The relationship between the resolution and the distance of the ball.
The RoboCup (Kitano et al., 1995) is an international joint project to stimulate research efforts in the field of artificial intelligence, robotics, and related fields. According to the rules for the 2009 RoboCup, in the league for kid-sized robots (Avalable, 2009), the competitions were to take place on a rectangular field with an area of 600 × 400 cm2 containing two goals and two landmark poles, as shown in Fig. 1. A goal was placed in the middle of each goal line, with one of the goals colored yellow and the other colored blue. As shown in Fig. 2, each goal for the kid-sized robot field had a crossbar height of 90 cm, a goal wall height of 40 cm, a goal wall width of 150 cm, and a 50 cm depth for the goal wall. The two landmark poles were placed on each side of the two intersection points between the touch line and the middle field line. The landmark pole was a cylinder with a diameter of 20 cm. It consisted of three segments, each 20 cm in height, stacked on top of each other. The lowest and the highest segments have the same color as the goal on its left side, as shown in Fig. 3. The ball is the standard size orange tennis ball. All of the above objects are the most critical characteristics in the field, and they are also the key features which we have to pay attention to.
The functions of humanoid robot vision system include image capturing, image analyses, and digital image processing by using visual sensors. For digital image processing, it is to transform the image into the analyzable digital pattern by digital signal processing. We can further use image analysis techniques to describe and recognize the image content for the robot vision. The robot vision system can use the environment information captured in front of the robot to recognize the image by means of the technique of human vision system. An object recognition algorithm is thus proposed to the humanoid robot soccer competition.
Generally speaking, object recognition uses object features to extract the object out of the picture frame, and thus shape (Chaumette, 1994) and (Jean & Wu, 2004), contour (Sun et al, 2003), (Kass et al., 1988), and (Canny, 1986), color (Herodotou et al., 1998) and (Ikeda, 2003), texture, and sizes of object features are commonly used. It is important to extract the information in real-time because the moving ball is one of the most critical object in the contest field. The complex feature such as contour is not suited to recognize in our application. The objects don’t have the obvious texture which is not suited to use in the contest field. However the object color is distinctive in the contest field, we mainly choose the color information to determine the critical objects.
Although this approach is simple, the real-time efficiency is still low. Because there is a lot of information to be processed in every frame for real-time consideration, Sugandi et al. (Sugandi et al, 2009) proposed a low resolution method to reduce the information. It can speed up the processing time, but the low resolution results in a shorter recognizable distance and it may increase the false recognition rate. In order to improve the mentioned drawbacks, we propose a new approach, adaptive resolution method (ARM), to reduce the computation complexity and increase the accuracy rate.
The rest of this study is organized as follows. Section 2 presents the related background such as the general color based object recognition method, low resolution method, and encountered problems. Section 3 describes the proposed approach, ARM. The experimental results are shown in Section 4. Finally, the conclusions are outlined in Section 5.
2.1. Color based object recognition method
An efficient vision system plays an important role for the humanoid robot soccer players. Many robot vision modules have provided some basic color information, and it can extract the object by selecting the color threshold. The flow chart of a traditional color recognition method is shown in Fig. 4. The RGB color model comes from the three additive primary colors, red, green, and blue. The main purpose of the RGB color model (Gonzalez & Woods, 2001) is for the sensing, representation, and display of images in electronic systems, such as televisions and computers, and it is the basic image information format. The X, Y, and Z axes represent the red, green, and blue color components respectively, and it can describe all colors by different proportion combinations. Because the RGB color model is not explicit, it can be easily influenced by the light illumination and make people select error threshold values.
