Section 7 - Input Image Data

Background Pixels vs. Image Pixels

Q) How does OrthoVista distinguish between background pixels and pixels which are part of the image?
A) OrthoVista utilizes the pixel intensity to determine if a pixel should be considered part of the image or if it should be treated as a background value. For versions 2.1.x and earlier, intensity values of 0(b/w), 0,0,0(color), or 255(b/w), 255,255,255(color) are considered to be background pixels while all other values are considered to be part of the image.
For versions 2.2.x and later, the threshold can be set using the OrthoVista "Setup-Preferences" menu command. This produces a dialog box which allows you to enable/disable background pixel checking and to control the threshold values used in the checking. Enabling background checking and setting the min/max intensity threshold to 0 and 255 respectively reproduces the behavior of earlier versions.

Image Configuration and Overlap

Q) How does OrthoVista utilize input image overlap?
A) OrthoVista utilizes input image overlap in two ways: 1) during computation of multi-image radiometric corrections; and 2) during automated composition of images into mosaics. Note that the single-image radiometric corrections such as "hot spot" removal are computed for each image individually - and therefore are not affected by image overlap geometry.
During multi-image radiometric computations (e.g. "global tilting" algorithm), OrthoVista utilizes overlapping pixel regions to compare radiometric characteristics of input images at corresponding geometric locations. The geometric correspondence within the overlap area is important to ensure the statistical similarity of the underlying scene content between the images. If images do not overlap, radiometric image differences may be due to changes in scene content (for example, adjacent image boundary could be along a section-line which separates bright wheat fields on one side, from dark asphalt parking lot on the other side).
During automated mosaic generation, OrthoVista utilizes the input image overlap region to provide flexibility in where the input images are 'stitched together' to form the single mosaic image. If there is no overlap, then the composition operation is limited to simple catenation of the source images.
Q) What is the recommended amount of overlap?
A) For typical projects the suggested overlap amount is in the range of 10-15% of the individual orthophoto dimension along each edge.
Q) What capabilities are available for input images which do not overlap?
A) As noted above, the single image radiometric corrections (e.g. "Hot Spot Removal") are independent of image overlap and can be effectively utilized for projects without input image overlap. However, since all multi-image algorithms (e.g. "Global Tilting") require overlap, these algorithms will have no effect on images which do not overlap. In the absence of input image overlap, the various mosaicking methods effectively reduce to the "Plain Mosaic" method.