Compression Quality Factors

You can specify a quality factor when saving an image file to LEAD, JFIF, LEAD2JFIF, LEAD2JFIF, JTIF, LEAD1JTIF, and LEAD2JTIF. The quality factor (Q factor) is a number that determines the degree of loss in the compression process. You can set a value from 2 to 255, where 2 is the highest quality and 255 is the most compression.

For JFIF and JTIF compression only, you can also use a factor of 0 to produce lossless JPEG files.

NOTE: JFIF  4:1:1 and 4:2:2 formats use subsampling for the color components. In the case of 411, the color components for 4 pixels are averaged during compression. This will cause a color shift, but the shift is tolerable for low compression ratios. If you have high compression and repeated savings, then the color shift will increase.  Due to inherent limitations of the JPEG algorithm, the only ways to avoid this are: (a) avoid repeated load and resave, or (b) use 4:4:4  format, which has no subsampling.

For LEAD CMP compression only, you can also use one of the following enhanced options:

Q Factor

Predefined options for LEAD only

PQ1

Perfect quality 1

This option compresses all colors of a 24-bit file and maintains the quality of the original image at a smaller file size. As the name suggests, there is no visual loss of color at this setting.

PQ2

Perfect quality 2

This will produce a smaller file size than PQ1 while maintaining similar quality.

QFS

Quality far more important than size

This option removes colors not noticed by the human eye. Most viewers cannot tell the difference between an image compressed at this level and the original image.

QMS

Quality more important than size

While more colors are removed, most of them are colors or shades that are close to one another. The compressed image will appear very much like the original image. It will take some study to see the difference.

QS

Quality and size equally important

This option provides the highest compression while maintaining good image quality. Try this option first, then move up or down the list accordingly to obtain acceptable compression size and image quality.

SQS

Size more important that quality (sharp)

This option is recommended for images that have sharp edges. For example, an image that has a text overlay or an image with many straight lines. While it creates a larger file than the next option, it retains better edges.

SQT

Size more important than quality (less tiling)

This option produces less tiling in the image than the previous options. However, it may distort text or straight lines. It is a good option for image database applications.

MCQ

Maximum compression keeping quality

This option provides maximum compression while maintaining as high as possible image quality.

MC

Maximum compression

This option gives maximum compression. It is equivalent to a Q factor of 255.

For the PNG format, the qFactor can be a value between 0 and 9. 0 = (default compression), 1 (fastest compression) ... 9 (highest compression ratio)

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