Rendering object attribute is one of the most important issue in generating realistic images for designing a large scale construction. Texture mapping is an excellent technique for rendering the object attribute, because it can generate nice visual effects with a simple process. However, if textures without any stains are mapped onto an object, the generated image loses the reality. In this case, the object looks too new, and it does not harmonize with the background. Therefore, it is important to generate textures taking into account secular changes of objects, such as water flow stains. Techniques of rendering such kind of object attributes are desired especially in generating high resolution images. In this field, we need a realistic texture containing stains which have been adhered to tracks of water flow in a long time.

This paper presents a method for generating textures containing water flow stains to generate realistic images of outdoor scenes. One of the targets for displaying outdoor scenes is a visual enviromntal assessment when large scale constructions, such as electric power facilities and buildings, are constructed; they consists of relatively simple shape objects.

Dorsey et al. calculated the tracks of water flow on the surfaces of complex three dimensional models such as statues by using a water particle model [1]. Although the method can generate realistic images with water flow stains even when objects have complex shapes, it needs a lot of calculation time. On the contrary, our method can generate natural looking textures with water flow stains in a short time. Our target, as mentioned above, is rendering relatively simple objects consisting of several planes. Therefore, our method calculates the tracks of water flow on approximated shapes of three dimensional objects.

The proposed method renders the stains by painting the tracks of water flow with dirt color. The water flow is calculated by water droplet model with a discretized surface [2, 3]. Observing actual walls containing stains, we found that water flow stains can be classified into two kinds of color patterns: uniform and gradation. The gradation means that the center of track is light color and the edge is dark. To represent the gradation pattern, we developed a method for painting stains to the tracks of water flow taking into account the shape of water droplets. That is, when projecting the water droplets onto a texture plane, the center of the water droplet is colored lighter, and the circumference is colored darker.

In order to represent different amount of stains in different positions, we also propose the extended painting method which considers the amount of dirt containing in a water droplet. In advance, we randomly supply some amount of dirt to both water droplets and the discretized surface. Then the transition of dirt in water droplets is calculated while getting the tracks of the water droplets. Moreover, in order to represent stains adhered along a rugged surface, we propose a method for specifying degree of affinity on the discretized surface [2, 3], which controls the direction of water flow.

For representing the effect of changes as years passed, we average several textures generated by the proposed method. We realized that this technique can simply get more natural textures containing stains.

Finally, to illustrate the usefulness of the proposed method, we show the texture generated by the proposed method. Figure 1 shows a resultant texture in which dirt color is adhered by the proposed method. Figure 2 (a) shows a scene of the electric power facilities without texture mapping, while in Fig. 2 (b), the resultant texture is mapped onto the electric power facilities.

Figure 1: Resultant texture.

Rendering without stains (a) and with stains (b).
Figure 2: Electric power facilities.