Modelling a face

How do window makers create a face? 

When reproducing a human face in a real stained glass window, more then one method can be used:

- A single pink glass pane can be painted with grisaille to line the anathomy part, as for example the nose or the eyes.

  - The face can be subdivided in different glass panes. This method is very uncommon though, but it can be used for special artistic needs. Moreover, it can be the result of a previous restoration, if done using mending leads.

- The glass pane can be painted as a proper "stained glass". This technique gives a remarkable effect, as evident form the image at [3]. The face details (lines of the eyes, nose, mouth, traits of the hair) are painted with grisaille. In transparency, a yellow shade is visible, due to presence of silver stain, generally in the position where hair or the halo is. Grisaille is painted on the interior face of the glass, to protect the drawing from weathering. Silver stain is found on the exterior face [2].

From the modelling point of view, one of them should be chosen. But, which one? It is evident that the first method is just a simplified version of the third one, therefore there is no point to reproduce it. 

The second method can be reproduced using the methods already published in previous posts, since the mending lead is just another kind of lead came.  Again, no point.

Instead, the third method is interesting. It involves at least two different surfaces: the grisalle one and the stained one. It can be considered as an elaborated version of the first method and, of course, it was used during the Middle Ages.

To get a good idea of the thing, it is always preferable to see a glass window from very close, hopefully in situ, instead of a simple picture. Luckily the Victoria&Albert Museum in London has a big collection of stained windows, very easy to see without need for special scaffolding or binoculars, and among them this stained glass dated to 1240-1380 [4 - Museum no. Circ. 95-1930] shows exactly what I want to reproduce. 

How to model the 3D stained glass face?

Let's think to how to model the face of the figure depicted in the stained window. 

The first idea is to try to reproduce it just as it is. Therefore the glass pane, already casted in order to obtain the proper shape of the face, needs to be divided in 3 different sub-objects: 
- the interior face (with the grisaille painted on), 
- the border (not sure if the border is going to influence the final result, since it is coated by the lead cames, however, in case of a close up on the border, also this feature would need to show a proper material)
- and the exterior face (to be partially stained).
Of course, every sub-object needs to be linked to a specific material. 

Before starting...

Take care that many materials can be used for the glass, and a specific work should be done in order to find which is the combination of material/renderer that gives the most realistic results in the smallest computational time. In fact, glass material is remarkable when rendered via Mental Ray, and many glass materials are present in the library too, but they not necessarly meet the requeasts of a Middle Age glass colour and, more important - since the colour can easily modified - the time consumption to render an image can be remarkable. Moreover, according to the material/render chosen, the material rollout changes, so, even if the result to be obtained is the same, the terminology could change and  be confusing (for example, opacity/transparency/cutoff maps seem to be work the same)

However, since the Medieval glass is a soda-lime glass, with no presence of lead to increase the refractive index, indipendently by the material to be chosen, its refractive index (R. I.) should be defined between 1.45 and 1.55, according to [1].
 
Another general rule is that the materials are not supposed to just "wrap" the surface, but they have to simulate the "filling" of the object. It means that, for example, to allow the silver stain (that would wrap only the exterior face) to be seen in transparency from the interior face, the "2-sided" option should be thicked. If the option is not present, it is important to check that the interior part is rendered properly.

Before starting, another thing needs to be considered. If the background is black, every transparent material will appear black in the final rendering! Therefore a more realistic background should be chosen. A good choice seems to be a gradient map with three different colours, a blue one for the sky, an almost white colour for the horizon and a green one for the grass. Of course, when a proper environment and lighting will be added, the materials will be properly corrected.

Defining a material for the border (clear glass)

The border is the easiest part for applying a material on, because its material is supposed just to represent the clear glass.

However, the level of clearness should not be the one of a modern glass, almost pure in silica and therefore colourless, but that of an ancient glass, with presence of some defects that actually reduce its transparency and give a pale tint.

