Creating Textures: Color Map (2) – Weathering

In this post I will work on the weathering effects of the color texture, while in the next one I will add scratches and some other remaining details.

The weathering effects that you can observe on the aircraft from WWII era are quite “dramatic”. The paints used in mid-20^th century were not as chemically “stable” as the contemporary coats, thus they could change their hues in few months of intense service. The archival color photos below show an extreme case of this effect (Figure 75‑1):

Figure 75-1 Examples of heavy weathered aircraft

These photos were taken by Frank Sherschel on 14^th November 1942, for the “Life” magazine. The SBD-3s depicted on the pictures belonged to VMSB-241 squadron, stationed at Midway in that time. Marines received these aircraft in July 1942, but all of them were already used before – most probably on the U.S. Navy carriers. I think that in November 1942 these SBDs had accumulated about 10-11 months of the war service. I will use them as an extreme case of the weathering. (It is always good idea to recreate such an ultimate case in the texture, because you can always make your model “newer” by decreasing intensities of the weathering layers. On the other hand, you cannot use more than the 100% of their intensities, thus you cannot make your model “older” than you initially painted).

Having historical color photos of several airplanes used by the same squadron, you can easily determine the general pattern of the stains, smudges and scratches. This pattern repeats with some random variations on every aircraft. It seems that such a radial engine like R-1820 emitted a lot of oil – in their exhaust fumes and in the air flowing from the NACA cowling. (Because of the high “oil consumption” of the R-1820 the oil tank in the SBD was quite large). The non-specular paint of the two-color Navy camouflage absorbed this oil mixed with the soot from exhaust fumes, creating characteristic dark traces along the fuselage and the center wing. The crew most often walked on the center wing, thus you can see on its upper surface the lighter traces along dome rivets seams and darker, “trodden” spaces in between. There are also some scratches in the paint. Some of them exposed the yellow primer, while the others reached the bare metal of the aircraft skin. (Thus I assume that the first layer of the Douglas primer had a yellow/orange color).

You can also notice some white splashes on the outer wings (traces of the coral sand from the atoll?), as well as the repainted areas around the tactical numbers. (On a black-and-white photo, it would be extremely difficult to distinguish these repainted areas from the oil/fumes traces).

What is interesting – in spite of the non-specular camouflage of these aircraft, you still can see a specular highlight at the wing root (Figure 75‑1b).

The key elements of the weathering effect still depend on the technical details of the aircraft skin: rivets and panel seams, bolt heads, inspection doors. However, their pattern is so random, that I cannot recreate them in the SVG image, as I did in the case of the reflectivity map. Thus we need a reference picture of the skin details for painting the color (diffuse) texture. It has to be mapped in the same UV layout (UVMap) as the color texture. To do this, I composed from the key layers of the SVG drawing an auxiliary image, mapped in the UVTech layout. Then I quickly transformed it into UVMap layout using the Bake feature (Figure 75‑2):

Figure 75-2 Baking the reference texture

The old Blender Renderer engine is a better tool for such a direct transformation of a single texture than the Cycles Renderer. In the Blender file that accompanies this article, I created a dedicated screen layout for this purpose. It is named Texture Baking. You can find there a simple script that I used to switch the UV layout of all model meshes between UVTech and UVMap. To bake a texture from UVTech to UVMap layout, you have to switch the current rendering engine to Blender Renderer and disable nodes in the B.Skin.Camouflage material. Then assign the image (mapped in the UVTech coordinates) to its single Blender Render texture assigned to B.Skin.Camouflage material, named Image for Baking. When you click the Render:Bake button, it will generate the resulting texture (in the UVMap coordinates) in the Test image, which you can see in the UV/Image Editor. You can use Image:Save As command to save it to an external raster file.

I created such a reference image for each SBD version I have modeled (SBD-1, SBD-3, and SBD-5), and placed all of them in GIMP (Figure 75‑3):

Figure 75-3 Using technical reference in GIMP

The darker areas in this weathering appear between the rivets seams. I decided that it will be easier to recreate them using one layer of darker camouflage color, overlaid by partially opaque layer that contains the weathering pattern (in white). First I painted in this way the right outer wing (Figure 75‑4):

Figure 75-4 Painting the test fragment

I used this “sample” for testing if such a pattern looks good in the final image (Figure 75‑5):

Figure 75-5 Checking the test pattern on the render

I think that it looks acceptable. Thus I started to paint in this way the weathering of the whole upper surface.

