The instrument is chosen in the group composed of a pen and of an eraser.
By pen is here meant any instrument that can be manipulated directly by the hand of a human being so as to draw on a physical writing support a trace. Typically, the trace is visible to the naked eye either directly on the physical support or on a display screen. To leave a trace which is directly visible on the support, the pen deposits a colored liquid or solid to form the trace directly visible by a human being on the writing support. The colored liquid is typically an ink or paint. The colored solid is for example graphite.
By eraser is here meant any instrument that can be manipulated directly by the hand of a human being so as to draw on a physical writing support a trace which, when it encounters a trace left by the pen, erases the trace left by this pen. The erasure can be a digital erasure of data of a trace recorded with the aid of a pen or be a physical erasure of the trace on the support. For example, the physical erasure of the trace on the support can stem from a chemical reaction and/or from a mechanical effect such as abrasion. In contradistinction to the pen, the trace drawn by the eraser is generally not visible to the naked eye.
Here, “drawing” encompasses the fact of traversing the writing support with a tip so as to write or erase.
The writing support is a sheet of paper, a cloth or any other support on which it is possible to draw.
To correctly log the trace drawn by this tip on a writing support, it is very important to correctly detect the existence of the point of contact between the tip of the instrument and the writing support. Indeed, each time that the tip is no longer in contact with the writing support, the logged trace must be interrupted.
Known methods for logging the trace drawn by a tip of an instrument on a writing support, comprise:
1. —the measurement of the position and of the orientation of a magnetic object fixed without any degree of freedom on the instrument,
2. —the computation of the position of the tip as a function of the measured position and of the measured orientation of the magnetic object,
3. —the detection of a point of contact between the tip of the instrument and the writing support,
4. —each time that a point of contact is detected, the recording of the computed position for the tip, the successive recording of these computed positions forming the log of the trace drawn by the tip of the instrument on the writing support.
For example, such a method is disclosed in patent application WO 02/043045. In this patent application, the detection of the point of contact is carried out by measuring the pressure which is exerted on the tip. Indeed, the pressure which is exerted on the tip of a pen is much larger when the tip is bearing on the writing support that when it is in the air. Accordingly, the tip is mounted slidingly displaceable inside the pen and the pen comprises a transducer capable of converting the displacement of the tip into a signal indicating the existence of a point of contact.
Thus, the necessity of detecting the point of contact between the tip of the pen and the writing support is manifested by the addition of a displacement transducer inside the pen, thereby complicating the production thereof.
Prior art is also known from FR2952450A1 and US2008/106520A1.