An HSV (HSV stands for hue, saturation, and value) color model relates the representations of pixels in the RGB color space, which attempts to describe perceptual color relationships more accurately than RGB. Because the HSV color model describes the color and brightness component respectively, the HSV color model is not easily influenced by the light illumination. The HSV color model is therefore extensively used in the fields of color recognition. The HSV transform function is shown in eqs. (1)-(3) as follows:
where “max” indicates the maximum value in the RGB color components and “min” indicates the minimum value in the RGB color components. Hence, we can directly make use of
Step 1: Scan the threshold image
By using the above-mentioned procedure, the objects can be extracted. Although this method is simple, it is only suitable for low frame rate sequences. For a high resolution or noisy sequence, this approach may need very high computation complexity.
2.2. Low resolution method
To overcome the above-mentioned problems, several approaches of low resolution method were proposed (Sugandi et al., 2009), (Cheng & Chen et al., 2006). The flow chart of a general low resolution method is shown in Fig. 5. Several low resolution methods, such as the approach of applying 2-D discrete wavelet transform (DWT) and the using of 2×2 average filter (AF), were discussed. (Cheng & Chen, 2006) applied the 2-D DWT for detecting and tracking moving objects and only the LL3-band image is used for detecting motion of the moving object (It is suggested that the LL3-band is a good candidate for noise elimination (the user can choose a suited decomposition level according to the requirement, and actually there is no need to do the reconstruction for these applications). Because noises are preserved in high-frequency, it can reduce computing cost for post-processing by using the LL3-band image. This method can be used for coping with noise or fake motion effectively, however the conventional DWT scheme has the disadvantages of complicated calculation when an original image is decomposed into the LL-band image. Moreover if it uses an LL3-band image to deal with the fake motion, it may cause incomplete moving object detecting regions. In (Sugandi et al., 2009) proposed a simple method by using the low resolution concept to deal with the fake motion such as moving leaves of trees. The low resolution image is generated by replacing each pixel value of an original image with the average value of its four neighbor pixels and itself as shown in Fig. 6. It also provides a flexible multi-resolution image like the DWT. Nevertheless, the low resolution images generated by using the 2×2 AF method are more blurred than that by using the DWT method. It may reduce the preciseness of post-processing (such as object detection, tracking, and object identification), because the post-processing depends on the correct location of the moving object detecting and accuracy moving object.
In order to detect and track the moving object more accurately, we propose a new approach, adaptive resolution method (ARM), which is based on the 2-D integer symmetric mask-based discrete wavelet transform (SMDWT) (Hsia et al, 2009). It does not only retain the features of the flexibilities for multi-resolution, but also does not cause high computing cost when using it for finding different subband images. In addition, it preserves more image quality of the low resolution image than that of the average filter approach (Sugandi et al., 2009).
2.2.1. Symmetric Mask-Based Discrete Wavelet Transform (SMDWT)
In 2-D DWT, the computation needs large transpose memory and has a long critical path. On the other hand SMDWT has many advanced features such as short critical path, high speed operation, regular signal coding, and independent subband processing (Hsia et al, 2009). The derivation coefficient of the 2-D SMDWT is based on the 2-D 5/3 integer lifting-based DWT. For computation speed and simplicity considerations, four-masks, 3×3, 5×3, 3×5, and 5×5, are used to perform spatial filtering tasks. Moreover, the four-subband processing can be further optimized to speed up and reduce the temporal memory of the DWT coefficients. The four-matrix processors consist of four-mask filters, and each filter is derived from one 2-D DWT of 5/3 integer lifting-based coefficients.
In the ARM approach, we can select only the LL-band mask of SMDWT (The moving object is low-frequency energy). Unlike the conventional DWT method to process row and column dimensions respectively by low-pass filter and down-sampling, the LL-mask band of SMDWT can be used to directly calculate the LL-band image. The matrix function of the LL-mask is shown in (6) and the coefficients of the LL-mask are shown in Fig. 7 (Hsia et al, 2009). SMDWT (using the LL-band mask only) can reduce the image transfer computing cost and remove the noise. Besides, this approach can have accurate object tracking for various types of occlusions.
