WO2010083018A1

WO2010083018A1 - Segmenting grass regions and playfield in sports videos

Info

Publication number: WO2010083018A1
Application number: PCT/US2010/000004
Authority: WO
Inventors: Jesus Barcons-Palau; Sitaram Bhagavathy; Joan Llach; Mithun George Jacob
Original assignee: Thomson Licensing
Priority date: 2009-01-16
Filing date: 2010-01-04
Publication date: 2010-07-22

Abstract

A method for segmenting grass and playfield regions in sports videos entails: generating likelihood grass and non-grass masks; generating a grass mask using the likelihood masks; performing a first clean-up of the grass mask to generate an intermediate grass mask; generating a playfield mask using the intermediate grass mask; refining the intermediate grass mask; performing a second clean-up of the refined intermediate grass mask; constraining the grass pixels of the grass mask to be with the playfield mask; and performing a third clean-up to generate a final grass mask. The final grass mask provides a robust and accurate representation of grass regions in the video frame. A soccer application is described.

Description

SEGMENTING GRASS REGIONS AND PLAYFIELD IN SPORTS VIDEOS

Related Patent Applications

[0001] This application claims the benefit under 35 U.S. C. § 1 19(e) of United States Provisional Application No. 61/205,333, filed January 16, 2009, the entire contents of which are hereby incorporated by reference for all purposes into this application.

Field of Invention

[0002] The present invention generally relates to digital image analysis, and more particularly to the detection of features in video images.

Background

[0003] In sports video analysis, such as the analysis of video of a soccer match, the detection of certain features, such as grass in the playfield, is one of the first steps for a wide variety of applications that deal with such content. Applications that can benefit from accurate grass detection include, for example, object detection/highlighting technologies. In the particular case of detecting and highlighting objects of interest inside the playfield (such as the ball and the players), a mask of the grass provides information about where such objects can be found. [0004] Another application for grass detection includes the classification of scenes into different camera views (e.g. far-view, close-up, etc.) by evaluating the characteristics of the playfield and the objects inside it.

[0005] A further application includes video compression, in which the grass mask can be passed to the encoder as metadata. Such information may be used, for example, to control the bit budget allocated to encode such regions.

[0006] And yet another application includes video reframing. Since the action of a soccer match is supposed to happen inside the playfield, the grass mask can be passed as metadata to a reframing application to preserve zones of interest.

[0007] Most previous approaches to the detection of grass in a playfield detect the grass pixels in the playfield using color segmentation and post-processing with morphological operations, such as connected component analysis, in order to limit the search area. (See, e.g., S. Choi et al., "Where are the ball and players? Soccer game analysis with color-based tracking and image mosaic," Intl. Con/, on Image Analysis and Processing, Sep 1997; Y. Liu et al., "Playfield Detection Using Adaptive GMM and Its Application," IEEE ICASSP '05, pp. 421- 424, March 2005; X. Tong et al., "An Effective and Fast Soccer Ball Detection and Tracking Method," /CPi?'O4, pp. 795-798, 2004; O. Utsumi et al., "An object detection method for describing soccer games from video," IEEE ICME '02, 2002; Y. Huang et al., "Players and Ball Detection in Soccer Videos Based on Color Segmentation and Shape Analysis," International Workshop on Multimedia Content Analysis and Mining (MCAM'07), in conjunction with ICMF07, pp. 416-425, June, 2007.)

[0008] A simple way to represent the color of grass is by using a constant mean color value that is obtained through prior statistics over a large data set. The color distance between a pixel and the mean value of the field could be used to determine whether the pixel is to be classified as grass or not. (See X. Tong et al. cited above.) Since the soccer field is roughly green colored, the hue component defined in Smith's hexagonal cone model can be used to detect the green colored pixels given a certain range. (See O. Utsumi et al., cited above.)

