Section 5 - Processing Options and Algorithms

Plugin Modules

Q) What data file formats are supported?
A) OrthoVista utilizes software "plugin" modules to support various image and georeference file formats. To determine which plugins are available use the OrthoVista "Help-About Plugins" menu command. This will open a dialog box which displays information on all available plugin modules.

Image Resampling

Q) What resampling methods are used?
A) OrthoVista resamples imagery if there is relative rotation. In that case it uses a bilinear method. For display it uses nearest neighbor sampling for speed. For output of downsampled imagery it uses either nearest neighbor (fast and good for overviews) or boxcar averaging (slow but 'smoother' result) per checkbox toggle in 'output' section of processing options dialog.

Mosaicking Methods

Q) What does the Adaptive Feathering "alpha grid size" do?
A) The alpha grid size determines the size of features that will be avoided. Larger alpha grid values will make it able to avoid smaller mis-matched areas such as relief features. This isn't very scientific, because so many things go into the computation. But, in general, smaller values (much faster) are satisfactory for sparse distribution of relief features whereas larger values (much slower) can improve results for densely distributed relief features.
Q) The Alpha Blending Grid numbers range from 50 to 200. Would you expect any problems if numbers out of range, such as 500, were used? (Other than the amount of time it would take.)
A) No, it's mainly a processing time issue. But, the time increases with order of N^4. So, doubling the size of the grid takes around 8 times longer to compute. Therefore, 500 would be about 40 times (2.5^4) slower than 200 for only a 6 times (2.5^2) increase in quality.

Image and Georeference Formats

Georeference Plugins
Q) The georeference data option I want is unavailable in the processing dialog selections. What is going on? How to I create mosaic data in the format I want?
A) To create image georeference files containing complex/ancillary data, OrthoVista copies ancillary information (e.g. producer organization name, datum description, etc.) from the input georeference files. Therefore, to output images with "complex" georeference data contents, it is necessary to provide input imagery which have this ancillary data contained in their georeference files.
Georeference Plugin - Tiff World Format
Q) Is there a way to change the numeric format (precision) of data in the TFW file?
A) Yes in versions starting with 2.2.4 there is an "Advanced Parameter" in the configuration file. The "TiffWRptPlugin::fieldFormat" keyword allows you to specify the format used to print the numbers in the TFW file. For more detailed description of the configuration file and of each keyword parameter, refer to Section 3.
Georeference Plugin - GeoTiff Format
Q) How do I save images using GeoTiff format for georeference information?
A) To output mosaic data with GeoTiff georeference information, you must provide OrthoVista with input imagery which have GeoTiff data. This is necessary so that OrthoVista can read ancillary data from the input georeference file and copy it to the output georeference file.
Q) What GeoTiff Tags are Supported
OrthoVista uses the TIFFTAG_GEOTRANSMATRIX(34264) tag to determine the shift, scale and rotation associated with a GeoTiff image.
In general, the TIFFTAG_GEOTIEPOINTS(33922) tie point form is a weak and ambiguous expression of the image<->world relationship. For example if there are 4 tie points, should the image<-> world relationship be defined by a least-squares conformal transform, a least squares affine, a fractional polynomial rectification tranform, a least absolute magnitude shift and scale only, should rubber sheeting be used to fit all points exactly, if so what math model,... etc, etc...
Note that the TIFFTAG_GEOPIXELSCALE(33550) tag is also not supported because it is overridden by the information in the TIFFTAG_GEOTRANSMATRIX tag.
A warning message is reported if an unsupported GeoTiff tag is encountered. However, in some versions of the program it may continue to process the imagery with an incorrect orientation.

Georeference Plugin - ERS Format
Q) What level of support is available for ERS georeference file formats?
A) The fundamental georeference content of the ERMAPPER "ERS" file format is supported. However, rotation is not supported in the current version (2.2.2).

Color Maps (CMAPs)

Q) In general, if using hot spot removal and global tilting in a large color mosaic, and I need to apply a cmap to all the images, should I apply the cmap before the balance, during the balance, or after the balance? My tests with this mosaic indicated that if I applied the cmap with the same run of OrthoVista when hot spot and global tilting was also applied, the results were less satisfactory than if I balanced the images first with hot spot and global tilting, then ran those balanced images through the cmap.
A) The CMAPs were designed to be run before balancing/tilting - which is why OrthoVista applies them first. But, the cmaps are fairly powerful in what they can produce, and it may be that they are better for use after balancing/tilting in this case.
Basically, the cmaps just describe a mapping of one color space to another. So, for example, if the input images vary greatly in colors, then a single cmap won't be as useful for applying to the input images. But, after balancing and tilting, the colors should be fairly uniform over the image, so your cmap may make more sense there.
The cmaps are most effective when there is a known color problem that is consistent among input images. For example, if you were scanning negatives instead of positives, then a cmap could be used to convert between them - but that's an example of a consistent color space mapping problem. If the input images have random color shifts, then a cmap won't be nearly as effective.