3. The proposed method
3.1. Adaptive Resolution Methos (ARM)
ARM takes advantage of the information obtained from the image to know the area of the ball and chooses the most suitable resolution. The operation flow chart is shown in Fig. 8. After HSV color transformation, ARM chooses the most proper resolution by the situation at this moment in time. The high resolution approach brings a longer recognizable distance but with a slower running speed. On the other hand, the low resolution approach brings a lower recognizable distance but with a faster running speed. When we got the area information of the ball from the image last time, we could convert it as the “sel” signal through the adaptive selector to choose the appropriate resolution. The “sel” condition is shown in (7):
|320×240||0(original)||404.6 cm||18 pixels|
|160×120||1||322.8 cm||54 pixels|
|80×60||2||121.3 cm||413 pixels|
3.2. Sample object recognition method
According to the above-mentioned color segmentation method, it can fast and easily extract the orange ball in the field, but it is not enough to recognize the goals and landmarks. The colors of the goals and landmarks are yellow and blue, and by color segmentation the extraction of goals and landmarks may not be correct as shown in Fig. 10. Therefore we have to use more features and information to extract them. Since the contest field is not complicated, a simple recognition method can be used to reduce the computation complexity. The landmark is a cylinder with three colors. Let us look at one of the landmark with the upper and bottom layers in yellow, and the center layer in blue; this one is defined as the YBY-landmark. The diagram is shown in Fig. 11. The color combinations of the other one are in contrast of the previous one, and the landmark is defined as the BYB-landmark. The labels of the YBY-landmark can be calculated by (8). The BYB landmark is in the same manner as the YBY-landmark.
According to the above-mentioned labeling procedure, we labeled all of the yellow and blue components in the frame and assigned the numbers to those components. Where is defined the pixels of the
where is the pixel of the
where is the pixel of the
3.3. Coordinate transformation
Because our proposed approach, ARM is using the different resolutions in the object recognition, we transform the coordinate into the original resolution by level-based of DWT when the object information is outputted. The transform equation is defined in (11).
4. Experimental results
In this work, the environment information is extracted by the Logitech QuickCam Ultra Vision (Using the monocular vision technique). The image resolution is 320×240, and the frame rate is 30 FPS (frame per second). For the simulation computer, the CPU is Intel Core 2 Duo CPU 2.1GHz, and the development tool is Borland C++ Builder 6.0. The graphical interface is shown in Fig. 15.
This work is dedicated to the RoboCup soccer humanoid league rules of the 2009 competition. In order to prove the robustness of the proposed approach, many scenes of various situations are simulated to verify the high recognition accuracy rate and fast processing time. For the analyses of recognition accuracy rate, it is classified as a correct recognition if the critical object is labeled completely and named correctly such as the objects of Goal[B] and Ball shown in Fig. 16(a). On the other hand there are two categories for false recognition, “false positive” and “false negative”. “False positive” means that the system recognizes the irrelevant object as the critical object, such as the Goal[Y] shown in Fig. 16(b). “False negative” means the system cannot label or name the critical object, such as those balls shown in Figs. 16(c) and 16(d).
4.1. Low resolution analysis
Several low resolution methods, such as down-sampling (DS), AF, and SMDWT, were implemented and simulated in this experiment and the noise removing capabilities with these methods were analyzed. The flow chart of noise removing for the low resolution approaches is shown in Fig. 17. The input frame resolution is 320×240, and the resolution turns to be 160×120 after the low resolution processing. The noise numbers under different low resolution methods were counted. The contents of the simulated scene are obtained by turning the camera to left to see the YBY-landmark and keeping turning until the YBY-landmark disappeared from the camera scope. In this situation the background of the scene produces noise very easily. The hue threshold values of the orange, yellow, and blue colors are set as 35~45, 70~80, and 183~193, respectively, and the saturation threshold of the orange, yellow, and blue colors values are all set as 70. The experimental results under different low resolution methods, DS, AF, and SMDWT, are shown in Figs. 18-20, respectively.