[0009] Some statistical detection methods involve learning a Gaussian or mixture-of- Gaussian (MoG) color model for the grass inside the playfield, which can be incrementally adapted using the Expectation-Maximization (EM) algorithm. (See Y. Liu et al. cited above.) A non-parametric color model for the grass is the histogram. [0010] Another approach to grass detection is to find the dominant color, assuming that the field has a uniform color of green and occupies the largest area in each frame. (See S. Choi et al. cited above.) [0011] Yet another approach uses ground truth data of grass and non-grass pixels to create two 3D histograms. (See Y. Huang et al. cited above.) Then, the probability of a given pixel being grass or non-grass is evaluated using the data from the learned histograms. The limitation of this method is that the success of the grass detection is very dependent on the ground truth used for learning the histograms. Summary

[0012] In an exemplary embodiment in accordance with the principles of the invention, a method is provided of segmenting grass regions in an image, such as a frame of a sports video. The exemplary method includes: generating likelihood grass and non- grass masks; generating a grass mask using the likelihood masks; performing a first clean-up of the grass mask to generate an intermediate grass mask; generating a playfield mask using the intermediate grass mask; refining the intermediate grass mask; performing a second clean-up of the refined intermediate grass mask; constraining the grass pixels of the grass mask to be with the playfield mask; and performing a third clean-up to generate a final grass mask. The final grass mask provides a robust and accurate representation of grass regions in the video frame.

[0013] In view of the above, and as will be apparent from the detailed description, other embodiments and features are also possible and fall within the principles of the invention.

Brief Description of the Figures

[0014] Some embodiments of apparatus and/or methods in accordance with embodiments of the present invention are now described, by way of example only, and with reference to the accompanying figures in which: [0015] FIG. IA is an illustrative image frame of a playfield; and FIGs. IB and 1C are a grass mask and a field mask, respectively, derived from the image of FIG. 1 A;

[0016] FIGs. 2 A and 2B show a flowchart of an exemplary grass and playfield detection method;

[0017] FIG. 3 is a flowchart of an exemplary texture-based grass detection method; [0018] FIG. 4 is a flowchart of an exemplary RGB histogram-based grass detection method;

[0019] FIG. 5 is a flowchart of an exemplary HSV non-grass detection method;

[0020] FIG. 6 is a flowchart of an exemplary local maxima non-grass detection method; and [0021] FIG. 7 is a block diagram of an exemplary system embodiment of the present invention. Description of Embodiments

[0022] Other than the inventive concept, the elements shown in the figures are well known and will not be described in detail. For example, other than the inventive concept, familiarity with digital image processing is assumed and not described herein. It should also be noted that embodiments of the invention may be implemented using various combinations of hardware and software. Finally, like-numbers in the figures represent similar elements.

[0023] FIG. IA is an illustrative image frame of a play field, such as may be grabbed from a video of a sporting event such as a soccer match. The image of the playfield contains the grass regions of the field, as well as objects in the field or in the foreground, such as field lines, players, referees, balls and the like. For each frame, an exemplary method described below, outputs a grass mask (GM) specifying pixels belonging to grass regions inside the field, and a playfield mask (PM) specifying all pixels lying inside the playfield. FIG. IB shows an illustrative grass mask and FIG. 1C shows an illustrative playfield mask.

[0024] FIGs. 2A and 2B show a flowchart of an exemplary method 200 for segmenting grass pixels and playfield pixels from a given soccer video frame. At a high level, the method entails generating likelihood grass and non-grass masks, and then using the masks to perform grass and playfield detection. More specifically, the exemplary method includes: at 210 generating likelihood grass and non-grass masks; at 220 generating a grass mask using the likelihood masks computed in 210; at 230 cleaning up the grass mask; at 240 refining the detection of grass pixels in the grass mask; at 250 cleaning-up the refined grass mask; at 260 detecting the playfield and generating a playfield mask using the cleaned-up grass mask from 230; at 270 constraining the grass pixels of the grass mask to be inside the playfield; and at 280 further cleaning up the grass mask by deleting isolated pixels. Each of these steps or procedures will now be described in greater detail.

[0025] In the exemplary method 200, the generation of likelihood grass and non- grass masks at 210 includes: texture-based grass detection 211, which outputs a binary mask GM_teχ_t; RGB histogram-based grass detection 212, which outputs a binary mask C/Mi_rgb; HSV non-grass detection 213, which outputs a binary mask NGM^, and local maxima (bright spot) detection 214, which outputs a binary mask

The input to each of functions 21 1-214 is a frame grabbed from a video stream depicting a soccer match. The illustrative frame is in a YUV colorspace format but may be in any suitable colorspace format, such as HSV or RGB. Functions 211-214 may be carried out in parallel, sequentially or any combination thereof. Moreover, even though four masks are generated in this particular embodiment, a different number of masks can be used. In addition, these masks are not limited to being binary. [0026] Exemplary implementations of functions 211-214 will now be described. Texture-based grass detection

