There are three pixel cache options; Image, Index and Warp.
TheImagecaching strategy involves caching the pixels from images as they are decoded. This strategy may be useful for sites that have many different images to serve and are not using the spatial index or WMS features of Express Server. LizardTech recommends enabling this strategy for non-WMS sites.
Chapter 4: Configuring Express Server
TheIndexcaching strategy involves caching the pixels decoded from a spatial index. A spatial index is a special type of Express Server catalog that dynamically combines a collection of spa- tially related images into a single mosaic. WMS GetMap requests use the spatial index. When this strategy is employed, the pixels from the index are cached, rather than from the individual images that are included in the index. LizardTech recommends enabling this strategy for WMS sites.
TheWarpcaching strategy involves caching pixels after they have been reprojected or “warped”. This only affects WMS GetMap requests that require reprojection. LizardTech recommends enabling this strategy for WMS sites in which most requests require reprojection.
To specify a pixel cache:
1. Open the Express Server Manager (seeAccessing the Express Server Manageron page 21).
2. ClickSettingson the top navigation bar, then select theCachetab on the left-hand nav- igation and clickEdit.
3. Select the checkbox for the desired cache, make any adjustments to the cache's prop- erties, then clickSave.
4. If you have no further settings to edit, clickPublish.
NOTE: The Image, Index and Warp caches can be used in combination with each other, but to reduce the performance costs associated with filling the caches, we recommend that you enable only one pixel caching strategy.
Fine Tuning the Pixel Cache
Four settings enable you to fine tune any of the pixel caching strategies to optimize performance. These settings specify the width and height in pixels of each block saved in the cache, the min- imum allowable ratio of requested scene pixels to decoded tile pixels, and the maximum image resolution at which caching occurs.
Width and Height of Cached Blocks
TheWidthparameter indicates width in pixels of each block saved in the cache. The default value is 200.
TheHeightparameter indicates the height in pixels of each block saved in the cache. The default value is 200.
LizardTech Express Server 9 User Manual
Pixel Cache Utility
The pixel caches sort an image into tiles of fixed width and height. When Express Server receives a request to decode an image (i.e. the getimage or GetMap requests), the pixel cache will decode all the tiles that intersect with the specified scene. In most cases the area of all the tiles will be lar- ger than the area of the requested scene. The result of this is that more pixels will be decoded than are necessary to satisfy the given request, initially hurting the performance of the server. However, because these tiles are then cached, subsequent requests will not have to decode any pixels, improving performance of the server.
TheUtilityparameter specifies the minimum allowable ratio of scene pixels to tile pixels. If the number of pixels in the requested scene divided by the number of pixels required to decode the intersecting tiles is less than this value, then the tiles will be clipped. The valid range is 0.0 – 1.0.
Some examples:
A utility of 1.0 specifies that no extra pixels will ever be decoded, and only those tiles that fall com- pletely within the bounds of a requested scene will be cached. This setting causes the least initial overhead, but because fewer tiles are cached, performance improvements resulting from the cache will take longer to accrue.
A utility of 0.0 indicates that all intersecting tiles will be decoded, regardless of the size of the requested scene. This is the default value. This may cause some initial performance degradation, but since all tiles are cached, performance will quickly improve.
A utility of .85 specifies that the pixel cache will always make sure that 85% of the pixels it decodes for a particular scene will be used in that scene. If the number of pixels required to fill the intersecting tiles exceeds that percentage, then they will be clipped.
The default and recommended value for the Utility parameter is 0.0.
Pixel Cache Maximum Magnification (Max Mag)
TheMax Magparameter specifies the maximum image resolution or "magnification" at which caching occurs. All requests for scenes at magnifications higher than this value will not be cached; all requests for scenes at or below this magnification will be cached. Decimal values greater than 0 and less than or equal to 1 are valid.
The default value is 1.0, indicating that decode requests at full resolution (1.0) will be cached, as will all decode requests at lower magnifications (i.e. zoomed out).
Chapter 4: Configuring Express Server
Requests at magnifications greater than 1 will not be cached. To fine tune the pixel cache
1. Open the Express Server Manager (seeAccessing the Express Server Manageron page 21).
2. ClickSettingson the top navigation bar, then select theCachetab on the left-hand nav- igation and clickEdit.
3. Enter or edit values forWidth,Height,UtilityandMax Mag, then clickSave. 4. If you have no further settings to edit, clickPublish.