MORPHING-BASED VECTORIZED CANDLE ANIMATION FOR LASER GRAPHICS. PURKHET Abderyim, MENENDEZ Francisco. J, HALABI Osama,CHIBA Norishige

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1 MORPHING-BASED VECTORIZED CANDLE ANIMATION FOR LASER GRAPHICS PURKHET Abderyim, MENENDEZ Francisco. J, HALABI Osama,CHIBA Norishige Department of Computer Science Graduate School of Engineering Iwate University {ohalabi,nchiba ABSTRACT Raster and vector are two common forms of graphics display systems. In this paper we introduce a new approach to display vector graphics by using a laser projector. The laser beam does not diffuse nor lose intensity over distance. These characteristics make laser projection suitable for large scale graphics. Laser projector is different from vector-based display in a way that it has its own properties such as the speed of laser beam and inertia of the reflective mirror. In this paper we present a method for creating vector data from an image and projecting using laser projector. The process starts by first extracting the binary image of the raster image and find contours then obtaining polygonal approximation of raster shape. Through this process, we converted a raster image according to the levels of its grey scale to a vector image that can be displayed using a laser projector. A Morphing technique is also proposed to create laser animation from a single candle vector data. A recursive vector subdivision algorithm is used to achieve smoother result. 1. INTRODUCTION In the early 1960s graphic display devices were developed which were called vector or calligraphic. A vector graphics display is basically a computer controlled oscilloscope. It draws lines and curves by moving the electron beam exactly where the line is supposed to go. To draw a line from point a to point b, it moves the electron beam to point a, turns it on, moves it directly to point b, and turns it off. They draw line segments as their primitive display operation [1]. Because of disadvantages such as the difficulty to avoid flickering and color capabilities, vector displays were replaced by raster displays which display graphics by arranged vertical and horizontal pixels. Although raster displays can resize their image as they are moved away from the screen, there is a limit on how much this resizing can happen. Beyond a certain point, the pixels being to blur out and lose quality. Furthermore, raster projectors should be displayed in special screens. Otherwise, the colors and intensity will fade. With laser projectors, these two problems do not occur. Laser projectors are using two fast moving mirrors that create the illusion of lines and shapes. Because of fast mirrors inertia laser project can create more smooth graphics than vector display. The laser beam has three properties that make it suitable for a large scale graphics. First, the light is monochromatic, or one pure color (although some lasers emit more than one color, each is pure, not muddy). Second, laser light is collimated, meaning that it does not spread out and diffuse like other types of light (such as incandescent). Third, the laser beam does not lose intensity over distance [2]. Lasers have been used in the entertainment especially in laser shows. There are two categories of the laser effect. One is Beam Effects show the laser beam that progressing in the air to the audience, the other is Screen Effects show the laser graphics on the screen that drawn in moving a laser spot on the screen. However there is little or none previous research done on laser graphics, therefore, it is worth to explore many problems and application that is inherented to laser projection displays. In this paper we use a laser projector [3][4] to draw graphics objects. Since the projector only understands vector data, the algorithms we need are different from those used in nowadays raster display devices. We use line segments as primitive display operation instead of pixels. Smooth candle animation for laser graphics was created by using vectorization and laser graphics techniques. First convert raster image to get vertices, generate vectors and project them using a laser projector. Second, using a raster grayscale image, we obtain vector information to be displayed by the projector. Also, we adapt one raster morphing technique so it can be used with vector graphics. 1.1 Previous Work Vector and raster are the two major approaches to create graphics. A vector device draws lines to create whole graphics, while a raster device uses an array of closely spaced dots to form a picture. Early computer graphics works on vector device such as Tektronix Basic graphics concept were introduced based on vector graphics [7] such as clipping and viewports, etc. The famous video game Asteroids utilized a monochrome vector graphics display, which was capable of fast moving objects made of very sharp lines Combined with great game play it became the biggest selling of game of its time. Vector display offers high resolution and allow varying levels of intensity [8]. 127

2 The advantage of vector graphics is that lines and curves look much cleaner than their rasterized equivalent. They suffer much less from "aliasing", or "stepping" which is a result of breaking a diagonal line up into dots, resulting in something that looks like a staircase in some cases. Because the electron beam actually moves diagonally from the beginning of the line to the end, the resulting line has very straight, clean, diagonal edges. Formally, there is little or no previous research done on laser graphics. However, there has been a big amount of work done. In industry, laser cutting is used to cut metals and other materials. Lasers have been used for marking products in a production line. A printing laser is used to mark things such as lot and serial numbers, as well as manufacturing and expire dates [12][6]. In medicine, the laser scalpel is used for laser vision correction and other surgical techniques. In the music field, lasers have been used for a while as part of laser shows in dance clubs [2] and laser hobbyists. Yet these shows usually employ very simple, static animations (they do not change once they started and repeat endlessly) or the lasers simply move to match the rhythm of the music playing. There is also an association called ILDA [9] (International Laser Display Association), which is the world's leading organization dedicated to advancing the use of laser displays in the fields of art, entertainment and education. It has shown some efforts into moving towards a standard file format and hardware specifications, these standards have a commercial goal only, as they benefit the exchange of products and services. 2. EXPERIMENTAL LASER PROJECTOR A laser projector can be built in many ways. The most common way to control the laser beam is by rotating mirrors with its corresponding galvanometers. The basic components of our laser projecting system consist of the following: galvanometers and also communicates the position feedback to the scan controller. Power: supply power to laser projector. Galvanometers and mirrors: galvanometers turn mirrors to control the x and y direction of laser beam. Laser beam: A laser beam of a given color and intensity. The operation of the projector is achieved by sending instructions into the controller. These instructions are sent to the corresponding servo controllers that rotate the X and Y mirrors by changing the current on the galvanometers. The laser beam stays static and it is the reflection on these two fast moving mirrors that creates the illusion of lines and shapes on the projection screen. The projector used in our experiments was built using the following components: [3][5][6] Scan Controller: GSI Lumonics SC2000 Servo Controllers: GSI Lumonics MiniSAX (2 units) Galvanometers: GSI Lumonics VM-500 Laser: Coherent MVP Control Diode Laser Module. Wavelength: 670nm±10nm (Red). Power: 0.95mW 3. LASER GRAPHICS RELATED ISSUES In constant laser beam intensity, if two lines of different length drawn in the same interval, the shorter line will look brighter than the longer one. Because in same time interval drawing longer line, laser projector moves mirror faster than drawing shorter one. The slower the mirrors move the brighter laser beam line come into sight (in Figure 2 a). (a) Different brightness Scan controller : It is responsible for communicating with PC and axis servos, and it is has some memory to store programs before sending them to the axis servos. Scan Controller Laser beam (b) Same brightness Power Galvanometer s and mirrors Figure 2: Mirrors speed effects Axis servos controler Figure 1: Laser Projector Axis servos controller: It converts the digital signals from the main controller into analog currents for the Laser projectors have the ability to control not only the coordinates from and to the laser beam should move, but also the time in which this movement happens. Constant brightness can be achieved either by moving the 128

3 laser beam with a constant velocity for all vectors by adjusting beam intensity for different vectors, or by constant laser beam intensity with adjusting moving speed. In this paper we calculate the moving speed according to the length of the lines which will be drawn. In this way, all the drawn lines have the same brightness (Figure 2 (b)). (a) (b) To achieve this, we first binarize the raster image, find its contours and store them in the chain format for polygonal approximation of the chains [10]. From the obtained polygon vertices, we generate vectors and project them using the laser projector. In this way, we avoid to turn the laser beam off and on that is to achieve less flickering. To demonstrate our vectorization method, we carried out experiments with various objects from images with different complexity. Figure 4 shows the process of processing a raster image to get vertices, then generating vectors and projecting them using a laser projector. The entire raster image consists of three objects which are represented by three single line graphs. The circle object was vectorized as a polygon because in laser graphics curves have to be represented by a group of line segments. (c) Figure 3: Inertia effects (d) Original raster image Laser projector is different from vector display in the way they display a line segment. Laser projector presents line segments by controlling two mirrors, one for X and another for Y dirction.. When the mirrors try to quickly switch from one position to the next, the mirrors inertia effect will appear, which produces a curved effect on straight corners. This enables us to represent smooth curves with less line segments. For example, to represent the circle shown in Figure 3 (a) with line segments, we used same numbers of line segments (Octagon) but with different moving speed (different inertia ) as can be seen in Figure 3 (b)( c), and (d), In Figure 3(b) the shape represents a polygon rather than a circle, moving speed is 10 frame per second (fps). Curved effect appeared on the angle points in Figure 3(c) when 40 fps was set. In Figure 3(d) we redraw the shape with 100 fps, and the inertia influence could be clearly observed as the octagon become more and more rounded and closer to circle. 4. CANDLE ANIMATIONS In contrast to raster graphics, where images are represented by a two-dimensional array of pixels, laser projectors are vector based devices. This means that objects can only be represented using straight line segments between addressable points. 4.1 Vectorization Since laser projectors are vector-based displays, they perform best when the drawing is composed of only one contour and the laser does not need to be turned on and off. Having this in mind, we convert the raster image to a vector image using Contour Retrieving techniques. Approximate polygon Laser image Figure 4: Laser drawing process Vectorization algorithms only find contours in the binary raster images. In order to display grayscale images with lasers we can convert the original image into a set of binary images by using different thresholds, and later apply Contour Retrieving techniques described before to each image of in the set. 129

