JPG speed /TIF speed

[Sean Gillies]

On Mar 26, 2006, at 11:51 AM, Matthew Perry wrote:

> On 3/26/06, Sa=9Ao Celarc <saso.celarc at hermes-plus.si> wrote:
>> Does somebody has an explanation of this? My intuition says that  
>> jpeg sho=
> uld
>> be faster.
>
> jpegs are typically MUCH slower than tiffs because they have to be
> completely decompressed before mapserver can access any pixel values.
> For larger rasters, this can mean 10+ seconds of decompression just to
> be able to access the data wheras the tiff can be scanned and read
> immediately and efficiently.
>
> You might look at ECW compression which is much faster and better
> quality than jpeg.  But the typical advice is to simply use
> uncompressed tiffs, internally tiled with overviews:
>
>  gdal_translate  -of GTiff -co "TILED=3DYES" input.tif output.tif
>  gdaladdo -r average output.tif 2 4 8 16 32 64 128
>
> It takes alot of disk space but it's orders of magnitude faster.
>
> --
> Matt Perry
> perrygeo at gmail.com
> http://www.perrygeo.net

My understanding is that you can also use compression within TIFF  
tiles. You can save on disk space if you need to do so, but also  
won't have to decompress much more imagery than you actually need.

cheers,
Sean

---
Sean Gillies
http://zcologia.com