When compressing images there is a tradeoff between the degree of compression achieved and the quality of the resulting image. The highest rates of compression can only be achieved by discarding some less important data from the image, known as lossy decompression. In the ERDAS ECW JPEG2000 SDK, the target compression ratio is an abstract figure representing your choice in this tradeoff. It approximates the likely ratio of input file size to output file size, given certain parameters for compression.
It is often unwise to try and force each image into a compressed file of the same size. This is because the resulting compressed images will have widely varying quality levels which may reduce their usefulness and any subsequent processing will have decreased value when combining different image tiles of different quality compression together.
Below is a table taken from the ERDAS IMAGINE 2010 Help (to be released later in 2011). This table was created from in-house testing and customer feedback.
|Imagery||Target Compression Ratio|
|Visually Lossless RGB Image||Crisp Image Interpretation (2x zoom)||4:1|
|Near Visually Lossless RGB Image||Clear Image Interpretation (2x zoom)||6:1|
|RGB Image||High Quality Printed Maps and typical GIS applications||15:1 to 25:1|
|RGB Image||Internet or Email Distribution||15:1 to 40:1|
|Visually Lossless Panchromatic Image||Crisp Image Interpretation (2x zoom)||2:1|
|Near Visually Lossless Panchromatic Image||Clear Image Interpretation (2x zoom)||3:1|
|Panchromatic Image||High Quality Printed Maps and typical GIS applications||10:1 to 15:1|
|Panchromatic Image||Internet or Email Distribution||15:1 to 30:1|
|Visually Lossless Multispectral Image||Crisp Image Interpretation (2x zoom)||3:1|
|Near Visually Lossless Multispectral Image||Clear Image Interpretation (2x zoom)||4:1|
|Numerically Lossless RGB, Panchromatic, and Multispectral (JPEG 2000 Only)||Imagery with perfect numeric reconstruction||1:1|