4 To create the feeling of grayscale using lasers, we must change the intensity of the beam for each contour. To achieve this, we calculate the moving speed not only according to the length, but also based-on grayscale level of each contour (Figure 5). In image morphing, each pixel in the source image is mapped to an appropriate place in the destination image. However, since in laser graphics, segments are the primitive operation, instead of applying morphing for every pixel, we only transfer vertices. After we get vector data using vectorization, we apply feature-based image metamorphosis to obtain new vector data. original image contours laser image Figure 5: Contour for different grayscale In Figure5 instead of obtaining one single polygon we get several polygon depend on the grayscale level of the image. The candle flame can be represented by several individual single line graphs. Also each curve brightness was calculated according to it is grayscale value. Since we cannot define a single edge for the candle flame, vectorization occurred on different grayscale levels. In this example we use three different grayscale level thresholds (50, 128, and 250) respectively. 4.2 Vector morphing Morphing is an image processing technique used to smoothly change one image into another. The idea is to get a sequence of intermediate images which, when put together with the original images, would show the transformation. The simplest method of converting one image into another is to cross-dissolve between them. In this paper we used the feature-based image metamorphosis [11] to create new vector data animation from original vector data that obtained from the raster image. We then use noise to move the control line in order to generate the destination image. Because we apply morphing only to the starting and ending vertices of the line segments, we lose some smoothness of the generated image. For example, using the morphing method on a vertical line in raster mode with three horizontal control lines, the straight line becomes smooth curve after the transformation (Figure 6, left). However, in laser morphing line remains straight after the transformation. This occurs because in laser morphing only start and end points are moved, resulting in loss of accuracy. To solve this problem, we subdivide the original vector into smaller vectors, depending on the resulting morphed coordinates. Figure 7 shows how to accomplish this task. We first subdivide vector at the midpoint (O) and calculate it is morphed position (O 1 ), if O 1 is not on (or near to) P 1 Q 1, we divide the vector PQ into two vectors then apply morphing process again. The algorithm steps are as following: Step 1. From vector PQ and its middle point O calculate morphed points P 1,Q 1 and O 1. Step 2. Calculate angle P 1 O 1 Q 1 Step 3. If the angle is smaller then threshold angle Insert O 1 to vertexes list and replace P 1 Q 1 with O 1 P 1 and O 1 Q 1 respectively then go to step 1. If the angle is bigger than a threshold then finish subdivision. Q 1 O 1 O Q Original line After morphing P 1 O 1 Q 1 angle P 1 P Figure 7: Subdivision in Laser Morphing In Figure 5 vector PQ became 3 subdivided vector. Control line Raster morphing Laser morphing Figure 6: Smoothness problem of vector 5 RESULTS We were able create a candle animation with our laser projector from a raster image. To create the feeling of grayscale using lasers, we changed the intensity of the beam for each contour. To achieve this, we calculated the 130

5 moving speed not only according to the length, but also based on the grayscale level for each contour. To solve this problem, we subdivided the original vector into smaller vectors, depending on the resulting morphed coordinates. Figure 8 is an example of the laser morphing processing feature. Both edges of rectangle were subdivided and the number of vectors increased after morphing. original without subdivision with subdivision Figure: 8 Figure 9 shows the animation frames of the candle animation displayed using our laser projector. Each picture was every 10 steps. We can clearly distinguish the continuity of the candle animation. Vectorization process was employed only once to obtain the original shape. From this vector data, the rest of the animation was created by changing the position of the vertices. This change depends on morphing produced by applying noise to the control line. 4. CONCLUSIONS AND FUTURE WORK In this paper we introduced the laser graphics for displaying computer graphics using a laser projector and implemented some basic application such as converting raster image to vector image that can be displayed using a laser projector and laser morphing. As for the future work, there is a lot of areas that this research can be further extended. After converting raster image to vector laser graphics we can use it for cartoon drawing. By using grayscale information with laser power density it is possible to research on non-photorealistic rendering with laser projector. REFERENCES [1] Eugene L. Fiume. the Mathematical structure of raster graphics. Academic Press (1989). ISBN: [2] Laser Light Shows and Laser Displays: [3] NASUKAWA Norihiro, ABDERYIM Purkhat, MENENDEZ Francisco, HALABI Osama, CHIBA Norishige, 1E-04, Development of Experimental Laser Projector, Tohoku-Section Joint Convention of Institutes of Electrical and Information Engineers 2006, (in Japanese) [4] MENENDEZ Francisco, ABDERYIM Purkhat, NASUKAWA Norihiro, HALABI Osama, CHIBA Norishige. 2A-16 Research Towards Laser Graphics and Related Problems. Tohoku-Section Joint Convention of Institutes of Electrical and Information Engineers [5] GSI Lumonics. Optical Scanners and Controllers: [6] Coherent Lasers: [7] FOLEY J.D., VAN DAM A. Fundamentals of Interactive Computer Graphics. Addison-Wesley (1982). ISBN: [8] DENNIS Harris. Computer Graphics and applications. Chapman and Hall(1984). ISBN: [9] International Laser Display Association (ILDA): [10] OpenCV Reference Manuals [11] T. Beier and S. Neely. Feature-Based Image Metamorphosis. Computer Graphics (SIGGRAPH '92), pp , July [12] Epilog Laser: ACKNOWLEDGEMENTS We would like to thank Mr. Norihiro Nasukawa for his valuable technical support by building and maintaining the laser projection hardware. 131

6 (0) (10) (20) (30) (40) (50) (60) (70) (80) (90) (100) (110) (120) (130) (140) (150) Figure 9 : The frames of the candle animation using laser projector The step number is shown between parenthesis 132