The experiment data are listed in Table 2. According to Table 2, the DS approach has the worst noise removing capability; the 2×2 AF approach also has a bad noise removing capability for big noise block even though this method can make the image smoother. On the other hand, the SMDWT approach (using LL-mask only) has a better noise removing capability than the other methods, and it can retain the information of low-frequency component and remove the noise of high-frequency component in the image.
In order to improve the noise removing capability of the whole system, we added the opening operator (OP) of mathematical morphology after labeling in the flow chart of Fig. 17. The results after adding the opening operator are shown in Figs. 21-23.
The experiment data after adding the opening operator are shown in Table 3. Compared with the results of Table 2, the noise numbers are reduced significantly after adding the opening operator, and it can reduce the unnecessary computation. The SMDWT approach has the best performance and the frame rate can be as high as 30 FPS. Therefore this work adopts the SMDWT approach as the low resolution method.
|DS + OP||153||408||2.67||38.01 FPS|
|AF + OP||334||2.18||37.73 FPS|
|SMDWT + OP||60||0.39||30.95 FPS|
4.2. Adaptive Resolution Method (ARM) analyses
In this experiment, we try to verify that ARM does not only retain high recognition accuracy rate, but also can raise the system processing efficiency. The hue threshold values of the orange, yellow, and blue colors are set as 35~45, 70~80, and 183~193, respectively. The saturation threshold values of the orange, yellow, and blue colors are all set as 70. To verify the ARM approach, the camera is set in the center of the contest field. The scene tries to simulate that the robot kicks ball into the goal and the vision system will track the ball. The results under resolutions of 320×240, 160×120, 80×60, and ARM are shown in Figs. 24-27, respectively.
The experiment data of the accuracy rate and average FPS under different resolutions and ARM are shown in Table 4 and Fig. 28. According to Table 4, although the 320×240 resolution has a high accuracy rate, the processing speed is slow. The 80×60 resolution has the highest processing speed, but it has the lowest accuracy rate. By this approach, it gets high accuracy rate only when the object is close to the camera. On the other hand, the proposed ARM approach does not only have a high accuracy rate, but also keeps high processing speed. According to Fig. 28, the result shows that ARM selects the most proper resolution when the ball is in different distances. ARM uses the 80×60 resolution when the level is equal to 2 and uses the 160×80 resolution when the scale level is equal to 1. As the scale level is equal to 0, ARM selects the original input frame size (320×240).
4.3. The critical objects recognition analysis
In this experiment, several scenes were simulated to improve the robustness of feature recognition approaches proposed in this work.
4.3.1. Landmark recognition analysis
According to (8), the landmark is composed of two same color objects in the vertical line, and the bias value
In this experiment, different values of
The experiment data of landmark recognition is shown in Table 5. According to this table, we can have a higher recognition accuracy rate when
|β||Total frame||Object frame||Correct recognition||Accuracy rate||Average frame rate|
4.3.2. Goal recognition analysis
The goal is the largest critical object in the field, and hence the camera always captures the incomplete goal in the frame when the robot is walking in the field. It causes a false recognition easily by using the feature of the shape ratio to recognize the goal. We improve this drawback by using the proposed method in Section 3.2 and the experimental results are shown here. The camera is set in the center of the contest field. The scene tries to simulate that the robot raises its head to see the goal and turns right to see the YBY-landmark and then turns left to see the BYB-landmark. The hue threshold values of the orange, yellow, and blue colors are set as 35~45, 70~80, and 183~193, respectively. The saturation threshold values of the orange, yellow, and blue colors are all set as 60. The results are shown in Fig. 34 and the experiment data are listed in Table 6. According to the result, the system can make the correct recognition of goal even though the goal is occluded.
