1. A method for constructing a digital image of a three-dimensional surface of a physical object, this method comprising the operations consisting in: choosing a surface on the object; placing, facing said surface, a projector equipped with a light source, an optical axis, and a mask defining a speckle pattern comprising a multitude of points with predetermined light intensities and/or colors, directing the optical axis of the projector towards the surface to be imaged, projecting, along the optical axis, the speckle pattern onto the surface, acquiring and storing a two-dimensional image of the speckle pattern projected onto the surface and deformed by the latter, by means of an optical sensor disposed in the optical axis of the projector, comparing, for at least a selection of points from the speckle pattern, the image of the deformed speckle pattern with an image of the non-deformed speckle pattern, as projected onto a reference plane, calculating, for each point of the selection, at least the depth coordinate, measured parallel to the optical axis, of the projection of this point on the surface, wherein, in the calculation operation, the depth coordinate, defined as the distance between the projection of the point onto the object and the reference plane, is calculated by the following formula: z ⁡ ( M ) = ( x ⁡ ( P ) + Δ ⁢ ⁢ x ) 2 + ( y ⁡ ( P ) + Δ ⁢ ⁢ y ) 2 - x ⁡ ( P ) 2 + y ⁡ ( P ) 2 ( x ⁡ ( P ) + Δ ⁢ ⁢ x ) 2 + ( y ⁡ ( P ) + Δ ⁢ ⁢ y ) 2 AB - x ⁡ ( P ) 2 + y ⁡ ( P ) 2 f where: P is the point selected within the image of the speckle pattern as projected in the reference plane, M is the image of the point P on the surface of the object; B is the perpendicular projection, onto the reference plane, of the point in the mask corresponding to the point P, f is the distance between the mask and the reference plane, A is the image of the point M on the sensor; x(P) and y(P) are, respectively, the abscissa and the ordinate of the point P within an orthogonal system linked to the reference plane, Δx and Δy are, respectively, the abscissa and the ordinate measurements, within the reference plane, of the deviation undergone by the point P owing to the deformation of the speckle pattern by the surface of the object.