[0027] FIG. 3 shows a flowchart of an exemplary method 300 for implementation of a texture-based grass detection procedure such as used in the method 200 of FIGs. 2 A and 2B described above. The purpose of method 300 is to prevent foreground objects such as players, goalmouths, lines, and balls from being detected as grass. [0028] Texture-based grass detection method 300 isolates possible grass areas based on the assumption that grass areas have a smooth appearance. The standard deviation of a neighborhood of pixels can be used as a local texture descriptor. The standard deviation provides a measure of spatial activity, with a small standard deviation indicating low spatial activity and therefore a smooth surface. [0029] As shown in FIG. 3, at 310, the luminance 7 of the frame is normalized by its maximum value in the frame, so that the range of Y in the frame becomes [0, I]. This is followed by a processing loop 320-370 beginning with step 330 in which a local standard deviation image I_s,d is generated using a neighborhood of N_si_d ^χN_std pixels around each pixel in the normalized Y_norm. [0030] At step 340, the value of I_std computed for each pixel is compared to a threshold T_std- If I_std(^χ _>y) ≤ T_sld , the pixel at (x, y) in the binary mask GM_teM is set to 1 at step 350, otherwise, it is set to 0 at step 360. In an exemplary embodiment for QVGA or 320x240-pixel video of a soccer match, the following parameter values were used: N_s,_d = 5, T_s,d = 0.2. In various embodiments, values for N_std have a range of 3 to 7, and values for T_s,d have a range of 0 to 2.5 for pixel values of 0-255. [0031] Although the generation of binary mask GM_text can be sensitive to local variations in the color or intensity of the grass surface and the resultant mask can be noisy, it provides valuable information about the grass texture. Moreover, by using this mask in combination with other masks and applying one or more clean-up operations, as described below, the effects of any noise can be reduced or eliminated.

RGB histogram-based grass detection method

[0032] FIG. 4 shows a flowchart of an exemplary method 400 for implementation of RGB histogram-based grass detection procedure such as used in the method 200 of FIGs. 2A and 2B described above. Method 400 relies on the assumptions that: 1) the playfield is the biggest area containing the most dominant color in the frame, hence, a pixel quite close to the dominant color is likely to be grass; and 2) a very green pixel is likely to be grass.

[0033] As shown in FIG. 4, the YUV frame is converted at 410 to the RGB color space. To detect the most dominant color in a frame, a histogram with N_hi_st bins is computed at 412 for each color component -R (red), G (green) and B (blue)-of the frame. [0034] Then, at 413, the bin with the largest value (peak) in each of the three histograms is determined. Additionally, the values of the bin centers corresponding to the peaks of the R, G and B histograms are determined. These values are designated Rpeak, Gpeak and B_peak, respectively. [0035] Operation then proceeds to processing loop 420-470 in which each pixel (x, y) of the frame is classified as grass at 440 (i.e., GM_hrgb(*,_.y) = 1 in the output binary mask) or non-grass at 460 (i.e., GMhrgbføjO - 0 in the output binary mask) depending on the results of comparison tests performed at steps 430 and 450. More specifically, a pixel (xj/) is classified as grass at 440 if it is determined at 430 that: *(x, y) < G(x, y) , and (1)

5(x, y) < G(x, y) , and (2)

|#(x, y) - *_Pe<*| < 7;_ω , and (3)

|G(χ, y) - G_peoA| < r_AOT , and (4)

or if it is determined at 450 that:

S_faclR(x, y) < G(x, y) and S_faclB(x, y) < G(x, y) , (6) where 7_h,_st is a threshold to bound the difference between the peak and a value of a given component, and 5_fact is a scale factor. Otherwise, if a pixel (x, y) fails both tests at 430 and 450, it is classified as non-grass at 460.

[0036] In an exemplary embodiment, the following values are used: Nh,_st = 100, T^_st = 12 and S_facl = 1.1. In various embodiments, for a pixel intensity range of 1-256, values for Nhis_t have a range of 75 to 150, values for Zh,_st have a range of 10 to 20 and values for S_fact have a range of 1.1 to 1.3.

[0037] The comparisons of R, G, and B values in steps 430 and 450 are based on the assumption that a grass pixel is "more green" than other pixels. This implies that the G component should be higher than the R and B components in a grass pixel. This assumption is tested in the first two comparisons (1) and (2) of step 430 and in the comparisons (6) of step 450, in which the R and B components are scaled and compared to the G component.

