Monday, July 19, 2010

Understanding the Target Compression Ratio when using the ERDAS ECW JPEG2000 SDK

It is important to note that when using the ERDAS ECW JPEG2000 SDK, the ECW and JPEG2000 target compression ratio makes no guarantees about the actual output size that will be achieved. Images with certain features (for example, air photos showing large regions of a similar color, like background values, oceans or forests) are easier to compress than others (completely random images).

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.

ImageryTarget Compression Ratio
Visually Lossless RGB Image Crisp Image Interpretation (2x zoom) 4:1
Near Visually Lossless RGB ImageClear Image Interpretation (2x zoom)6:1
RGB ImageHigh Quality Printed Maps and typical GIS applications15:1 to 25:1
RGB ImageInternet or Email Distribution15:1 to 40:1
Visually Lossless Panchromatic ImageCrisp Image Interpretation (2x zoom)2:1
Near Visually Lossless Panchromatic ImageClear Image Interpretation (2x zoom)3:1
Panchromatic ImageHigh Quality Printed Maps and typical GIS applications10:1 to 15:1
Panchromatic ImageInternet or Email Distribution15:1 to 30:1
Visually Lossless Multispectral ImageCrisp Image Interpretation (2x zoom)3:1
Near Visually Lossless Multispectral ImageClear Image Interpretation (2x zoom)4:1
Numerically Lossless RGB, Panchromatic, and Multispectral (JPEG 2000 Only) Imagery with perfect numeric reconstruction1:1

No comments: