Resampler Algorithms

The resampler algorithms are used in interpolating or "resampling" pixels when reprojecting an image and when scaling an image from source to output dimensions.

The options are the same for both scaling and reprojecting:

• Nearest neighbor – this is the fastest algorithm in terms of CPU time, but it yields the poorest quality.
• Bilinear – "bilinear interpolation" takes more CPU time than Nearest neighbor but yields better quality.
• Bicubic – "bicubic interpolation" is the most CPU-intensive algorithm but produces the best result for reprojection.

By default Express Server uses bilinear interpolation (Bilinear) both for scaling and for reprojection. Bilinear is a good balance of speed and quality that serves most imagery fairly well. In cases where image quality or speed is less important you may wish to change this to one of the other algorithms.

Setting the Scaling Algorithm

1. Open the Express Server Manager (see Accessing the Express Server Manager).
2. Click Settings on the top navigation bar, then select the Basic tab on the left-hand navigation and click Edit.
3. Choose an algorithm from the Scaling Algorithm drop-down menu, then click Save.
4. If you have no further settings to edit, click Publish.

Setting the Reprojection Algorithm

1. Open the Express Server Manager (see Accessing the Express Server Manager).
2. Click Settings on the top navigation bar, then select the Basic tab on the left-hand navigation and click Edit.
3. Choose an algorithm from the Reprojection Algorithm drop-down menu, then click Save.
4. If you have no further settings to edit, click Publish.

Setting the Scaling Bias

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 scaling bias is a bias toward the next-higher-numbered (lower-resolution) 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 corresponds 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, and that the scaling bias is set at 0.5. Express Server adds the scaling bias 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. A scaling bias 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 scaling bias strengthens the inclination toward higher numbered (lower resolution) zoom levels, which results in generally quicker decodes but also in lower image quality. Lowering the bias favors higher image quality over performance.

To set the scaling bias:

1. Open the Express Server Manager (see Accessing the Express Server Manager).
2. Click Settings on the top navigation bar, then select the Basic tab on the left-hand navigation and click Edit.
3. Click and drag the Scaling Bias slider to the desired value, then click Save.
4. If you have no further settings to edit, click Publish.

By default the scaling bias is set at 0.5.

Setting the Reprojection Precision Factor

The reprojection precision factor specifies the maximum error allowed when choosing a faster reprojection algorithm. In order to reproject pixels from one coordinate reference system 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 transform, a simple function that is much faster than the reprojection function. Express Server's reprojection 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. As examples, a value of 0 specifies that the linear transform is never used; a value of 0.25 specifies that the linear transform is used when it yields an error of 1/4 pixel or less; and a value of 2.0 specifies that the linear transform is used when it yields an error of 2 pixels or less.

To set the reprojection precision factor:

1. Open the Express Server Manager (see Accessing the Express Server Manager).
2. Click Settings on the top navigation bar, then select the Basic tab on the left-hand navigation and click Edit.
3. Enter the desired value in the Reprojection Precision field, then click Save.
4. If you have no further settings to edit, click Publish.

The default and recommended reprojection precision factor is .333.