[0038] Step 430 also incorporates the assumption that a majority of the pixels in the field image are grass pixels. Therefore, pixels within an interval 7h,_st about the peaks of the R, G and B histograms should belong to grass. This is tested in the last three comparisons (3)-(5) of step 430.

[0039] Occasionally, the dominant color (histogram peaks) may correspond to a non- grass area leading to an incorrect detection result. In order to reduce this possibility and to increase the likelihood of isolating the color of grass, the aforementioned histograms are preferably computed using pixels only from the bottom-half of the frame. In most soccer videos (particularly in the case of far-view scenes) the audience is usually at the top of the frame and the playfield lies between the audience and the bottom of the frame. Therefore the color of grass is likely to be dominant in the bottom half of the frame even if it does not dominate the entire frame. This knowledge helps improve the robustness of detection and reduce the complexity of histogram computation as there are fewer pixels to consider. As such, at 411, the bottom half of the image frame is selected for computation of the histograms at 412.

[0040] In some embodiments, several histograms could be computed for different portions of the frame; for instance: left, right, top, bottom and center. Such portions of the frame may overlap each other. Then, the histogram with the highest, greenest peak would define the dominant color.

HSV non-grass detection

[0041] The aim of texture-based grass detection 21 1 and RGB histogram-based grass detection 212 is to detect regions likely to be grass. HSV non-grass detection 213 detects pixels with a very low likelihood of being grass. This information is used, as described below, to ensure that non-grass pixels are not labeled as grass, thereby reducing the occurrence of false alarms and increasing the accuracy of grass detection. [0042] FIG. 5 is a flowchart of an exemplary method 500 for implementing HSV non-grass detection 213. At step 510, the input frame, if not already in the HSV colorspace, is converted to HSV colorspace format. In the HSV colorspace representation, the hue component H takes on values in the range [0, 360). Although hues in the range [0, 240] contain some amount of green, grass pixels are more likely to have a hue in the range [60, 180]. Using the knowledge that grass is green, method 500 generates the non-grass mask NGM^ as follows: if H_m]n < H(x, y) < H_max, then NGM_hsv(x, y) = 0, else 1.

As shown in FIG. 5, this operation is carried out over all pixels of the image in processing loop 520-560. In an exemplary embodiment the following values are used: •#_mi_n ⁼ 30 and H_max = 150. In an alternative exemplary embodiment H_max = 210.

Local maxima non-grass detection [0043] Like HSV non-grass detection 213, the purpose of local maxima non-grass detection 214 is to detect non-grass pixels. This operation ensures that small bright spots in the playfield (e.g. ball, field lines, goalmouth) are not incorrectly detected as grass. Bright spots are isolated by locating pixels corresponding to local maxima in the luminance component /. [0044] FIG. 6 is a flowchart of an exemplary method 600 for implementing local maxima non-grass detection 214. In order to generate the non-grass mask NGM_\ocmax based on the local maxima, the luminance of the frame is convolved at step 620 with a normalized Gaussian kernel G_nk generating the result C|_Um_a:

The non-grass mask NGMι_ocmaχ is generated as follows: if Y(x, y) > C,_uma(x, y) + T_c], then NGM_]0Cmax(x, y) = h else 0.

As shown in FIG. 6, this operation is carried out over all pixels of the image in processing loop 610-660. In an exemplary embodiment, G_n^ is a 9 *9 kernel and T_c\ = 0.1. In various embodiments, G_n^ has values of 5, 7, 9 and 11.

Grass and playfield detection

[0045] Referring again to FIGs. 2A and 2B, using the likelihood grass and non-grass masks generated at 210, grass detection is performed at 220 to generate an initial grass mask GΛfj_nJt indicative of pixels representing grass in the playfield. [0046] As shown in FIG. 2 A, in a processing loop 221-225, for each pixel (x, y) in the video frame being processed, the corresponding values of the grass masks GM_text and GMhrgb and the Y component of the frame are tested at 222. At 223, the corresponding initial grass mask value GM_mφc,y) is set to 1 if it is determined at 222 that: GM_itxt(x,y) = 1, and GM_hrgb(x,y) = 1, and Y(x,y) > Y_tM. If not, at 224, GM_mα(x,y) is set to 0. [0047] Once all pixels have been processed in 220 to generate GM_ιnα, at 230 an opening morphological operation (using, for example a 3x3 kernel) is applied to

in order to remove small objects. The resultant intermediate grass mask is labeled GM_mχ_tτ. [0048] Operation then proceeds to 240 in which the intermediate grass mask is refined to generate refined grass mask GM_refined. As shown in FIG. 2B, in a processing loop 241-246, for each pixel (x,y) in the video frame, the corresponding values of non- grass masks NGM_hsv and NGM]_QCmax are tested at 242. At 243, the corresponding value of the refined grass mask GM_τenneά(x,y) is set to 0 if it is determined at 242 that NGMhsv(x,y)

~ 1. If not, operation proceeds to 244. If it is determined at 244 that Sfact x R(x,y) < G(x,y), and Sf_act x B(x,y) < G(x,y), and Y(x,y) > 7_thr2, then at 245, GM_Kr_med(x,y) is set to 1. [0049] Once GM_Kfmt_ά has been generated upon completion of refinement 240, a further opening morphological operation (using, for example a 3x3 kernel) is applied at 250 to GMr_efin_ed in order to remove small objects. [0050] Using intermediate grass mask GMm_er generated at 230, playfield mask PM is generated at 260. Playfield mask PM indicates those pixels that are in the playfield. In an exemplary embodiment, playfield mask PM is generated by performing a morphological closing operation on intermediate grass mask GM_mtcτ- Any holes in the resulting binary image are closed and the largest connected component is found and output as the playfield mask PM. Playfield mask generation 260 can be carried out before, in parallel or after 240 and 250.

[0051] At 270, using the playfield mask PM, the grass pixels as indicated by the refined and further cleaned-up grass mask are constrained to lie inside the playfield. This operation is based on the assumption that the only grass areas of interest lie inside the playfield. This operation can be omitted for applications in which this constraint is not required. At 270, all of the pixels of the grass mask where the playfield mask PM is 0 are set to 0 as well. In other words, all pixels outside the detected playfield are set to non- grass.

[0052] At 280, components smaller than a given threshold size are removed. In an exemplary embodiment this threshold size is two pixels, whereby isolated, individual pixels are removed. For larger threshold sizes, noisy clusters of connected pixels would be removed as well. The result of this operation is the final grass mask. [0053] In an exemplary embodiment the following parameter values are used: S_faα = 1.1, Ythri ⁼ 50, and 7_thr2 ' 80. In various embodiments, values for Sf_act have a range of 1.0 to 1.3, values for F_1J11-_I have a range of 40 to 70, and values for F_thr2 have a range of 60 to 90.

[0054] Note that for applications that do not require limiting the grass mask to those grass pixels in the playfield, this operation can be omitted. If the playfield mask PM is not required, the processing loop in 220 to create the initial grass mask of pixels and the processing loop in 240 to refine the grass mask can be merged into one loop. [0055] Furthermore, in the exemplary embodiment shown, playfield mask generation 260 uses intermediate grass mask GM_mi&, instead of, for example, the Final grass mask GM, because intermediate grass mask GM_mtr is more likely to have the field as one connected component. The operations applied to the grass mask at 240 and 250 are focused on the rejection of pixels classified as grass and may cause the field to break up into multiple pieces. [0056] FIG. 7 is a block diagram of an exemplary system 700 in accordance with the principles of the invention. The system 700 can be used to generate grass and playfield masks, GM and PM, respectively, from a video stream of a sports event such as a soccer match. The system 700 comprises a frame grabber 710 and a digital video editor 720. Frame grabber 710 captures one or more frames of the video stream for processing by digital video editor 720 in accordance with the principles of the invention. Digital video editor 720 comprises processor 721, memory 722 and I/O 723. In an exemplary embodiment, digital video editor 720 may be implemented as a general purpose computer executing software loaded in memory 722 for carrying out grass and playfield segmenting as described above. [0057] Exemplary embodiments in accordance with the principles of the invention can be readily applied to surfaces other than grass covered playfields. For example, surfaces such as clay for tennis or ice for ice hockey can be detected with minor modifications adapting the criteria to the features of the surface. Moreover, as can be appreciated, the method can be applied to still images as well as frames from a video stream.

[0058] Additionally, in an exemplary embodiment, the likelihood masks, GM_ex_t, GMhtgb, NGM_hsv and

may be non-binary and real-valued. In such an embodiment, the real values from these masks could be combined in determining whether pixels should be classified as grass pixels, as opposed to simply checking if the mask values are 0 or 1, as in steps 222 and 242 of the exemplary method depicted in FIGs. 2A and 2B.

