Digital pen using visible image and autocorrelation of substrate pattern

A digital input device, comprising:

• an elongated body defining a distal tip configured for depositing a substance onto a substrate;
• at least one light source oriented on the body to direct light toward the vicinity of the distal tip;
• at least one detector oriented on the body to receive light from a substrate against which the distal from time to time can be positioned;
• at least one contact sensor on the body;
• and
• at least one pen processor on the body, the processor communicating with the detector and the contact sensor to determine, each cycle period, whether to generate delta signals and to store in memory onboard the body, each update period greater than a cycle period, a key frame image of the substrate, wherein the pen processor has a local memory and wherein when a confidence threshold is reached as defined by a comparison between a current frame and a last frame, a delta signal is stored in the memory onboard the body when the contact signal indicates the writing tip is positioned against the substrate, and otherwise the delta signal is stored in the local memory of the pen processor.

A computer-implemented method for digitizing strokes of a writing tip across a substrate, comprising:

• detecting light from a substrate each cycle period;
• comparing a frame of light received from the substrate in a second cycle to a frame of received light in a first cycle immediately preceding the second cycle to generate a delta signal;
• occasionally but not every cycle generating at least one key frame;
• storing the key frames and selectively storing delta signals in memory in a pen body associated with the writing tip;
• transferring the key frames and delta signals to a host processor;
• and
• using the host processor to correlate the delta signals and key frames to a digital representation of writing input, wherein when a confidence threshold is reached as defined by a comparison between the frame of light received in the first cycle and the frame of light received in the second cycle, storing a delta signal in the memory in the pen body when the contact signal indicates the writing tip is positioned against the substrate, and otherwise storing the delta signal in a local memory.