The tint would be almost invisible in transparency, thanks to the low thickness of the glass pane. But it would be visible  when watching at the glass pane in glazing angle (for example, when standing close to the wall of the window, looking from the very bottom of an high window) or when watching it from the edge instead of the face (possible only when the glass pane is detached from the lead, or when the leads are so bended that the window is no more plane). However, this increase in the tint should be already been calculated by the software.

The tint should be chosen between bluish/greenish/brownish (to represent the excess of iron in different oxidation states) and slightly purple (to represent an excess of manganese, used as a decolourant to contrast the colour given by iron [5]). For a general idea, some realistic tints can be inferred from reference [6] (Of course, take care to look only the bottles that were produced without adding any other colourant a part from iron/manganese!)

A starting point could be to use a olive green tint, that is very common in ancient glass (see [7]) as a test colour for the entire pane. When everything works well, the colour of the faces should be changed to an almost uncoloured tint.  

Defining the material for the interior face (grisaille)

This material should be composite of the following:
- grisaille (with a degree of transparency that allows to see the clear glass material)
- and the clear glass itself.  

Clear glass should be defined as the diffused colour, while grisaille should be represented by a map, created via a classic 2D photo editor, to be allocated in the diffused colour slot. The map should be in black and white (considering black for grisaille). Otherwise the grisaille could be coloured accordingly to its correct brownish/black colour and the glass to be left white. 

To maintain information on transparency (and avoid the glass to get a "milky" appearance, due to the presence of the pure white colour in the .jpg), the grisaille map should have, in addiction to RGB channels, an alpha channel, in which the area where the grisaille is present should be opaque, while the area of the clear glass should be transparent to allow to see through. Since .jpg doesn't support this option, only .tiff or .tga should be used for the map. 

To give a sense of roughness to the grisaille, a bump map can be applied. Unfortunately the use of a bump gives a certain 3D sense, but it also add an "ancient and weathered" appearance to the glass pane, that is pretty interesting when trying to reproduce an ancient glass, but it is not if the glass is supposed to be contemporary. Of course, it can be seen only with grazing angle, so it is pretty useless when the window needs to be seen in front. However it could be taken in consideration when modelling a weathered pane.

Another solution could be to use a displacement map for the grisaille. Displacement doesn't involve a change in the colour of the pixels, but only a motion of the points from the glass to the exterior, to simulate the light roughness of the surface. However, to get a good result, the number of polygons at the surface should be increased and therefore the general computational time could became excessive for such a small thickness (grisaille is less then 1 mm thick!).

Defining the material for the exterior face (silver stain) 

Now it all becomes easier. The silver staining, for the needs of a model, is just a different kind of grisaille, with a different colour and transparency. So everything goes the same, except to take extra care to apply correctly the diffused colour map (n.b. the exterior face of the glass pane is specular to the interior one!)

Final considerations

The glass pane should look like well now, a part from the colour of the grisaille/stain, that could be not as intense as it is supposed to be.

In fact, with this method, grisaille is applied thanks to a .tiff image with its own alpha channel dedicated to transparency. In addiction, the degree of transparency of the entire material needs to be homogeneously decreased, in order to see the pale colour of the glass! (Otherwise grisaille would be painted over a colourless pane, that is not the case I want to reproduce). So, the resulting colour of the grisaille could turn not as intense as it is supposed to be. Therefore, instead of creating a new map with a semitransparency information for the clear glass in the alpha channel (it would be too challenging to menage once applied on the object!) the output of the grisaille map can be increased in order to reproduce the requeasted dark colour. 

When using the mental ray material for glazing, the problem is easily solved because the general colour slot has a specific slider for the degree of gray in the alpha channel. It also allows to easily reproduce the frosted effect caused by surface weathering simply moving the "glossiness" slader to low value. Again, even in this case, if the grisaille fades too much, its output should be increased.

The image (fig.3) shows the final result.

