1 Introduction

With the development of aerospace and computer technologies, the digital visual interfaces have been used into the avionics system. Head-up display (HUD) is the main flight display of the modern aircraft [1]. HUD as a new display, it can project flight data onto the transparent display in front of the pilots, and so that pilots could Head-Up get more airplane flying information. HUD as a transparent display interface, it is particularly important to accurate and clearly overlay information to a complex and real-time environment. It plays a huge role in getting flying information especially during the emergency condition.

The information of the existing HUD interface is generally presented with single green color, while the background environment will change because of the time and region. During the process of actual use, the display information will be affected by the external background color and light conditions [2]. So there is a serious problem that the cognition of flying information projected to the HUD interface will be affected in the green environment such as grassland and forest. This condition make the pilots not easy to distinguish information clearly, even lead them missing information or take in something wrong and then may make airplane in danger. Due to the change of background environment, HUD foreground color becomes an important research in the HUD interface design. In developed countries like America, certain research has been conducted in pertinent fields. Such as the HUD of F-15C has (red/yellow/green; two kinds of color depths) six color modes. But the study of foreground color adaptive system of aircraft HUD is very less. In consideration the importance and insufficiency of this problem, this study is aimed to explore the way of extracting main color from the background image and then give the appropriate HUD foreground color.

2 Basic Theory of HUD Color Design

2.1 Color Space

Processing color images, the most basic job is to choose the right color space. Color space is a color model that represented by mathematical method. With the development of color image processing technology, the common color space are RGB, HSV, NTSC, YCbCr. They have their own characteristics during image processing. HSV (Hue, saturation, value) is one of the color systems used by people to choose colors from the palette or color wheel, HSV system is more approximate to people’s experience and perception than others, and thus more close to the characteristics of human observation. So in this paper, HSV model is used in the main color minutia extraction. HSV color space is created by A. R. Smith based on the intuitive nature of color. It can also be called a hexagonal pyramid model [3].

2.2 Human Cognition

The Relationship Between Hue and Cognition.

Hue is one of the main properties of a color. In the system of color, usually use twelve ring hue circle to present color hue (Fig. 1). The twelve ring is composed of 12 basic colors, including the first three primary colors.

Fig. 1.
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Hue circle (Color figure online)

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Two colors separated by 180° are called complementary colors. Complementary color combinations have strongest contrast, allowing users to produce a sense of irritation and instability. Colors separated by 15° are called analog color. Analog colors are low contrast and user will be in a calm mood. Colors separated by 60° are called adjacent colors. The combination of adjacent colors is elegant and gentle and has a good recognition [4].

The Relationship Between Value and Cognition.

Value is the feeling of brightness and it also can represent the brightness of color [4]. Value can exist independently, even if there is no hue, value can also show without color (Fig. 2).

Fig. 2.
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Value change

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Any kind of color in nature has value. And the way that improve or reduce the value is also suitable for color. In other words, adding black will reduce the lightness and adding white will improve the brightness. In real life, a color will appear dark color in the weak light and will appear bright under the glare of the light (Fig. 3).

Fig. 3.
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Green value change (Color figure online)

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The Relationship Between Saturation and Cognition.

Saturation refers to the degree of color. The saturation of color is calculated based on the degree of gray, and it will become low if color mixed with black or white (Fig. 4).

Fig. 4.
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Color saturation change (Color figure online)

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The color with high saturation is bright and have strong visual impact. It easy to cause the visual attention but if long-time visual easily lead to visual fatigue and then increasing user load. On the contrary, the color with low saturation only has weak visual impact on user. Although it is suitable for long-time attention but not easy to cause the user visual attention.

2.3 Color in Avionics System

Color is one of the basic elements of avionics system interface design. On the one hand, color can highlight the important information in the avionics interface to guide the pilots to complete their tasks well and improve the efficiency of the system, on the other hand, user psychological feeling is affected by different color significantly.

The HUD color design should follow the standard of color application in avionics system. At early 1980s, many countries began to explore the colorful head-up display, and carried out some relevant researches. At present, the color head-up display technology is mature. The different information should be expressed in different color. Such as for the abnormal information showing, the red is better than yellow and green is not appropriate [5]. For this study is about the main color of HUD, we should avoid the warning color and fluorescent color.

