Creating Textures: Bump Maps (3)

In previous post I have enhanced the bump map texture effect, using two different images. This is the continuation on this subject.

Have you ever noticed that the classic stressed skin of a real aircraft is not ideally smooth? It is more visible in the areas where the skin is thinner, especially on an old, “weary” aircraft (Figure 70‑1):

Figure 70-1 Deformations of the stressed skin (in two different aircraft)

The wing on the left (Figure 70‑1a) belongs to a SBD-4 (BuNo 10518) from Yanks Air Museum in Chino. This wing was recovered separately from Guadalcanal (circa 1980), and restored a few years later. This aircraft is in flyable condition (registered as N4864J), but has not flown since its restoration.

The  wing on the right (Figure 70‑1b) belongs to a SBD-5 (BuNo 28536) from Planes of Fame, also in Chino. This wing was also recovered from Guadalcanal, in the same time as for BuNo 10518. This aircraft was restored, registered as N670AM, and made its first flight in 1987. Since that time it has been flying during various air shows.

I assume that the skin of the SBDs that were flying in 1940-44 resembled the skin of the wing from Figure 70‑1a). Note that the leading edge and the central panels have no visible deformation. (However, their skin still could deform a little in the flight). This is because they were created from relatively thick (0.032”) sheet metal. The buckling of the skin is more visible on the panel behind the rear spar, because it was made from a thinner (0.025”) sheet.

It is quite easy to obtain this effect using textures (Figure 70‑2):

Figure 70-2 Buckling of the model skin

To do it, I re-used the contents of the Rivets layers from the source Inkscape image. However, before I did it, I drew additional, thick gray lines below the rivet seams. I placed these lines on a separate layer, named Shadows (Figure 70‑3):

Figure 70-3 Additional element of the source image: additional lines under rivet seams

Once this was done, I could compose the final texture image using these lines and clones of the Rivets sublayers (Figure 70‑4):

Figure 70-4 Composing the image of the second bump map (nor_blur.png)

First I altered the color of the white Rivets:Dome elements, using a simple SVG filter that blackens everything. Then I blurred this composition, using another SVG filter: cascading Gaussian blur. (For details of this solution, see “Virtual Airplane” guide, chapter about Inkscape, section titled “Using filters”).

Finally, to decrease the influence of this texture on the forward part of the wing, I covered it with a gradient-filled shape (Figure 70‑5):

Figure 70-5 Additional adjustments of the final texture image

As you have noticed, in this composition I re-used contents of the Rivets layers, using their clones. Using such clones in the final texture image allows me to easily modify contents of these pictures in the future. When you alter any element in the source layer, Inkscape immediately updates all its clones. Thus I rearranged the structure of the SVG file (see the layers pane in Figure 70‑5). I grouped all the source layers (Rivets, Panels, Covers, Bolts, etc.) into a layer group named Source. Then I created another layer group, named Result. Each of its sublayers contains the composition of one final texture image (Holes, Nor-Details, Nor-Blur). Their contents is composed from clones of the Source sublayers, with altered opacity and (sometimes) applied various SVG filters. (See the source  Inkscape file)

When I am working on such a drawing, I am drawing new elements (or modifying existing ones) on the Source sublayers. Then from time to time I export the final images generated by the Result sublayers to the raster files, used by Blender (holes.png, nor_details.png, nor_blur.png).

In the process of creating textures, the most troublesome areas are those along seams, especially when such a seam contains a corner. Some time ago I tried to avoid breaking the skin panel edge along such a UV seam (see this posts, Figure 67‑3). Now I can see that this was a bad idea (Figure 70‑6):

Figure 70-6 Deformation of the rivets line near the UV seam

The rivets in the line that runs along the UV seam are skewed. They also have different sizes. All of this has occurred because of the high shape distortion of the bottom faces that belong to the large wing fillet.

I placed the small part of the fuselage inside the UV seam at the center wing. This fragment is undistorted. The remaining triangle (marked in orange in the figure below) is an area where the mesh faces mapped onto UV surface have high distortion (Figure 70‑7a):

Figure 70-7 Altering the UV map of the mesh fuselage

After some deliberations, I decided that it is much easier to join the few rivet lines that run across an UV seam, than to improve these skewed rivets produced by the current UV mapping. (Well, as you can see, the “improvement” of the seam line that I made some time ago was a bad idea). Thus I had to shift the UV seams to the outer edges, and “glue” some additional mesh faces to the center wing (Figure 70‑7b). This time I took care to minimize deformation of the faces that remained outside the mesh seam.

Figure 70‑8a) shows, that I was able to precisely match the rivet lines across this new seam. It was not as difficult as I thought. Figure 70‑8b) shows the UV map of this area and the original image of the panel seams and rivet lines:

Figure 70-8 Final UV mapping and drawing of the skin details

Note that this time only small number of rivets occur in the highly deformed area. On the other hand, because the degree of deformation is lower than in the previous case, these rivets are not ideal, but look “acceptable”, at least.

Figure 70‑9 shows both bump map images, that I mix to obtain the texture of the technical details:

Figure 70-9 Texture images – current state

At this moment, I filled with appropriate details all the common surfaces, and the elements belonging to the SBD-3. As you can also see, I already drew some asymmetric elements on these textures. However, before I map them, I have to apply the Mirror modifiers to the appropriate meshes of my model. I will do in the next post. (I delayed this operation as long as I could, because presence of the Mirror modifiers allowed me easily alternate the model shape. (I had to modify its left side only. Blender took care on updating of the right side). However, after so many months of various checks I can only hope that the shape of this model “seasoned” enough, so I will not have to modify it in the future).

Figure 70‑10 shows my model. (To make the effect of the bump textures more visible, I significantly increased their intensity):

Figure 70-10 Bump maps on the model (increased effect intensity)

Strangely enough, I obtained such an intensity increase by setting control nodes of these two textures to negative values: Moderate:Range = -1 (nor_blur.png) and Range From Min:Min = -3 (nor_details.png).

Actually, the textures of this model are symmetric, which means that there are many missing/wrong details on the fuselage right side. In the next post I will introduce asymmetry to these meshes.

In this source *.blend file you can evaluate yourself the current version of the model, and here is the source Inkscape file of its textures.

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