I create the basic pattern of the lighter traces along rivet seams in four steps (Figure 75‑6):

Figure 75-6 Painting the weathering around rivet seams

First I painted the “overall noise” with an “acrylic brush” tool (Figure 75‑6a). Then I changed the tool shape to “pencil” and draw thin lines along rivet seams (Figure 75‑6b). In the next step I used Eraser to make this pattern more random (Figure 75‑6c). Finally, I filled it again with light touches of an irregular brush, to lighten the overall effect (Figure 75‑6d). (This last step is optional, depends how this fragment looks like on the reference picture).

Figure 75‑7 shows the resulting weathering pattern on the fuselage:

Figure 75-7 Weathering of the upper surfaces

As you can see, it differs from the pattern on the center wing. The lighter traces along rivet seams are thinner, the color is more uniform. Using a separate layer, I added a yellow tint to the darker areas.

Unfortunately, in Frank Sherschel’s collection there is only one, small photo which shows the bottom surfaces of an SBD from this squadron (Figure 75‑8):

Figure 75-8 Archival photo of the bottom surfaces

You can identify there some smudges on the outer wing and rear fuselage. However, you cannot determine the dirt pattern for the center wing and the engine cowling. (You can only say that they are dirty).

Well, in this case I had to use another, black and white photo as the reference. This photo shows clearly the center wing section of a typical aircraft (Figure 75‑9):

Figure 75-9 Oil stains on the bottom of the SBD

As you can see, I recreated in my image the exact copy of the smudges from this SBD-4 center wing. They are placed on another layer (Flow) in the Stains layer group.

Finally, I enriched this basic dirt pattern with all the additional details visible on the reference photos: white “burnouts” on the fuselage sides, discrete traces of soot. There are also irregular, darker lines along some of the panel seams (Figure 75‑10):

Figure 75-10 Additional elements of the image and their layers

Each of these elements has its own color and layer. Note that the darker lines along panel seams extend across the upper and lower surfaces. I painted them using the same color, but on the bottom surfaces they are more transparent, to obtain the appropriate contrast. (Painting them, I used a 50% opaque mask for the bottom surfaces).

Figure 75‑11 shows this “weathered” diffuse texture on the model:

Figure 75-11 Updated textures on the rendered model

Frankly speaking, I am still not satisfied with these results. This weathering requires some minor adjustments. For example: on the reference photos it has a slightly different hue. The fuselage below the tailplane also requires some fixes.

While painting all these weathering effects, I came to conclusion that I cannot re-use them without any modification in the three-color camouflage, used in the SBD-5s and -6s. Thus I will not split the color texture into three interchangeable parts, as I announced in my previous post. I will have to prepare few alternate color textures instead:

• A colorful pre-war painting scheme (orange wings!), without weathering for the brand-new SBD-1s from the Marines squadrons (+ eventually the later single Light Gray color scheme, as visible in color photos preserved in the Smithsonian Air And Space Museum);
• Two-color Navy scheme for the SBD-2,-3 and -4s (described in this post);
• Three-color semi-gloss Navy scheme for SBD-5s and -6s (+ eventually the white variation of this  for the SBD-5 and -6 scheme, using on the Atlantic areas);

Of course, I will reuse fragments of the weathering pattern described here in the three-color scheme for the SBD-5s and -6s. However, before I do it, I have to finish this color texture. Thus in the next post I will fix the minor flaws described above, and recreate the scratches visible on the reference photos. Then I will apply the “decals” – national insignia, tactical numbers, etc.

In this source *.blend file you can evaluate yourself the current version of the model, here is the GIMP source file of its textures. Because of the large size of the original GIMP file (*.xcf), this post is accompanied by its smaller version (2048x2048px), packed into *.zip file. I think that such a version is sufficient for checking all the details of this image (the structure of its layers, their opacities and mixing functions). The resulting textures (4096×4096) are packed into accompanying Blender file.