Section 6 - Production Workflow Techniques

MetaData File Use

Q) What/Where are MetaData files and how does OrthoVista use them?
A) MetaData files are where OrthoVista temporarily stores various radiometric correction parameters. In general, there are metadata files for each of the processing options and for each of the images being processed. The location where these files are stored is the "MetaData Directory" specified from the main processing dialog.
Since radiometric correction data can be stored in metadata files, it is possible to compute the corrections during one processing run and then save these corrections (by way of metadata files) and apply them later during a subsequent processing run. This is termed "two-step" radiometric processing and is discussed in more detail in the Multi-Step Processing section.
At the start of a processing run (e.g. when you press the "process" button), OrthoVista takes inventory of the sequence of processing operations which need to be performed. For example candidate processing steps include image/background region detection, radiometric color corrections, intensity adjustments, etc. OrthoVista then looks in the MetaData directory for relevant metadata files that it can use - thereby avoiding the need to recompute the relevant values.
A strong test is applied to determine if a metadata file can be used in place of a new computation. This test includes the simple requirement that the metadata filename and extention match the image name and processing operation. Also the current processing options and parameters must match exactly the options and parameters which are recorded in the metadata file. If all tests match, then processing results are read from the metadata file and that information is used instead of being recomputed.
Note that there is a significant intentional exception to the exact matching process just described. This exception is required to support Multi-Step Processing. The exception is that _no_ check is performed on which images where used during computatin of the particular metadata file values. Thus, it is possible, for example, to compute radiometric corrections using "all" images - then apply these corrections during subsequent processing runs involving only a "few" images each.
After computing new correction and adjustment parameters, OrthoVista checks if a metadata directory has been specified (in the processing options dialog). If a directory has been specified, then the newly computed parameters are written to the appropriate metadata file in that directory. If a file with that name already exists, it is overwritten with the new parameter values!
The metadata file data are stored in a 'normalized' format. This means that metadata values can be computed from downsampled (e.g. thumbnail) imagery - and then later applied to full resolution images. This provides the possibility to compute a "simultaneous" radiometric adjustment for very large numbers of images (having only small thumbnails available) then to later apply the results to full resolution image data on an image-by-image (or more likely mosaic tile by mosaic tile) basis.
Note that unless you pay special attention to what you are doing (e.g. intentional Multi-Step Processing), it is generally recommended that you either do not specify a metadata directory or that you always begin processing with an empty metadata directory. This will ensure that you do not inadvertently apply radiometric corrections computed from a previous run... As a bad example - an easy mistake is to 'test process' only a few images before starting the 'real run'. If the metadata directory is not empty upon starting the second ('real') run, then the results computed from only the few test images are re-applied during the second run. In this case, you would be effectively (and probably unintentionally) embarking on multi-step radiometric processing.

Multi-Step Processing

Q) How do I use MetaData file in "Two-Step" processing
A) As described in Metadata File Use, before computing various radiometric corrections, OrthoVista will first check existing metadata files. You can utilize this behavior to "transfer" the radiometric results of a first-stage processing run into a second-stage processing run.
In the two-step model, some or all of the metadata files are created during the first stage processing run and are then "applied" during a second stage processing run (or subsequent multiple runs).
The first stage run could include, for example, simultaneous radiometric adjustment of a very large number of images by processing only the small reduced resolution versions of the images. Alternatively it could consist of processing all images in a geometric sub-block (e.g. a group of contiguous flight lines) with the intent to 'transfer' the radiometric characteristics of "edge images" from this sub-block into an adjacent sub-block.
During a second-stage processing run the existing metadata files (i.e. computed during first stage run) provide a mechanism for holding the radiometric characteristics of an image fixed at the values computed during the first-stage processing run.
Therefore, for example (to continue the first stage examples) - after computing metadata using downsampled images, individual mosaic tiles can be generated one at a time from high resolution images - but using the radiometric results recorded in metadata files computed during the simultaneous adjustment of _all_ reduced resolution images. Or... a sub-block can be processed and the edge images will be adjusted to match the radiometric properties of adjacent images for which metadata files exist in the metadata directory (e.g. computed during processing of previous sub-block).
Note that this two-step process is made possible by OrthoVista and it can be highly effective. However, the technique is not fully integral to the program architecture. Thus, the successful application of the multi step radiometric processing is strongly dependent upon the organization and systematic procedures you utilize to employ it. Therefore, it is recommended that you incorporate a systematic procedure (e.g. checklists, metadata directory naming conventions, etc.) to ensure that the correct metadata files are computed and applied as intended.

Image Dodging

Q) Is there any way to get non-georeferenced .tif images into Orthovista and dodge them?
A) Only if you create some "fake" (e.g. identity) georeference files.... It's not particularly clean, but you can create an identity TFW file easily as follows:
----- "Identity" Tiff World File (*.tfw)
1.0
0.0
0.0
-1.0
0.0
0.0
----- End of file (File contains only the 6 lines with numbers on them)
If you do this, make sure you use ONLY Single Image Balancing - turn Tilting OFF. Enable save balanced images. Turn mosaic output OFF.
On unix systems, you can create symbolic links for each <image>.TFW file to the identity file. On Windows, you'll need to copy it.
Although this is a complete hack, it is a completely effective way to gain access to the "HotSpot" and "Intensity Dodging" algorithms.