[mapserver-users] Image Speed Questions---Generic

[Norman Vine]

Paul Ramsey writes:
>
>Could I get some enlightenment from the raster experts on the benefits
>of various image formats?
>
>Right now, I have 25metre relief for all of BC, in about 120 files, in
>PNG format. I am guessing that I can radically improve my speed by
>changing to a format which does not require decompression. 
>Would TIFF be
>a faster format? Assuming it is, Ed mentions this concept of
>strips-and-lines with respect to TIFF files: is this some way of making
>random access of the tiff files faster? What do the concepts refer to,
>and what is the best balance of the variables?

IMHO Generally speaking a Run Length Encoded (RLE) 
tile based Image format will give the best overall performance 
with 8 bit images.

I have found that compression of some sort greatly helps disk seek 
and read times never mind storage requirements, and that RLE 
decompresses VERY quickly.

There are other compression schemes that also decompress
quickly, mostly codebook based, but RLE works

The trick is to store LARGE images with many tiles 
< keeping  the tile dimensions a power of two >
and only requesting the necessary tiles for the subimage
requested.

FYI, The Tiff library supports a RLE tiled image format
and subimage requests, greatly simplifying the code you
have to write :-)

Cheers

Norman Vine