.

===Sample photographs===

[[File:Visual impact of a jpeg compression on Photoshop.jpg|thumb|Visual impact of a jpeg compression on Photoshop on a picture of 4480x4480 pixels]]

For information, the uncompressed 24-bit RGB bitmap image below (73,242 pixels) would require 219,726 bytes (excluding all other information headers). The filesizes indicated below include the internal JPEG information headers and some [[metadata]]. For highest quality images (Q=100), about 8.25 bits per color pixel is required. On grayscale images, a minimum of 6.5 bits per pixel is enough (a comparable Q=100 quality color information requires about 25% more encoded bits). The highest quality image below (Q=100) is encoded at nine bits per color pixel, the medium quality image (Q=25) uses one bit per color pixel. For most applications, the quality factor should not go below 0.75 bit per pixel (Q=12.5), as demonstrated by the low quality image. The image at lowest quality uses only 0.13 bit per pixel, and displays very poor color. This is useful when the image will be displayed in a significantly scaled-down size. A method for creating better quantization matrices for a given image quality using [[PSNR]] instead of the Q factor is described in Minguillón & Pujol (2001).<ref>{{cite journal|author=Julià Minguillón, Jaume Pujol|title=JPEG standard uniform quantization error modeling with applications to sequential and progressive operation modes|journal=Electronic Imaging|date=April 2001|volume=10|number=2|pages=475–485|hdl=10609/6263|bibcode=2001JEI....10..475M|doi=10.1117/1.1344592|s2cid=16629522 |url=http://openaccess.uoc.edu/webapps/o2/bitstream/10609/6263/6/jei-jpeg.pdf|access-date=2019-09-23|archive-date=2020-08-03|archive-url=https://web.archive.org/web/20200803042010/http://openaccess.uoc.edu/webapps/o2/bitstream/10609/6263/6/jei-jpeg.pdf|url-status=live}}</ref>

::{| class="wikitable"
|+ align="bottom"| Note: The above images are not [[Institute of Electrical and Electronics Engineers|IEEE]] / [[ITU-R|CCIR]] / [[European Broadcasting Union|EBU]]&nbsp;[[standard test image|test images]], and the encoder settings are not specified or available.
|-
! Image !! Quality !! Size (bytes) !! Compression ratio !! Comment
|-
| [[File:JPEG example JPG RIP 100.jpg|120px]]
| Highest quality (Q&nbsp;=&nbsp;100)
| 81,447
| 2.7:1
| Extremely minor artifacts
|-
| [[File:JPEG example JPG RIP 050.jpg|120px]]
| High quality (Q&nbsp;=&nbsp;50)
| 14,679
| 15:1
| Initial signs of subimage artifacts
|-
| [[File:JPEG example JPG RIP 025.jpg|120px]]
| Medium quality (Q&nbsp;=&nbsp;25)
| 9,407
| 23:1
| Stronger artifacts; loss of high frequency information
|-
| [[File:JPEG example JPG RIP 010.jpg|120px]]
| Low quality (Q&nbsp;=&nbsp;10)
| 4,787
| 46:1
| Severe high frequency loss leads to obvious artifacts on subimage boundaries ("macroblocking")
|-
| [[File:JPEG example JPG RIP 001.jpg|120px]]
| Lowest quality (Q&nbsp;=&nbsp;1)
| 1,523
| 144:1
| Extreme loss of color and detail; the leaves are nearly unrecognizable.
|}

The medium quality photo uses only 4.3% of the storage space required for the uncompressed image, but has little noticeable loss of detail or visible artifacts. However, once a certain threshold of compression is passed, compressed images show increasingly visible defects. See the article on [[rate–distortion theory]] for a mathematical explanation of this threshold effect. A particular limitation of JPEG in this regard is its non-overlapped 8×8 block transform structure. More modern designs such as [[JPEG 2000]] and [[JPEG XR]] exhibit a more graceful degradation of quality as the bit usage decreases&nbsp;– by using transforms with a larger spatial extent for the lower frequency coefficients and by using overlapping transform basis functions.