Why the ERDAS IMAGINE ECW Compression target compression ratio settings never works in the actual compression ratio in the output.
Currently there is no way of directly controll the output file size when compressing images into the ECW or JPEG 2000 file format in ERDAS IMAGINE products and SDKs. This is because the output size is affected not only by the compression algorithm used but also by the specific low-level character of the input data.
In ER Mapper ‘s ECW SDK compression, images are compressed by ‘Quality layer ratio’. The target compression ratio is used during the compression process is to determine how much of that high frequency information is to be discarded. Therefore IMAGINE / ER Mapper uses a ‘Target Compression Ratio’ NOT the actual compression ratio. The actual compression ratio depends on the variability in the input data. Certain kinds of images are simply easier to compress than others. An image with few changes in texture and contrast (e.g. an aerial photograph of grasslands) will compress to a higher actual compression ratio than a very complex image (e.g. an aerial photograph of a dense urban area). The actual compression ratio achieved depends on the amount of information in your image. The ECW / JPEG2000 SDK method of compression keeps the compression quality the same for any image that is compressed with a specific target compression. The focus is in similar output quality not similar output files sizes. The ECW / JPEG2000 SDK balances the need for compression with the need for similar output quality.