Method of watermark with hybrid halftone dots
A method of watermark with hybrid halftone dots that is suitable for a printed shadow image. The watermark patterns that are composed of amplitude modulation (AM) and frequency modulation (FM) halftone dots (their concentration has been calibrated) cannot be perceived beyond a certain distance by the human eye. This is done through outputting the density calibration chart to choose the proper density parameters, which are applied to the manufacture process of the watermark with hybrid halftone dots. The hidden patterns of a watermark need to be pre-processed first, and thereby making the image blocks of AM and FM halftone dots to be combined smoothly. The copy machine, the lenticular lens and the optical scanner can detect the hidden watermark in the printed shadow image. Each secured document possesses a different serial number to achieve the document-version control.
1. A method of making a watermark with hybrid halftone dots, comprising the steps of:
combining an amplitude modulation (AM) and a frequency modulation (FM) halftone dots through a hidden-pattern mask;
using the hybrid halftone dots to make a printing with the watermark;
deciding a density of the AM halftone dot first, then using a calibration chart to choose a corresponding density of the FM halftone dot;
pre-processing the hidden-pattern mask to achieve a boundary of a combined image to be smooth;
making a secured document by outputting the watermark in a shadow image whose density of AM/FM halftone dots is forming a hidden pattern becomes visible after duplicating;
generating hidden patterns of different serial number to achieve a document-version control; and
detecting a hidden watermark by simple tools.
2. The method of claim 1 , wherein the calibration chart of density is produced automatically.
3. The method of claim 1 , wherein the hidden watermark is detected by a copy machine.
4. The method of claim 1 , wherein the hidden watermark is detected by a lenticular lens.
5. The method of claim 1 , wherein the hidden watermark is detected by an optical scanner.