Facsimile of Queen Maria Luisa on horseback (1799)
by Francisco de Goya

Museo del Prado, Madrid, 2018

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Oil on canvas, 338 x 282 cm

The 3D and colour recording of Goya's Queen María Luisa on horseback – a complex and difficult challenge undertaken by Factum Arte – at the Museo del Prado, Madrid, was the first step towards the production of a high-resolution facsimile that can be used for the study, conservation and dissemination of the painting. Over a number of years, Factum Arte has established a fruitful collaboration with the Museo del Prado, employing digital technology to record some of the most important works in their collection. The purpose of these digitisations has been to facilitate access to the Prado's treasures for experts, artists and the general public, and to produce an archive for current and future generations.

Queen María Luisa on horseback Ⓒ Museo del Prado

The recording of Queen María Luisa on horseback has been made possible by renowned American artist and film-maker Julian Schnabel, whose work can be found in the collections of leading museums around the world. Schnabel's films, including Basquiat and Before Night Falls, have won major awards at the Venice Film Festival and the Cannes Film Festival, amongst others. Julian Schnabel is one of the most important contemporary painters whose works are embedded in a deep knowledge of the past.

The production of a facsimile of Queen María Luisa continues in the Prado's long tradition of encouraging both great artists and students to make copies of works in the collection as a way to study the old masters. In this case, however, the 'copy' will take advantage of the latest technologies, allowing us to extract high-resolution information from the painting and transform it using digital tools in order to create a new interpretation of the original. The process of creating a facsimile requires the active collaboration of experts from different disciplines - it is rethought for each project and often leads to new insights into the work of art.

The reproduction is being carried out in three phases: (1) recording colour and relief, (2) processing the data, and (3) fabrication of facsimile.

1. Recording colour and relief

Queen María Luisa was recorded by a team from Factum Arte at the Museo del Prado in early March, when the painting was removed from its frame and transported from the gallery to a dedicated conservation space within the museum. The canvas was secured and fixed to the wall in preparation for digitisation with non-contact methods.

Panoramic composite photography – in which hundreds of overlapping, high-resolution images of a surface are taken from a single view-point – was used to capture colour data. With the camera at 6 m from the painting, it took over six hours to capture the entire surface.

Photographing the surface of a painting at high-resolution is a difficult task. The varnish on this particular painting resulted in reflections and the featureless areas made it hard to achieve a perfect focus. A second photography session, which took place in the museum's gallery space in May, was necessary to complete the recording at the quality that was requiered.

A second photographic recording session was carried out in the gallery space

A second photographic recording session was carried out in the gallery space

A second photographic recording session was carried out in the gallery space

Capturing relief data for Queen María Luisa on horseback was also a great challenge. It is one of the largest paintings digitised to date with the Lucida 3D Scanner, a system developed by artist and Engineer Manuel Franquelo and Factum Arte, with financial and logistical support from Factum Foundation. Lucida is a tool specifically created for conservation and accurate surface recording. It obtains 3D information of the surface of paintings and other low-relief objects, producing data that reveals the complexity of the surface. Lucida has been used in dozens of museums and collections around the world, including leading institutions like the Museo del Prado (Madrid), The National Gallery (London), and the Musée du Louvre (Paris), digitising more than 140 paintings since 2011.

A painted surface can be considered an object with 2.5 dimensions – that is, a flat plane with shallow relief. It is the recording of this subtle relief that will introduce a material quality to the data and the facsimle: the result is not only an accurate copy of the colour and composition but also a representation with all the material evidence of the painting. The Lucida captures relief information by understanding the relation between tone and relief. The virtual model generated by the scanner is a grayscale depth map image that assigns different heights to different grey tones: thinking of volumetric information as an image facilitates the series of transformations that occur in the digital processing phase.

In order to minimize disruption to the museum and to return the painting to the gallery as soon as possible, the recording was limited to three days. The timeframe was tight, which meant it was only possible to capture the central area of the canvas - a 40 cm perimeter was recorded only using composite photography. During data processing, 3D and color information from the original will be transformed to recreate the rich character of the surface.

The most significant issue with the recording of Queen María Luisa was that the enormous canvas vibrated throughout the 3D scanning process. The vibration was caused by the air conditioning in the museum and the movement of air as people walked past the painting. The data captured showed an artificial pattern of lines, affecting the quality of the information. As it will be described in the next section, it was necessary to think outside the box to come up with creative processing solutions in orider to minimize the interference of the vibration pattern with the true relief data.

2. Data processing

A series of digital transformations are necessary to prepare the data for eventual re-materialisation. The process consists of: (a) editing (cleaning, optimizing) the single files obtained by each system (photography and 3D scanning), (b) stitching the single files together in software like PTGui (both 3D scanned tiles and images), and (c) registering the color information on top of the relief information to create a multilayer file.

The following section describes the specific process followed for Queen Maria Luisa, one of the most difficult projects ever faced by Factum Arte:

2.1 Processing colour

Nearly 1,000 colour photographs of the painting were taken during the recording – these are used to produce a single, high-resolution image of approx. 500 dpi at 1:1. Each photograph shares more than 50% overlap with adjacent images; in a semi-automatic process, PTGui software finds common points between these in order to create a continuous panorama.