Fig.3

stained glass pane (isolated from the rest of the window)

References
1. Newton R., Davison S., Conservation of Glass. 1989. Butterworth ed. 
2. Corpus Vitrearum Medii Aevi glossary - 
http://www.cvma.ac.uk/resources/glossary.html
3. Wikipedia, Silver Stain definition - 
http://en.wikipedia.org/wiki/File:Silver_stain.jpg
4. Victoria&Albert Museum -  
http://www.vam.ac.uk/content/articles/s/stained-glass-1240-1380/
5. Colours of the Medieval stained glass
http://en.wikipedia.org/wiki/Medieval_stained_glass#Colour
6. Bottle colours - 
http://www.sha.org/bottle/colors.htm
7. Olive colour - 
http://antiques.about.com/od/madeofglass/ss/Colored-Antique-Glass-What-Gave-It-Those-Beautiful-Hues_4.htm
 

Modelling a rondel - "rullo veneziano" type

What is a rondel? 

In general, a roundel is a glass pane with a round shape. The simplest method to create it is to cut a bigger glass pane to a round shape. However, this method is not very interesting from the point of view of 3d modelling. 

The second and most interesting method is to create a new glass pane after a proper modelling, each piece one by one. This second production technique [8] is used in the typical Venetian "rulli" or "rui" and it is a very ancient technique. Just as an example, a picture of this kind of glass pane refers it as dated to the sixth or seventh century [1]. 

The method is very similar to the one used to produce crown glass [9], but it is used to create singular small panes (about 15 cm diameter) instead of a big sheet of glass to be cut in regular pieces with a grozing iron. 

How to model a "rullo veneziano"?

A simple method consists to create a line drawing the half section of the rondel and then use the "lethe" modifier to create the 3D object. To have a general idea of the shape it is important to have a picture as reference. The ideal situation would be to have also a proper section of the rondel, but, to obtain it, a broken glass should be necessary! Since in my project I have no access to such material, the thickness I will choose is an hypothetical one. 

However, some features can be reproduced properly. For example, it is important to give a "broken aspect" to the pontil mark (i.e the exterior part of the bull's eye, where the glass was detached from the pontil). On the opposite side, where the glass is smoothed, the model should be in accordance. 

The edge should also be thicker then the surrounding glass, because the method involves a particular attention to it. The image [1] suggests that a U bended sheet edge is present, even if the picture is not very clear. It is likely the the edge is not bended, but just thicker then the rest. 

From the video, it is evident a certain attention to the edge, but its cross section is obviously invisible. If the edge bends, it is supposed to bend in the direction of the pontil mark, because it would be the most comfortable direction according to the position of the glass-maker when creating the rondel. 

However, in case a proper section of such kind of glass becomes available, and if a close up is needed (remember that the edge is covered by the lead!) the cross section should be shaped accordingly.
 

 

fig. 4
half section of the genuine rondel (grid = 1 mm)

fig. 5 
3D model of the rondel with pontil mark in evidence  (grid = 1 mm)

fig. 6 
3D model of the rondel on the opposite side  (grid = 1 mm)

References

1. Newton R., Davison S., Conservation of Glass. 1989. Butterworth ed.
8. production of a rondel at Lamberts - 
http://youtu.be/TuDl27MZDnY
9. production of a crown glass, as shown in Alan Macfarlane Youtube channel -
http://youtu.be/AYzrBIv0VbU

Modelling the glass panes

What is the shape of a glass pane? 

The glass panes have different shapes and thickness according to the production methods used during the ages: handmade flat glass, bull's eye, plated glass, cathedral glass, float glass...

However, the most common panes for stained windows, realistically used also during the Middle Ages, are made of handmade glass, generally produced by the cylinder method. (this technique involves blowing a big glass cylinder - muff [2]- and then cutting it in order to obtain a rectangle to be shaped accordingly to the figures of the window). Therefore the aspect of an ancient glass pane is that of an almost flat plane, with a thickness that can realistically be up to 5 mm [1].

