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 is 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 (8-bit, 12-bit and 24-bit only), you can also use one of the following enhanced options:

Q Factor	Predefined options for LEAD only
PQ1	(-1) 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	(-2) Perfect quality 2 This will produce a smaller file size than PQ1 while maintaining similar quality.
QFS	(-3) 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	(-4) 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	(-5) 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	(-6) 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	(-7) 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	(-8) Maximum compression keeping quality This option provides maximum compression while maintaining as high as possible image quality.
MC	(-9) 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).

For JP2 and J2K files, the qFactor can be a value between –1 and 255. –1, and 0 represent lossless compression, while all the values between 1 and 255 are considered as compression ratios. For example a qFactor of 10 is a compression ratio of 10. A qFactor of 1 gives the best lossy quality, while a qFactor of 255 gives the highest compression.

If the compression control used by the toolkit when saving J2K files is J2K_COMPRESSION_QFACTOR, then the compression used is based on the Qfactor passed to the L_SaveXXX function. For more information about setting the options for saving J2K files, refer to the FILEJ2KOPTIONS structure, L_GetJ2KOptions, or L_SetJ2KOptions.

For the ABC compression (FILE_ABC and FILE_TIF_ABC), the qFactor can be one of the following values:

Q Factor	Predefined options for LEAD only
ABCQ_LOSSLESS	[0] Lossless compression. This option compresses a 1-bit file and maintains image data unchanged. Highest quality.
ABCQ_VIRTUALLOSSLESS	[1] Lossy compression. This option removes the image's noisy pixels and compresses it to a smaller file size than ABCQ_LOSSLESS.
ABCQ_REMOVEBORDER	[2] Lossy compression. This option removes the image's border, if it exists, and compresses it to a smaller file size than ABCQ_ VIRTUALLOSSLESS.
ABCQ_ENHANCE	[3] Lossy compression. This option cleans up the image, removes its border if it exists, and compresses it to a smaller file size than ABCQ_REMOVEBORDER.
ABCQ_MODIFIED1	[4] Lossy compression. This option cleans up the image, removes its border if it exists, and compresses it to a smaller file size than ABCQ_ENHANCE. However, it may distort text or straight lines.
ABCQ_MODIFIED1_FAST	[5] Same as ABCQ_MODIFIED1 without the border remove or image clean processes.
ABCQ_MODIFIED2	[6] Lossy compression. This option cleans up the image, removes its border if it exists, and compresses it to a smaller file size than ABCQ_MODIFIED1. However, it may distort text or straight lines. Maximum compression.
ABCQ_MODIFIED2_FAST	[7] Same as ABCQ_MODIFIED2 without the border remove or image clean processes.
ABCQ_MODIFIED3	[8] Lossy compression. Provides faster encode and decode speed than ABCQ_MODIFIED1 or ABCQ_MODIFIED2. Provides the maximum compression.
ABCQ_MODIFIED3_FAST	[9] Same as ABCQ_MODIFIED3 without the border remove or image clean processes.
ABCQ_LOSSLESS_FAST	[10] Faster lossless compression than ABCQ_LOSSLESS. This option compresses a 1-bit file and maintains image data unchanged. Highest quality. Does not produce compression ratios as high as ABCQ_LOSSLESS.
ABCQ_LOSSY_FAST	[11] Lossy compression. This is the fastest ABC lossy compression option. Does not produce compression ratios as high as the other lossy options.