Signal processing for a thermal printer

In a continuous tone color thermal apparatus including a printhead having a plurality of heating elements for printing a line at a time of image pixels, each heating element being selectively energized in response to a digital dye density word to apply heat to a carrier having dye to cause such dye to transfer to a receiver to form a dye image pixel of a line, each dye image pixel having a density which is a function of the value of its corresponding digital dye density word, the improvement comprising:

• a. contrast and color correcting means responsive to each digital dye density word and transfer functions for producing corrected dye density words which are adjusted for contrast and color such correcting means including means for changing such transfer functions;
• b. edge enhancing means for comparing each corrected dye density word in a line with its corresponding correcting dye density word in an adjacent line for adjusting the value of each corrected dye denstiy word to provide an edge enhanced dye density word, said edge enhancing means includes computer means programmed with an edge enhancement algorithm which computes the difference .DELTA. between the value of a corrected dye density word to be printed and the previous coresponding corrected dye density word and in response to such difference .DELTA. adjust the value of the corrected dye density word to be printed by an enhancement term .delta. to produce an enhanced dye density word where .delta. will have a negative value if .DELTA. is less than a and a positive value if .DELTA. is greater than b, where a and b are negative and positive values;
• and
• c. control means coupled to the printhead and responsive to each enhanced dye density word for selectively energizing the heating elements of the printhead to form a line of continuous tone dye image pixels.