Processing of sorting a train of pixel values within the same line of a radiation image containing a gas part region or an air part region in a subject and rearranging the train of pixel values within the same line from the pixels having higher pixel values to the pixels having lower pixel values, is executed over the entire image of the subject. A characteristic value is calculated from a predetermined region of the image subjected to the sorting processing.

A method and system generates a histogram from a scanned image by creating a sample window defined by a number of scanlines to be sampled, a number of leading scanlines to be skipped in a slowscan direction before sampling, a number of leading pixels to skip in a fastscan direction before sampling begins, and a number of pixels within a scantine to be sampled. It is then determined if a number of pixels in the sample window is greater than a capacity of a histogram buffer. If it is determined that the number of pixels in the sample window is greater than the capacity of the histogram buffer subsample windows are created within the sample window. The image is scanned and a predetermined number of pixels within each subsample window is processed to generate a histogram, the predetermined number being less than a total number of pixels in a subsample window if it is determined that the number of pixels in the sample window is greater than the capacity of the histogram buffer, thereby processing a total number of pixels that is less than or equal to the capacity of the histogram buffer.

In contrast enhancement during image processing, a histogram equalization process or apparatus divides an equalization image screen into overlapping windows and performs histogram equalization on each window. Using windows improves contrast of pixel values that are rare throughout the entire image and reduces or prevents loss of information represented by the rare pixel values. Overlapping the windows reduces discontinuities at the boundaries of the windows. The size and overlap of the windows can be adjusted according to the time available for equalization and can be optimized according to the desired image quality improvement. The histogram equalization can be selectively performed on an entire image screen or any part thereof to reduce image degradation in the entire or part of an image. As a further aspect of the invention, to reduce the number of operations required for equalization, a histogram for a current window is obtained starting from the histogram for a previous window. To further improve image quality, a histogram equalization apparatus uses low-pass- or band-pass-filtered pixel data as a lookup table address to output a histogram-equalized value. This helps remove amplified thermal or quantization noise.

A method for storing map data in a database and a method of searching the database to find objects in a given area and to find objects nearest to a location. To generate the map data, a map plane is divided into a number of squares and the squares are numbered with spatial key numbers according to a space filling curve. Objects identifying places such as restaurants or hotels are placed in a main table of the database along with one of the spatial keys (object keys) intersecting an area of the map occupied by the object. A secondary table of the database is then created with one column including object keys corresponding to the main table, and other columns identifying ranges of spatial keys for objects identified by the object keys. To search the database to find objects in a given area, ranges of spatial keys are calculated for the given area and compared with ranges in the secondary table to identify object keys. The object keys identified are then used to obtain the desired objects from the main table.

In a contrast correcting apparatus 1 which divides an image into unit regions and carries out a contrast correction for each of unit regions, a gray level histogram calculation section 201 generates gray level histograms of the image, and a scene judgment section 202 makes a judgment on the state of the image. When the result of the judgment shows that the image is in a state such as overexposure, underexposure, low contrast or high contrast, a region size determining section 204 sets a greater size as the size of the unit regions. Based upon the size of the unit regions thus determined and the amount of contrast correction determined by the contrast correction amount determining section 203, gray level transformation curves are formed for the respective unit regions, and by using these, the contrast corrections are carried out on the respective unit regions. With this method, it is possible to properly reduce the occurrence of unevenness in gray levels in an image that tends to arise in the case when the size of the unit regions is small although the amount of contrast correction (the amount of emphasis) is great.