Method for modelling the glass panes 

Modelling every glass pane one by one would be very time requesting. Therefore a different method seems to be working well. It consists in creating an ideal glass pane of the dimension of the entire window, 5 mm thick, and then casting out (via proboolean tools, subtraction and cookies off) the single glass panes, substantially deleting the part occupied by the lead cames. Each single pane then to be detached and modified according to the requested colour, map (in case of grisailled glass), thickness or texture to generate single variations.

To recreate the Middle Ages supposed shapes, the lead sections used for casting are visible in figure 1, numbers 1-5 (fig 1). When using the more realistic lead sections (fig.1, numbers 4-5), the casting leds to a curious glass pane border, with a rounded shape. Obviously, this is not the appearance of a glass cut for stained windows! Actually, the glass is cut very precisely (typically a oil filled glass cutter), but in Middle Ages a very imprecise tool was used, called grozing iron [2], that caused the edge of the glass to be very irregular. However, in both cases, the border of glass will not be visible in the virtual model, because it is under the lead leaf. Therefore the simplest way should be chosen, and a special set of lead sections (called “simplified lead cames” in fig. 1, numbers 2-3) be used in order to virtually cast the glass and obtain a simple edge. In case one of such glass panes would be used to zoom in the border details, a simulated grozing iron border will be created on purpose. 

The putty problem 

Putty is commonly inserted between lead came and the glass edge, in order to prevent movements of glass and to increase the impermeability of the window. This object is however hidden in a real window, unless the lead is very deteriorated. Therefore even in this case, it will be not modelled in the virtual one, unless specially needed.  

Reference

1. Newton R., Davison S., Conservation of Glass. 1989. Butterworth ed.
2. Corpus Vitrearum Medii Aevi glossary

Choosing the subject

Which window should I recreate? 

The first idea would be to reproduce an existing stained window. Of course, for the purpose of this internship, such window should show some important characteristics: 

-  a simple drawing (for time problems), 

- a huge documentation (to distinguish original parts from restoration ones), 

- the window should be easy to rich in situ for a proper observation (since glass is transparent, and it obliges to be seen against the light, only a proper observation from very close could help to understand its typical features. Moreover, the fact that the leads are black, doesn’t help to visualize them unless from very close), 

- the support of experts (of primary importance in case the previous data were insufficient). 

Unless all such things are present, reproducing an existing stained window could led to a fake model.

The best idea would be to create a model with more levels, in which both the original supposed shape and the nowadays visible object could be shown. Unfortunately there is no time to perform such a big project in few months, especially if the drawing of the window is not simple.

Therefore the most sensed idea is to start from an arbitrary window (in order to not be confused with an existing one), but to build it with materials and methods known to be present in a specific historical period, in particular in the Middle Ages. In this way, no problem of faking objects would arise and the methodology would not suffer. The model so created would be complicated enough to show the biggest variety of features that a real Middle Age window could show, so that its features would be useful for future re-use. On the opposite, if the drawing was too complicated, simplifications would be allowed without leading to a fake object. 

So, since it is arbitrary, why not to start from my own drawing? The poetic licence would be that the the subject and the lead paths would not be typical of the Middle Ages, and that statical problems would not be taken in account. However, the work intends to be a starting point for the modelling of the details of a stained window and, when developing the project and applying such experience for the study of an existing window, such problems would not arise anymore. 

About the artwork used as a guideline for the 3d model of the stained window 

In order to have a reference drawing for the model, I choose the following artwork (fig.2). The artwork shows the working principle of a Scanning Electron Microscope. In this instrument, an electron beam interacts with the surface of the object under test generating secondary electrons (at the surface). Since secondary electrons arise only from the very surface of the object, their map is exactly the picture of the object under test.

In my drawing, the primary beam of the electron microscope is represented by the vertical lines in the background. The woman is the object to be studied. The secondary electrons are represented by the veil. The microscope image is therefore the map of the veil. Hollow objects, and in general everything under the veil, are invisible because secondary electrons come only from the surface of objects. 

The costume is inspired by the “Lucia di Lammermoor” opera.  

