Extending dynamic range of stored image database

DETAILED DESCRIPTION In accordance with the present invention, the above discussed problem is solved by extending the dynamic range of the digitized image database, so as to permit a variation or shifting of the encoded pixel values without `clipping`, to provide a limited window or range of values into which specular reflectance image points, such as a reflection from a car bumper or a specular reflection of sunlight off a water surface, may be encoded and stored, and to provide a limited window, or range of values into which unusually low reflectances or areas of objects in shadow light may be encoded and stored.
In particular, the present invention is directed to a method of enabling the dynamic range of the digitized image data base to be effectively extended beyond the range of values into which the digital codes output by the image scanner corresponding to the basic scene content are mapped by an image processing (scene balance) mechanism.
For this purpose, those digital codes into which the scanner output has been mapped by the image processing operator are converted into a set of `reduced-range` digital codes of the same digital resolution but having a smaller range of basic image content values than the dynamic range of the digitized image data base.
The code conversion mechanism operates to convert a maximum value of 100% white reflectance to an encoded value that is less than the upper limit of the dynamic range of the database.
For the foregoing example of an eight bit encoding and storage resolution, such a value may be somewhat less than the maximum of 255 (e.
g.
225), so as to leave a limited range or window of values (here 30 values) at the upper end of the encoding range, to allow for the placement of specular highlights that are beyond the 100% white reflectance maximum, and to accommodate shifts in the digitized imagery data.
The code conversion mechanism also operates to convert a defined minimum value of low reflectance to an encoded value that is greater than the lower limit of the dynamic range of the database