The first phase of the colour recording entailed specific concerns as there were too many pictures of areas of the painting that were out of focus. Due to the lack of details and contrasts on vast backgrounds, the cameras were technically unable to focus properly. It was thus necessary to adjust the white balance of all the pictures according to the colourchecker passport. A colour profile was generated and applied to all images that were then stitched through PTGui. During the stitching process, it was crucial to review that all the pictures were in focus and identify the ones that were not - in such case, it is possible to substitute them with one of the surrounding images. The stitched images were finally exported in a 16 bit TIFF format and the first colour and grey-scale corrections carried out, using the colourchecker passport in order to achieve an accurate exposure on all the areas with reliable and consistent results.

One of the 900+ individual shots taken in the composite photography recording

One of the 900+ individual shots taken in the composite photography recording

One of the 900+ individual shots taken in the composite photography recording

The process of aligning all the images in one panorama is carried out with PTGui

The assignment of control points to merge contiguous images is a semi-automatic process

2.2 Processing relief

The Lucida 3D Scanner records data 'tiles' measuing 48 x 48 cm, both as grayscale depthmap (32bit TIF) and shaded renders (8bit TIF). The tiles must be individually edited before stitching. Factum employs in-house software to reduce noise in the data (artifacts of the recording process that are not present on the surface of the painting).

Detail of 3D data before editing, showing artifacts not present in the real data

Detail of 3D data after editing, showing artifacts not present in the real data

Tile of 3D data before editing, in which the data look closer to the true surface

Tile of 3D data after editing, in which the data look closer to the true surface

Due to the dimensions of the painting, it was impossible to avoid micro-vibration in the canvas during the 3D recording process and as a result, the relief information showed a regular pattern of parallel lines that are not seen in the real surface. It was necessary to reduce the presence of these lines in the data, whilst keeping as much of the surface detail – captured with a resolution of 100 microns or 10,000 points/cm2 – as possible. Any artificial smoothing of the data must find a balance between cleaning and preserving surface detail in order to maintain the original character of the object. In this case, a frequency domain filter was applied directly to the TIF files (both shaded and depth map versions), with promising results.

Detail of the 3D data before applying the 'anti-vibrations' filter, where the pattern of vertical lines is visible

Detail of the 3D data after applying the 'anti-vibrations' filter, where the pattern has disappeared

So as to fully understand the quality of the recorded 3D data, it is usually recommended to reproduce it as a physical prototype. CNC milling in 2.5 dimensions or 'elevated printing' (a technique developed by Canon Production Printing) can be used to obtain a high-quality re-materialization of surface relief data. In this case, a series of relief details were milled on high-density polyurethane resin in order to study the surface prior to applying the filter. The same details were printed by CPP to study the results of the filter. The results of the comparison highly satisfactory, given that the line pattern was eliminated whilst the surface detail was mostly preserved.

Surface detail before and after 'anti-vibrations' filter, reproduced with CNC-milling and Océ's 'Elevated Printing'

Surface detail before and after 'anti-vibrations' filter, reproduced with CNC-milling and Océ's 'Elevated Printing'

[R] Closer surface detail before and after 'anti-vibrations' filter, with CNC-milling and Océ's 'Elevated Printing'

[L] Closer surface detail before and after 'anti-vibrations' filter, with CNC-milling and Océ's 'Elevated Printing'

Océ's 'Elevated Printing' samples of the surface texture

Océ's 'Elevated Printing' samples of the surface texture

After applying the 'anti-vibration' filter to each individual tile, the next step was stitching them together using PTGui. The process is similar to the one carried out for the colour data, here with grayscale shaded renders instead of colour photographs.

Panoramic composite software is based on finding and assigning common points between overlapping photographs, after which the software carries out a series of transformations and distortions to create a single, coherent image. In the first attempt to stitch the different tiles together in PTGui, an unexpected artifact appeared in the form of a beautiful geometric pattern, probably occurred because of an error in the default settings of the project.

The artifacts appeared as fake information, overlapping with the true surface detail

The artifacts appeared as fake information, overlapping with the true surface detail

After analyzing the resulting artifact, it was clear that it was actually relief information, not just a visualization issue. The process was repeated with proper settings, but these 'noise' could become the basis for further image research

The 3D recording of Queen Maria Luisa was carried out with two scanners working in tandem, which recorded the painting from two different positions, turning the canvas by 180º to record top and bottom. This resulted in were four independent sections, each corresponding to a quarter of the total scanned area. Once the stitching problem had been analysed and solved, the process in PTGui consisted in obtaining these four sections: position 1, scanner 1/ position 1, scanner 2/ position 2, scanner 1/ position 2, scanner 2.

Shaded render of the section 'Position 1, Scanner 1' (approx. 131x121 cm)

Shaded render of the section 'Position 1, Scanner 2' (approx. 131x121 cm)

Shaded render of the section 'Position 2, Scanner 2' (approx. 130x131 cm)

Shaded render of the section 'Position 2, Scanner 1' (approx. 134x132 cm)

Shaded render of the section 'Position 1, Scanner 1' (approx. 131x121 cm)

The next step of the process will involve producing the rematerialisation of the processed data.

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