This specifies the algorithm used for scaling pixels during reprojection (warping).
Possible
warpResample
values:
NN
Chapter 10: Reference
Specifies that the "nearest neighbor" algorithm be used. This is the fastest algorithm but yields the poorest quality.
BL
Specifies that "bilinear interpolation" be used. This algorithm is more expensive than nearest neighbor, but yields better quality.
BC
Specifies that "bicubic interpolation" be used. This is the most CPU- intensive algorithm, but produces the best result for reprojection.
warpErrorThreshold
This specifies the maximum error allowed when choosing a faster reprojection algorithm. In order to reproject pixels from one coordinate reference system (CRS, sometimes called spatial ref- erence system or SRS) to another, a complex mathematical formula must be applied to each point in the source image. In some cases, this formula may be roughly equivalent to a linear trans- form, a simple function that is much faster than the reprojection function. Express Server's repro- jection engine can compare the results of a linear transform with that of the reprojection, and if the results are close, then it will choose the faster linear transform.
The value of this attribute is a decimal number greater than or equal to 0. The default and recom- mended value is .333.
Example
warpErrorThresholdvalues:
0
Specifies that the linear transform is never used .
0.25
Specifies that the linear transform is used when it yields an error of 1/4 pixel or less.
2.0
Specifies that the linear transform is used when it yields an error of 2 pixels or less.
LizardTech Express Server 9 User Manual
scaleMagThreshold
In a MrSID or JPEG 2000 image, Zoom Level 0 (Zero) is full scale and has the highest resolution, and higher-numbered zoom levels represent lower resolution, on up to the tiny thumbnail or icon level. The scaleMagThreshold attribute is a bias toward the next-higher-numbered (lower-res- olution) zoom level in determining which zoom level to scale pixel data from.
Express Server assigns a decimal value to the resolution level of a requested scene that cor- responds to the numbering of the image's zoom levels, then adds the bias, and finally truncates the number after the decimal to arrive at a zoom level.
For example, let's say a requested scene has a resolution level of 1.6, which falls between Zoom Level 1 and Zoom Level 2. Express Server adds the default scaleMagThreshold value of 0.5, which brings the requested scene's resolution level to 2.1. Express Server drops the ".1" to arrive at Zoom Level 2. It then pulls the lower resolution pixel data from Zoom Level 2 and scales it up to satisfy the scene request. This scaled-up pixel data is not as sharp as that from Zoom Level 1, but it takes less time to retrieve.
For another example, let's say a requested scene has a resolution level of 2.4. The default scaleMagThreshold value of 0.5 brings the total number to 2.9, which is not enough to bump this scene's value "over the threshold" to Zoom Level 3. Express Server uses the higher resolution pixel data of Zoom Level 2 and scales it down to the requested dimensions.
Increasing the scaleMagThreshold value strengthens the bias toward higher numbered (lower res- olution) zoom levels, which results in generally quicker decodes but also in lower image quality. Lowering its value favors higher image quality over performance.
Accepted values for scaleMagThreshold are decimals between 0 (zero) and 1, inclusive. By default the value is 0.5.
The
ImageManagerElement
In order to enable maximum performance with minimal resource requirements, Express Server employs certain strategies in its image handling. TheImageManagerconfiguration element allows you to define parameters used in these strategies.
TheImageManagerelement contains the following editable attributes:
Chapter 10: Reference
itemCacheSize
The first time Express Server receives a request for data from a particular item, it loads data about the item into memory for quick access later. Since a given item may be gigabytes in size, it does not load the whole item into memory. Rather, it only loads information which allows it to access the item efficiently in response to subsequent requests. This information is stored in an internal memory cache called the "Item Cache". This attribute specifies the maximum number of items held in the Item Cache.
The default value is 1000.
NOTE: JPEG 2000 source imagery uses a lot of memory. Setting this value lower (150- 200) will help ensure that you don't run out of memory when JP2 source images are being used.
imgCacheSize
Each item contains one or more or images. These images are also cached for quick retrieval, in the image cache. TheimgCacheSizeattribute specifies the maximum number of images which can be stored in this cache.
The default value is 500.
maxConcurrentExtractions
The most CPU expensive operations on Express Server are extractions. These operations occur in response togetimagerequests in the Web API,GetMaprequests in the WMS API, andltc- sExtractfunctions in the C API. In these operations, Express Server decodes a portion of the source image, and reencodes to an output format such as TIFF or JPEG.
If Express Server is hit with a large number of extraction requests at one time, then server per- formance may suffer.
This configuration attribute specifies the maximum number of extractions that may be processed concurrently. Extraction requests occurring in excess of this number are placed in queue, and handled on a first-come, first-served basis.
If this attribute is missing, or its value is set to 0, then the number of concurrent extractions is not limited.
LizardTech Express Server 9 User Manual
The default value is 50.