[mapserver-users] WMS image display

[Rahkonen Jukka (Tike)]

Hi,

Overviews help at small scales. Let's assume that you have orthophotos with 1 meter native pixel size. If you want to show a 10x10 km area in a 500x500 pixel sized box in your application, it means that the pixel size of the output it 20 m. It is heavy to read 10000x10000 native pixels and resample those but if there happens to be readily available overviews with 2-4-8-16 m pixel sizes you can pick the 16 m pixels and use them for final resampling. 

Tile index is used for finding the right image file at some geographic area. And finally internal tiling of image files help at big scales, when you want to draw 500x500 m area in the 500x500 pixel sized box. If you have tiled tiff files it is possible to pick a handful of 256x256 pixel tiles from the middle of the image file. If you have a 10000x10000 pixel sized png file you must decompress all the 100 million pixels before you can pick the 250 thousand that you really need.

The key is to read as little extra pixels as possible, and therefore you need all three: internal tiling, overviews, and tileindes. Actually for country wide service even a fourth one: on overview image that covers the whole country with much bigger pixel size so you do not need to open hundreds of images from the tileindex for picking only a few pixels from each.

-Jukka Rahkonen-

________________________________________
Dash wrote:

> Our tiff images do not have overviews.  We are thinking that something may
have happened during the transfer of the image from our Windows machine to
Linux machine.  I highly doubt this is the case but it's worth a shot.

> I was just reading about creating overviews.  It seems this would increase
the amount of disk space needed but would it actually increase application
performance?  What is the difference between overviews and creating a tile
index using gdal?

Thanks.



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