|Goal Recognition||328||297||0||7||97. 64%||21.98 FPS|
4.3.3. Ball recognition analysis
For the ball recognition, the system determines the orange block which has the maximum pixels as a ball for preventing the influence of noise. In this experiment, two balls are used in the scene. One ball is static in the field, and the other one moves into the frame and then moves away from the camera. The hue threshold values of the orange, yellow, and blue colors are set as 35~45, 70~80, and 183~193, respectively. The saturation threshold values of the orange, yellow, and blue colors are all set as 60. The result is shown in Fig. 35 and the experiment data are shown in Table 7. The static ball is labeled absolutely if only one ball is in the field, and the result is shown in Fig. 35(a). Because another ball has a bigger area when it is moving into the frame, the system will label the moving ball and determine the static ball as noise, and the result is shown in Fig. 35(b). When the moving ball is distant from the camera, the static ball is labeled again, and the result is shown in Fig. 35(c).
|Ball Recognition||274||274||0||0||99.99%||30.93 FPS|
|Ball Occlusion||289||289||3||0||98.96%||20.69 FPS|
Besides, it can also handle the situation when the ball is occluded partially by using the feature recognition proposed. We use the scene that the ball is occluded by the landmark during the ball moving in the frame from left to right. The hue threshold values of the orange, yellow, and blue colors are set as 35~45, 70~80, and 175~185, respectively. The saturation threshold values of the orange, yellow, and blue colors are all set as 50. The results are shown in Fig. 36 and the experiment data are shown in Table 7.
4.4. Environmental tolerance analysis
The color deviation by luminance variation has the most influence to the result of the color-based recognition method proposed in this work. Before the robot soccer competition we usually have one day to prepare for the contest, and therefore we can regulate the threshold values easily by the graph interface according to the luminance of the field. The results under different luminance are shown in Fig. 37. The reference threshold values are shown in Table 8.
The system cannot only recognize the critical objects under different luminance, but it can also accommodate the light changing suddenly. This experiment simulates that the robot recognizes the BYB-landmark and ball in the field under the light changing suddenly. The hue threshold values of the orange, yellow, and blue colors are set as 33~43, 67~77, and 175~185, respectively. The saturation threshold values of the orange, yellow, and blue colors are all set as 50. The results are shown in Fig. 38 and the experiment data are listed in Table 9. According to the result, the proposed method has a good performance about environmental tolerance.
|Light Influence||1,219||1,219||0||0||99.99%||31.14 FPS|
4.5. Synthetic analyses
In this experiment, several scenes were simulated to compare the recognition accuracy rate and processing time between the 320×240 resolution and ARM. Scene 1: the ball is approach it to the camera slowly. Scene 2: the robot is approaching the ball after shooting the ball to the goal. Scene 3: the robot finds the ball and then tries to get approaching and kick it. Scene 4: the camera captures a blurred image when the head motor of the robot is rotating very fast. Scene 5: the robot localizes itself by seeing the landmarks. The hue threshold values of the orange, yellow, and blue colors are set as 35~45, 70~80, and 185~190, respectively. The saturation threshold values of the orange, yellow, and blue colors are all set as 50. The experiment data of these scenes are shown in Table 10 and the experimental results are shown in Figs. 39-43, respectively. According to the simulation results, our proposed method accommodates many kinds of scenes. It has the accuracy rate of more than 93% on average and the average frame rate can reach 32 FPS. It does not only maintain the high recognition accuracy rate for the high resolution frames, but also increases the average frame rate for about 11 FPS compared to the conventional high resolution approach. Furthermore, all of the experimental result videos mentioned in this section are appended in.
An outstanding humanoid robot soccer player must have a powerful object recognition system to fulfill the functions of robot localization, robot tactics, and barrier avoiding. In this study, we propose an HSV color based object segmentation method to accomplish object recognition. The object recognition system uses the proposed adaptive resolution method (ARM) and sample object recognition method, and it can recognize objects. The experimental results indicate that the proposed method is not only simple and capable of real-time processing but that it also achieves high accuracy and efficiency with the functions of object recognition and tracking. The method achieves a high accuracy rate of more than 93% on average, and the average frame rate can reach 32 FPS in indoor situations.
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