Method and apparatus for dynamically setting a background level

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS FIG.
2 shows an overview of data flow in an apparatus for generating a background level, which constitutes a part of an image scanning system.
Scan bar sensor 205 moves across the face of an image to be scanned such as sheet of paper 210, which is mounted on platen 215.
Scan bar sensor 205 is composed of multiple light detecting elements mounted along the axis of the sensor.
During the course of scanning sheet of paper 210 scan bar sensor 205 moves down the sheet of paper by way of mechanical means (not shown).
When scan bar sensor 205 moves to a new horizontal position, scan bar readout circuit 220 reads each light detecting element in scan bar sensor 205 during the course of a horizontal scan.
Scan bar sensor 220 presents a series of analog values to analog to digital converter 225, each analog value corresponding to the reflectivity sensed by one of the light detecting elements in scan bar sensor 205.
During the course of a horizontal scan, a multiplexer 230 steers its' B input value to its output, which is coupled to the address of a histogram memory 235.
It is presently preferred that histogram memory 235 be a random access memory having a reset line that can be clocked to clear all memory locations.
The data output of histogram memory 235 is connected to an upcounter 240 and the output of upcounter 240 is coupled back to the data input of histogram memory 235.
The elements just described, 205-240, build a histogram when controlled by control logic (not shown) of the current horizontal line.
As described earlier, in the BACKGROUND OF THE INVENTION, a histogram may be visualized as a graph, such as the one shown in FIG.
1, having reflectivity on the horizontal axis and the number of pixels having a certain reflectivity on the vertical axis.
Briefly, the histogram building process of the preferred embodiment involves performing a scan of the reflectivity of the pixels across a line of an image to be scanned, converting the results of the scan from analog to digital form, and digitally recording the number of pixels having each reflectivity value