Mapping Asymmetric Details of Aircraft Skin

Although the technical details of aircraft skin are symmetric in general, there are always exceptions. For example, look at the bottom surfaces of the SBD (Figure 71‑1 shows them on my model):

Figure 71-1 Asymmetric details on the bottom surfaces of the center wing and the fuselage

As you can see, there are several details that are not symmetric. (In addition, let’s do not forget about the asymmetric opening under bottom covers of the fuselage, visible on this picture – see Figure 70‑9 in my previous post).

So far I mapped only the symmetric half of the wing on the UVTech texture layout. It occupies a significant portion of the space. Such a size allowed me to draw all the technical details in higher resolution. The plan was that both wings will be mapped in the same points of the UV space, because most of their structure is symmetric. For the few asymmetric details, I was going to prepare additional areas, intended for the UV mesh faces that contain these elements.

Let’s see how it works in practice. I created the right side of the center wing by mirroring its left side (Figure 71‑2a). Initially, the texture image is symmetric, because mesh faces from both sides are mapped onto the same areas in the UV space:

Figure 71-2 Problems with mapping of an “L”-shaped fragment

Then I drew the asymmetric elements of the center wing on the image, and “flipped” an L-shaped selection of the corresponding UV faces onto this area (Figure 71‑2b). However, when I looked at the effect in the 3D space, I saw a huge texture deformation (Figure 71‑2c). Why did it occurr?

The reason of this deformation is the Subdivision Surface modifier that I used to smooth this mesh (as well as most of the other meshes in this model). To preserve proportions of the texture image, I enabled its Subdivide UVs option. When I turned on in the UV/Image Editor the preview of the modified (ultimate) UV faces, I saw the pattern as in Figure 71‑3a):

Figure 71-3 Side-effect of the UV-smooth option (caused by the Subdivision Surface modifier).

Edges of the ultimate, subdivided UV mesh faces are marked in yellow. As you can see, the Subdivide UVs option “smooths” all inner corners of the original UV layout! Well, I cannot disable this option, because it would deform the texture details, on all mesh faces. Still, it is possible to counter this “inner corner” effect by sharping selected seam edges (i.e. by increasing their Crease coefficient to 1.0). As you can see in Figure 71‑3b), I was able to fix most of the original deformation in this way. However, while I could mark as sharp any of the “rib” edges, I could not do the same for the perpendicular “stringer” edge, because it would change the wing shape. (It would alter the side view profile of the center wing).

All in all, the solution for the wings was to “cut out” from their UV layout “stripes” of the faces that span across whole wing chord. Such a stripe has no inner corners (Figure 71‑4a):

Figure 71-4 The solution: UV patches without inner corners

As you can see in Figure 71‑4b, it produces the desired effect. The drawback is that it occupies more precious UV space, and I had to replicate more details on this drawing (for the whole span of such a “stripe”).

There are also few differences between the left and the right outer wing (Figure 71‑5):

Figure 71-5 Asymmetric details on the left wing

Strangely enough, aircraft designers usually place all additional stuff like the aileron tab or landing light on the left wing. At this moment I just marked on the wing the contours of these two lights. During the next, “detailing” phase of this project, I will create all of these three details shown in the figure above as separate objects. However, I still have to modify the bump map texture, because of the different rivet pattern around these lights and frame around aileron trim tab. (When there is an element without influence on the rivets/panels pattern, I skip it at this moment. For example: in the left leading edge of the center wing there is small round inlet of the cockpit ventilation air. It does not alter the rivet seams, thus I will recreate it completely during the detailed phase).

Following the experiences with the UV mapping of the center wing, I stripped two full-span bands of the UV faces from the left wing and the right aileron (Figure 71‑6):

Figure 71-6 Mapped differences of the outer wings

Frankly speaking, drawing details of these additional strips in a way that they seamlessly fit the rest of the wing was quite difficult. As you can see, I also made small adjustment on the leading edge seam, on both wings. (It removed the deformation described some time ago in this post, Figure 64-9).

The UV layout depicted above contains three inner corners, all located on the leading edge. This is a kind of a compromise: I used sharp “rib” edges (Crease = 1.0) to minimize the overall deformation of the mesh UV faces around these points. They still bend the texture along their “stringer” edges (as in the case depicted in Figure 71‑3b). However, in these two particular cases I managed to “hide” this unwanted effect. Figure 71‑7 shows how I did such a thing:

Figure 71-7 UV mapping of the mesh fragment around the landing attitude indicator light

Figure 71‑7a) shows the fragment around the landing attitude light indicator and its faces in the UV space. This is a simple quad, without inner corners. As you can see, I mapped the inner wing edge as a straight line, to facilitate drawing of the multiple rivets and panel seams that run along it. Figure 71‑7b) shows the details of the corresponding inner corner in the main part of the mesh. I used a sharp “rib” edge along this seam. Still there is deformation along the perpendicular “stringer” seams, but it is practically invisible. There are two factors that “hide” it:

1. The edges adjacent to the seam edge are relatively close to each other, which minimizes the deformation size;
2. The seam edge runs in “safe” distance between nearest visible element of the texture image (a rivet seam), so the deformation in the UV mapping disappears before it reaches this image;

The possibility to “cut out” such a small part from the main body of the UV faces preserved precious UV space. It also allowed me to avoid duplicating on the texture picture of all the details along the inner edge of the left wing. (It would require a few hours, to fit such a separate fragment to the rest of the picture).

Apart the differences on the bottom of the fuselage, depicted at the beginning of this post (Figure 71‑1), there are also differences between its left and right side (Figure 71‑8):

Figure 71-8 Asymmetric details on the starboard

The circular door of the life raft compartment was located on the port side (you can see it in the last picture from the previous post – Figure 70‑10). The raft was packed in a tube riveted to the starboard skin, creating characteristic circular rivet pattern (Figure 71‑8). The door to the baggage compartment was also located on the starboard. There were also differences in the locations of the steps to pilot’s cockpit.

The shape of this fuselage is much more complex than the wing. I cannot mark any of its edges as sharp, because it would change the shape of this element. Thus, after the experiences with the wing, I decided that I need to map in the UV space the whole fuselage right side. Fortunately, I preserved some spare space on the original UVTech layout. Now I used it to fit this part (Figure 71‑9):

Figure 71-9 Modified UV layout for the hole/bump map textures

On the picture above, I marked the newly added objects in orange. The main dilemma was how to fit another fuselage silhouette by replacing as few drawing elements as possible. As you can see, I finally decided to “shuffle” the cowling panels from the left side of the original image into the spare area. It created enough space for the fuselage on the left. Note that I also added the right sides of the cowling panels (because they also were asymmetric: there were two inspection doors on the left side of the cowling).

Figure 71‑10 shows the source image of the bump textures adapted to this new layout:

Figure 71-10 Modified texture image

My experience tells me that in the future I will have to update some details of this picture, following new findings in the photo material (it is just a matter of time). Avoiding applying the same modification twice, I decided to join into a group all the originally drawn elements that are identical for both sides of the fuselage and belong to the same layer. Then I created a mirrored clone of such a group and placed it over the right side of the fuselage. After I “filled” this contour with all the required clones, I drew the asymmetric details. In the future, when I change contents of any of these groups on the fuselage port side, they will be automatically updated on the starboard.

I drew the other side of the elevator in the same way. In this case, the whole difference is a plate mounted between two ribs. It contains the hole for the trim tab actuator. Of course, I could “cut out” this very mesh fragment, as I did in the case of the aileron. However, in the SBD the elevator is smaller than the aileron, thus I decided to make the “full size” copy of its opposite side. (Just to make the eventual future modifications easier).

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.