TIF vs SID file size

[choctaw]

Halito,

At ver 6.1 I was able, using the default compression ratio (20:1) to compress a 140mb TIF down to 4-6mb SID file. Now at ver 7.0 I compress the same 140mb TIF - this time it comoress down to a 28mb file.

I have not changed any of the settings in GeoExpress 7.0, any thoughts on why my SID files are much larger at 7.0?

thanks

[rparker]

Hello,

20:1 is a ratio of what GeoExpress calls the "Input Image Size" to the "File Size".

The "Input Image Size" is the size of the raw data outside of any file format. This can be measured objectively by simple multiplication:

width * height * bands * bits = raw size in bits

For example, a 5000 pixel wide * 5000 pixel high image with 3 bands (RGB) at 8-bits per pixel would have an "Input Image Size" of about 71.53 MB. The file size of any compressed image can then be described as a ratio of that number.

20:1 would be (71.53 MB) / 20 = 3.58 MB
10:1 would be (71.53 MB) / 10 = 7.15 MB

So, if you set your target in GeoExpress 7 to be 20:1, GeoExpress does all it can to hit that target and is very accurate.

With all of that information in mind, please double check your data. What does GeoExpress report for the "Input Image Size" of your data? What is that value divided by 20?

If the "Input Image Size" is really 140 MB, let's look at what the outputs would be at 20:1 and what the ratio should be to produce a 28 MB file size.

(140 MB) / 20 = 7 MB

(140 MB) / X = 28 MB

X = (140 MB) / (28 MB) = 5

5:1

So it seems like you had the "lossless" box checked, because 5:1 is well within the range of what we can expect of the lossless setting.