Through the analysis of the relationship between hue and cognition (2.2.1), this paper choose the color separated 60° with the background color as our mainly color. In addition, after the exclusion of alarm color we could get the following figure with color (Fig. 5).

Fig. 5.
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Color matching diagram (Color figure online)

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3 Color Processing Method

3.1 Image Partition

Aircraft head-up display interface have different information layout, including the flight mode notification area, the airspeed indicator, the attitude guidance area, the height and glide deviation area and the course beacon deviation area [6]. (Figure 6) In this paper, we choose the HUD interface size of 480*600 px.

Fig. 6.
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The layout of HUD

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Depend on the Rockwell Collins Flight Dynamics Head-Up Guidance System (HGS®) installed on an in-service aircraft [7], we divide the aircraft HUD interface and the background image into the following dimensions (Fig. 7). The separated five parts correspond to the five functional areas of HUD interface. Part 1 is airspeed indicator, part 2 is the flight mode notification area, part 3 is the attitude guidance area, part 4 is course deviation area and the part 5 is altitude & slip/skid area. There are different kinds of information required by the pilots in different flight stages, so pilots need to observe the information in different regions of the HUD interface to guide their task execution.

Fig. 7.
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Image dimension partition

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3.2 Image Partition Main Color Extraction

Color histogram is one of the most widely used color feature extraction methods. Color histogram is the statistical distribution of color value for the image. The color range is the horizontal axis, and the vertical axis represents the proportion that the number of the same color accounted in the whole image [8]. There are many kinds of histogram, gray histogram can check whether it is appropriate in the distribution of available gray range; global histogram can describe the image features, such as color image in RGB space, because the RGB image is composed of three images with monochromatic brightness, can be directly get the global histogram in three component (RGB). Most of the color image is in RGB color space, while the description with HSV space is closer to human visual characteristics, so this paper will transform the image from RGB space into HSV space, and then processing the HSV color image, drawing the color histogram. The following is a section algorithm of the five partition, and the histograms obtained as shown in Fig. 8.

Fig. 8.
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Histograms of H component (Color figure online)

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Using the same way, we could drawing the histograms of S component and V component as following. (Fig. 9a and b)

Fig. 9.
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(a) Histograms of S component, (b) Histograms of V component (Color figure online)

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Next, calculate the average value of the three largest H, V, S value that obtained by histograms of H, S, V component. The average value is the H, S, V value of main color for this partition. For this Figure, the three largest H, V, S, and their average values were obtained by the histogram, and the HSV value of dominant color was obtained. Contrasting Fig. 5, it is concluded that the following color matching (Fig. 10).

Fig. 10.
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Color processing and matching (Color figure online)

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4 Processing Result

According to different regions, the environment of the forest and grassland is mainly green; when aircraft fly over the sea or a lake, the environment is mainly blue; over a desert, the environment is mainly yellow; over the city, the environment is mainly gray. In addition, according to the different time, the environment color of morning is soft, the color will become bright at noon, and it will appear dark at night. The same area in different seasons will be reflected in different colors, for example, in spring, because of the recovery of all things, the background gives a fresh feeling; in summer, everything becomes flashy; in the autumn, background is vicissitudes of environment, most things become yellow; in the winter, snow white is everywhere. The following Fig. 11 shows the typical task environment.

Fig. 11.
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Typical background environment

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Using MATLAB software to extract the main color of this group figure, then according to the color matching diagram (Fig. 5) to determine the foreground color of HUD interface (Fig. 12).

Fig. 12.
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The foreground color matching result

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5 Conclusion

This study put forward foreground color adaptive system of aircraft Head-Up Display based on the background real-time changes. And according theory of color design, this paper propose the color matching plans and an image main color partition extracting way based on the HUD interface information layout. The conclusion of this paper can guide the color design of aircraft HUD interface, but it can be only used as preliminary process. If it is to be practical application, it still need to be research deeply and verified with experiments.