**2. Projection Mapping Applications**

The ability of the projection mapping technique to alter the appearance of an object by projecting light on it has made its applications more and more common every day, as shown in Figure 2. Under the name of Shader Lamps, Raskar in 2001 introduced the idea of animating white objects without texture by projecting different graphic images on them, altering the appearance of these objects by varying their perception of colour and texture [8].

Amano, by means of superimposed projection, altered the appearance of contrast in real objects that already had their own colourimetric and texture properties [9].

In Wang et al. (2010), the concept of context-aware light source was introduced, defined by them as 'a light source that can modify its lighting depending on the sensor information obtained from the scene'. It is a projection mapping application, which works by means of the previous acquisition of the information of the scene by a camera, so that after the appropriate image treatment, the improvements are shown on the object through a projector. In this way, it allows improving the appearance of an object in real time, even if the user manipulates it. From a context-aware light source, they developed the concept of proxy light, designing a portable instrument which, as if it were a common flashlight, allows the user to illuminate the areas of the object they want, appreciating parts of the object that are not perceptible to the naked eye by humans. This instrument is a very valuable tool for museumgoers, archaeologists, and art restorers [10].

**Figure 2.** Outstanding applications of the projection mapping technique.

Later, in 2014, Revealing Flashlight emerged. It is an instrument that, like the one described by Wang et al., allows users to visualize hidden parts of an object. Wang et al.'s instrument works in real time, so it is limited by the camera resolution and 2D image processing, Revealing Flashlight performs 3D object pre-analysis, then the displayed images are not limited by the camera resolution or by analysis time, thus allowing a more detailed visualization of the object, obtaining the results shown in Figure 3 [11].

**Figure 3.** Image proposed by Ridel et al. [11] where the result is shown after illuminating a cultural artifact with the Revealing Flashlight technique.

Both Punpongsanon et al. and Kawabe et al. used the projection mapping technique in combination with psychophysical studies to alter the perception of movement through lighting [12,13]. Kawabe et al. developed a technique called Deformation Lamps to achieve the perception of the movement of objects while maintaining their original colour and texture [12]. Punpongsanon et al. used the mapping projection technique to alter the perception of bending stiffness of a fabric [13].

Among the applications mentioned above, the application related to the conservation and exhibition of cultural heritage objects will be described in more depth.

Some artworks have had their appearance affected due to the action of external agents, such as non-optimal humidity/temperature conditions or the incidence of electromagnetic radiation from the sun or different sources of artificial light over their years of exposure [14–16]. A restoration by a classical method can then be complicated. However, by means of the projection mapping technique, a restored vision of the artwork can be obtained by projecting an image on it, called in many studies the compensation image.

## *Heritage* **2020**, *3*

Lafontaine presented the precursor of this technique in 1986: he developed a projection technique by which an artwork can be visualized without affecting the yellowing that occurs in its varnish with the incidence of electromagnetic radiation on it [17].

The projection mapping technique was used in 2005 by Peral to virtually restore the appearance of the portico of Saint Mary's Cathedral in Vitoria (Spain) [18].

Aliaga et al. (2008) presented a technique to restore deteriorated objects by projecting light with multiple overlapping projectors. They proposed an interactive restoration algorithm, in which users could select those points they want to restore in a captured image. Then, using a restoration algorithm, one could obtain the compensation image and thus project it onto the object [19].

Aleksi´c and Jovanovi´c used the virtual restoration technique through projection mapping to obtain a restored view of Lazar Vozarevi´c's *Untitled* from 1961. To do this, they were inspired by the technique developed by Stenger in restoring Mark Rothko's murals, since both artists used very similar chemical compositions for the development of their work. But unlike Stenger's work, Vozarevic's work has a three-dimensional geometry, requiring a different restoration technique [20].

Recently, Vázquez et al. (2020) presented a method of restoring artworks by projecting light point by point, characterized by the previously acquired spectral reflectance of the artwork. For their procedure, they used a photograph of Sorolla's *Walk on the Beach* that was previously artificially aged using a Matlab algorithm, as shown in Figure 4 [21].

**Figure 4.** Images proposed by Vázquez et al. [21] where it is shown: (**a**) RGB color matrix that will be projected onto the aged image to compensate for the aged appearance of the artwork. (**b**) Photograph of Sorolla's *Walk on the Beach* artificially aged. (**c**) Photograph of Sorolla's *Walk on the Beach* after virtual restoration process.
