[Gvsig_english] QUESTION gvSIG -- Image files with fast rewrites [SEC=UNCLASSIFIED]

[Bruce Bannerman]

Simon,

Most of these formats are considered to be 'lossy', in that you lose something in the integrity of the base data.

This is not an issue if you just want to view an image as a backdrop.

It does become an issue though if you wish to undertake some form of digital image processing to analyse the imagery.

TIFF / GeoTIFF is probable the best format to store 3 band imagery (e.g. RGB), to preserve the integrity of the source data.

Bruce



On 15/09/10 1:20 AM, "Juan Lucas Dominguez Rubio" <jldominguez at prodevelop.es> wrote:

Hello, Simon.

Yes, I have seen reasonably fast GeoTiff/TIF files. You will probably get one of those by using the parameters suggested by Jukka Rahkonen.

As for the comparison between image formats, I have done myself some tests, and my current opinion is this:

JPG/JPEG: very good compression ratio but very poor performance in terms of speed and memory consumption. Should be used only for long term storage or with very small images (for example, less than 5 megapixels, wich includes tiles, as Google Maps does). Should not be used either for very large images (for example, more than 50 megapixels) because the amount of memory needed to manage them can be really scary. For small images, it can be faster than ECW.

ECW: very good compression ratio and very good performance in terms of memory consumption and speed. Good for any image size. It's based on the wavelets algorithm, and I suspect they have optimized significantly their computations thanks to many years of experience and very skilled C++ programmers.

JPEG2000: Uses wavelets too, but in my (little) experience it's significantly slower than ECW, probably due to lack of optimization.

GeoTiff, TIF: Lots of different options when creating them. Very resonable performance, I would say, though not as good as ECW in terms of compression ratio and speed.

As far as I know, JPG and ECW can only store RGB images, while GeoTiff can manage other interesting data types (long integers, float, etc.)

I don't know the details, but I think one needs to acquire some kind of license from Erdas (owner of the ECW format) when creating very large ECW images (more than 170 megapixels, I  think) for commercial purposes.

When I talk about compression ratio, I'm assuming a similar level of lossiness.

Another relevant format is MrSid, which I pressume is very similar to ECW, but less popular.

All in all, yes, I would say ECW is the best when using RGB images and deserves recognition.


Regards,
Juan Lucas Domínguez Rubio
---
Prodevelop SL, Valencia (España)
Tlf.: 96.351.06.12 -- Fax: 96.351.09.68
http://www.prodevelop.es <http://www.prodevelop.es/>
---

________________________________
De: gvsig_internacional-bounces at listserv.gva.es en nombre de Simon Cropper
Enviado el: mar 14/09/2010 14:52
Para: Users and Developers mailing list
Asunto: Re: [Gvsig_english] QUESTION gvSIG -- Image files with fast rewrites

Hi Juan Lucas,

On Tuesday 14 September 2010 6:41:13 pm Juan Lucas Dominguez Rubio wrote:
> - If I remember it well, the gdal you get by using apt-get or similar does
> not include the ECW compressor, but I think the binaries from the FWTools
> website do, though I cannot test it right now:
>
> http://home.gdal.org/fwtools/FWTools-linux-2.0.6.tar.gz
>
> Your experience suggests otherwise?

Ubuntu GDAL libraries keep resetting to version without ECW. Even the Ubuntu
FWTools has reset. I keep setting up the system only to have it reset by
itself.

> Which error message do you get when trying to create a ECW?

None really. It is just that the library does not recognise ECW format.

> gdal_translate -of ecw image.jpg image.ecw

Yeap. I found similar examples on the GDAL site and several HowTo websites.

The only way to get the FWTools to work was install it on a Windows machine
then using the export tool.

> - Have you tried compressed TIF?

No. Is it fast?

Do you know of anyone who has compiled a matrix of image formats against
compression, retention of image integrity (~loss of detail) and rewrite time
while panning (particularly in gvSIG)?

ECW files are touted as the best compression-image quality retention-rewrite
combination: certainly in Australia it is the primary format used in natural
resource management by consultants and government agencies. Is this hype, or
does the format really hold the title?

> ________________________________
>
> De: gvsig_internacional-bounces at listserv.gva.es en nombre de Simon Cropper
> (Botanicus Australia Pty Ltd) Enviado el: mar 14/09/2010 9:07
> Para: Users and Developers mailing list
> Asunto: [Gvsig_english] QUESTION gvSIG -- Image files with fast rewrites
>
>
>
> Hi,
>
> Most of my clients provide aerial photography in ECW format. These images
> provide high resolution, are relatively small and quickly rewrite on the
> screen when panning.
>
> Lately I have been getting JPG files. These files however are quite big and
> take significantly longer time to rewrite.
>
> What is an alternative image format to ECW in the 'open free' environment?
> Something GDAL can be used to convert from JPG without jumping through
> hoops.
>
> Ideally the file sizes should be low despite the high resolution, gvSIG
> able to read it natively and is quick to rewrite when panning.
>
> Any suggestions would be appreciated.
>
> PS. I know gdal_translate can, supposedly convert between JPG and ECW, but
> despite many attempts I have been unable to get this to work. I am looking
> for something else, if possible, that has the same intrinsic properties
> but is recognized by default in the gdal libraries so that every time I
> get an update of the 'free' Ubuntu version the FWTools or alternative
> 'commercially enabled' version is not replaced.

--
Cheers Simon

        Simon Cropper
        Botanicus Australia Pty Ltd
        PO Box 160 Sunshine 3020
        P: 03 9311 5822. M: 041 830 3437
        W: http://www.botanicusaustralia.com.au <http://www.botanicusaustralia.com.au/>
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