[0059] In view of the above, the foregoing merely illustrates the principles of the invention and it will thus be appreciated that those skilled in the art will be able to devise numerous alternative arrangements which, although not explicitly described herein, embody the principles of the invention and are within its spirit and scope. For example, although illustrated in the context of separate functional elements, these functional elements may be embodied in one, or more, integrated circuits (ICs). Similarly, although shown as separate elements, some or all of the elements may be implemented in a stored- program-controlled processor, e.g., a digital signal processor or a general purpose processor, which executes associated software, e.g., corresponding to one, or more, steps, which software may be embodied in any of a variety of suitable computer-readable media. It is therefore to be understood that numerous modifications may be made to the illustrative embodiments and that other arrangements may be devised without departing from the spirit and scope of the present invention.

Claims

1. A computer implemented method for segmenting grass regions in an image, comprising: generating at least one likelihood grass mask; generating at least one likelihood non-grass mask; generating a grass mask using the likelihood masks; performing a first clean-up operation of the grass mask to generate an intermediate grass mask; generating a playfield mask using the intermediate grass mask; refining the intermediate grass mask; performing a second clean-up operation of the refined intermediate grass mask; and constraining the refined intermediate grass mask to be within the playfield mask.

2. The method of claim 1, comprising: performing a third clean-up operation to generate a final grass mask.

3. The method of claim 1, wherein the image is a frame of a sports video.

4. The method of claim 1 , wherein generating at least one likelihood grass mask includes: comparing red, green and blue color components of pixels of the image; and designating pixels of the image as grass pixels if the pixels have a green color component that is greater than the red and blue color components.

5. The method of claim 1, wherein generating at least one likelihood grass mask includes: generating a histogram for each of a red, green and blue color component of the image; determining a peak of each of the histograms; and designating pixels of the image as grass pixels if the pixels have red, green and blue color components proximate to the peaks of the histograms.

6. The method of claim 5, wherein the histogram are generated using a lower portion of the image.

7. The method of claim 1, wherein generating at least one likelihood grass mask includes: determining texture information for the image.

8. The method of claim I, wherein generating at least one likelihood non-grass mask includes: performing local maxima detection; and designating local maxima as non-grass.

9. The method of claim 1, wherein generating at least one likelihood non-grass mask includes: designating pixels of the image as non-grass pixels if the pixels have a hue that is not green.

10. The method of claim 1, wherein performing each of the first and second clean-up operations includes performing a morphological opening operation.

11. A computer program recorded on a computer-readable medium, said program causing a computer to segment grass regions in an image by executing the steps of: generating at least one likelihood grass mask; generating at least one likelihood non-grass mask; generating a grass mask using the likelihood masks; performing a first clean-up operation of the grass mask to generate an intermediate grass mask; generating a playfield mask using the intermediate grass mask; refining the intermediate grass mask; performing a second clean-up operation of the refined intermediate grass mask; and constraining the refined intermediate grass mask to be within the playfield mask.

12. The computer program of claim 11 causing the computer to segment grass regions in an image by executing the further step of: performing athiid cVean-up operation to generate a final grass mask.

13. The computer program of claim 11, wherein the image is a frame of a sports video.

14. The computer program of claim 11, wherein generating at least one likelihood grass mask includes: comparing red, green and blue color components of pixels of the image; and designating pixels of the image as grass pixels if the pixels have a green color component that is greater than the red and blue color components.

15. The computer program of claim 11, wherein generating at least one likelihood grass mask includes: generating a histogram for each of a red, green and blue color component of the image; determining a peak of each of the histograms; and designating pixels of the image as grass pixels if the pixels have red, green and blue color components proximate to the peaks of the histograms.

16. The computer program of claim 15, wherein the histogram are generated using a lower portion of the image.

17. The computer program of claim 11, wherein generating at least one likelihood grass mask includes: determining texture information for the image.

18. The. computer pϊogtaαv of claim. 11 , wherein generating, at least one likelihood non-grass mask includes: performing local maxima detection; and designating local maxima as non- grass.

19. The computer program of claim 11, wherein generating at least one likelihood non-grass mask includes: designating pixels of the image as non-grass pixels if the pixels have a hue that is not green.

20. The computer program of claim 1 1, wherein performing each of the first and second clean-up operations includes performing a morphological opening operation.