Lossy JPEG Compression And Lossless Operations

JPEG compression, defined by an ISO norm in 1990, was developed for the purpose of efficient storage of photographic data. Lossy compression (such as JPEG) takes advantage of the human eye's imperfection and stores data with a certain loss of information, thanks to which it achieves much higher compression than lossless compression does. The mentioned imperfections include for example a greater sensitivity towards large changes than fine details, and towards brightness changes than color changes. JPEG takes advantage of this to reduce the size of color information by recalculating color channels to a lower resolution (subsampling). The amount of detail preserved by JPEG is configurable, and is measured by a "quality level" on various scales; Zoner Photo Studio uses a scale from 1 to 100. Higher values mean less distortion and a larger output file. Smaller values mean more distortion and a smaller output file. JPEG compression settings affect only the picture's quality and file size. Its size in pixels remains unchanged.

Choose compression levels based on how you plan for the picture to be used. Use values from 30 to 60 to maximize size saving. Use values from 70 to 80 for usual purposes, e.g. for pictures published on the Internet. Use values from 80 to 100 for desktop publishing. Although higher values give more detail, this relationship is not linear. Values above 90 give nearly imperceptible improvements while markedly increasing file sizes.

Because of rounding errors in calculations, and in some cases due to color subsampling, JPEG compression causes quality less even when you use a quality value of 100. Therefore, JPEG compression is inappropriate for pictures that require absolute precision (pen drawings, line drawings, etc.). However, for photographs, it is invaluable, since with proper settings, the changes to a picture are imperceptible. The JPEG Group also created an ISO standard for lossless JPEG compression, but this standard is unused. It is outdated today, since the more modern PNG standard generally gives better results than lossless JPEG.

Lossless Operations

It is sometimes necessary to rotate pictures because they were taken in portrait orientation (with the camera turned on its side). Every time you open, edit, and resave a picture with JPEG compression, some detail is lost. Therefore, Zoner Photo Studio enables lossless transformations wherever possible, that is, for mirroring pictures and for rotating them by 90 degrees.

Lossless transformations are only possible for those JPEG images that have both dimensions divisible by a number that can in turn by obtained by multiplying the basic JPEG-block size (8) and the color channel sampling values (1 or 2). This number determines the size of the blocks used for performing JPEG compression. The typical samplings are 2:1 in both directions, or 2:1 in the horizontal direction only. Typical block dimensions are 16 × 16, 16 × 8 and 8 × 8. Cameras normally produce photos with dimensions that are multiples of these values, and thus for uncropped, unresized photos, the above-mentioned transformations are always lossless in Zoner Photo Studio .
The program's behavior when these transformations are performed on photos whose sizes do not meet this requirement depends on a setting in Settings | Preferences | General: either the picture is cropped to meet the requirement, or the transformation is lossy rather than lossless.

Lossless transformations are only possible for operations performed from the Browser.
By its nature, work in the Editor involves decompressing a picture and then recompressing it before saving.

To eliminate unneeded quality losses when resaving in the Editor, use high JPEG compression quality values such as 90 to 95. To set what values are used automatically, use Settings | Preferences | General To set the value manually while saving, use "Save As" and then the control offered during the saving process. To avoid quality loss altogether, use a format with lossless compression, like PNG or TIFF. If you need support for EXIF picture information, do not use PNG: it does not support it.