Fig. 2
Title: 30.03.2012_SecondaryElectrons
Size: 100 cm x 35 cm.
Media: acrylic / china ink / coloured pencils on paper.
Year: 2012

Starting to model a 3d glass window: the lead cames

What is a stained glass window made of?

 

A stained window is a complex object, made of different materials. Strictly speaking: the glass panes, the lead cames (that join the glass panes together) and the ferramenta (the last one providing support to sustain the glass plane, that otherwise would be very weak, due to the weight of glass and lack of strength of the lead). Each feature needs to be modelled separately. 

What is the structure of a lead came? 

 

A came is a grooved strip of lead. Its section is generally shaped as a "H" letter, with two glass panes supposed to fit in the apertures (see a selection in fig. 1, grid = 1 mm). The "leaf" is the part that is always visible in a stained window and it is in direct contact with the faces of the glass panes. The "heart" is the part that is commonly invisible. It is in direct contact with the borders of the glass panes. The shape of the leaf can be round, in the case of panes used for round lines, or flat, if used for straight lines.  

Method for modelling the lead cames?

The more efficient method for modelling the cames of a stained window seems to be to create a shape (with the cross section of the lead came) and then loft it through a spline with the appropriate shape of the lead cames. To avoid intersection of lines, many different separated splines are supposed to be created. When sharp angles are obtained, they are supposed to be sliced, like in the real word, in order to avoid unrealistic intersections. 

Which lead section should be chosen for my model? 

The section of the came is variable. In particular it changes according to:
- technical problems (round cames are good for round lines, because they bend accordingly to the shape without wrinkling, while flat lines are used for straight lines, wideheart cames are used for plated glass, etc etc);

- the historical period of the window (for example, during the Middle Ages, very thin lead cames were in use);
- restoration needs (releading is a common restoration method);

- artistic needs (it means changing the thickness of the “black line” of the edges of panes according to the final aesthetic result)

So, which lead section shall I chose for my model? To answer this question is important to decide what is the aim of the project itself. It could be to create a model of an existing window in order to keep knowledge of its actual conservation state, and then visualise an hypothetical model of how the window should have look like when it was built. In this case, at least two different kinds of cames would be needed: the actual cames (generally introduced during a releading in previous restorations) and the original (or supposed to be original) ones. In my opinion, since only small Middle Ages windows are nowadays visible in their original appearance, due to corrosion and multiple releadings, this special period could be the most interesting to visualise.

 

Middle Ages versus Actual lead cames

Reference [1] states that during Middle Ages (fig 1, numbers 1-5) very thin leads were in use, up to 3 mm in leafwidth, while the standard leaf used in nowadays restoration is 6 mm (fig. 1, numbers 8-9). Middle Age used even 1.5 mm narrow lead cames for mending leads (leads used to compose a broken pane - it is the old way ancient restorators used to link broken glass, when silicone or other resins did not exist yet). Nowadays, if the aim of the restoration is to maintain this special feature, mending leads are 3 mm (fig. 1, numbers 6-7). 

The other dimensions (thickness of the leaf, of the hearth, depth of the heart) were stated because, so far, no other references were found on the subject. In particular, thickness of the leaf was stated to 1 mm for everything. Hearth thickness was stated to 1.2 mm when used for nowadays cames and decreased to 1 mm when used for the Middle Ages cames. The depth of the hearth was maintained at 6 mm for every section, in order to allocate glass panes that, in Middle Ages, could have been up to 5 mm thick, according to [1]. However, if visualising plated glass (i.e. two glass panes coupled in the same slot), at least 12 mm will be necessary.

fig. 1 Sections used in the modelling of the lead cames, 1 mm grid.

In details:

1- Middle Age mending lead.
2- Middle Age round came, simplified for modelling.
3- Middle Age flat came, simplified for modelling  
4- Middle Age round came, original.
5- Middle Age flat came, original.
6- Nowadays mending lead, round.
7- Nowadays mending lead, flat.
8- Nowadays round lead.
9- Nowadays flat lead.

Reference

1. Newton R., Davison S., Conservation of Glass. 1989. Butterworth ed.