How Laser Shows Work - Graphics Systems

Graphics systems are usually computer based and allow for the storage of words, pictures and animations for projection by the laser scanning system.

   The type of image display we are most familiar with in everyday life is the the CRT (Cathode Ray Tube) or LCD (Liquid Crystal Display) which is found in TV sets and computer monitors. This is a raster type of display as the information is displayed by drawing a series of closely spaced horizontal lines very rapidly and varying the brightness and colour of the pixels in the lines. This type of display requires very high bandwidth as there is lots of information in the image.
    The other most common type of display is a vector display. This requires lower bandwidth as only the end points and changes in the line direction are stored. You may have seen this kind of display in some older video games like Asteroids and Tempest. In a vector image, the picture is stored as a series of points and lines are drawn between the points - like a "connect the dots" picture. If you run through the points in the image fast enough, the eye is tricked by it's persistence of vision into seeing a picture rather then the points that make up the image.
    As we saw on the Scanning Systems page, laser images are crated by the mechanical movement of tiny mirrors. The scanning system can not move fast enough to give a TV quality raster image so laser systems use vector images. Even laser systems that are projecting raster like images are using vector technology to simulate a raster image. Laser systems draw mostly outline images with no filled or shaded parts.

Most graphics systems use point plot sequential frame animation for graphics storage. This means that the images are stored as a series of points (like a connect-the-dots picture), and that animation frames are played back in sequence to create the illusion of motion as in a cartoon. The points that make up the image are converted by Digital to Analogue Converters (DACs) or a display processor in the computer into analogue voltages that drive the scanning system.
The scanners use small mirrors mounted on galvanometers at right angles to each other to control the vertical and horizontal deflection of the beam. The points in the image are refreshed (re-drawn) many times a second by the scanners so that your eye is tricked into seeing an image - typical refresh rates are between 18 and 22 Hz. The rapid projection of a sequence of slightly different images gives the illusion of movement just as in traditional cell animation (cartoons).



Ten of the Key frames from the Unicorn animation shown above. These key frames were combined with "tween" frames to create the unicorn running (this is a .gif representation made from laser frame data and not actual photos of the laser image)

There are many and varied graphics systems from the older technology types that store image data on PROM's, to the newer multitasking real-time 3D systems with dedicated display processors that store laser images and animations on disk drives. The latest generation of graphics software uses vector storage where only the corners or angle changes in lines are stored (rather then every point in the image), in between points are calculated by the computer in real time as the frames is displayed.
Computer storage and playback of laser graphics and animations gives the highest image quality. The graphics and animations are always first generation as they have been produced from the digital data on site. Modern ADAT based tape storage systems allow for complex colour laser shows to be digitally recorded onto tape with minimal image distortion. In some planetarium based laser shows the majority of the show is recorded onto tape to insure a high quality, consistent display each time.

Laser Raster Graphics

Laser projection systems can project low resolution rater type images. This is accomplished by scanning the laser beam back and forth in a series of closely spaced lines. The colour information is changed as the beam moves along each line so as to display a TV like raster image. The scanning system is not fast enough to project more than 60 to 80 lines so the images are low resolution and have to be kept small to achieve close spacing of the lines.

Raster laser image
Photo courtesy of Pangolin

The process of making laser animations usually begins with the preparation of artwork. Each individual frame is usually drawn on paper or sometimes on a computer system. Each frame must then be hand digitised into the laser graphics system either by means of a digitising pad/tablet or with a mouse. To create one second of quality animation, the artist must draw 15 to 20 frames and the digitiser must convert them to to laser data. Since it can take up to half an hour to hand create one frame of animation, one second of animation on the screen represents about 7 to 9 hours of work.

Laser animation of a couple dancing
Courtesy of LaserAnimation Sollinger GmbH, Germany

Some computer animation tricks can be used such as generating 'tween' frames. In this process, two frames of a character in slightly different positions are provided to the computer which generates a third frame that is the average of the two. Other techniques include cycling a short animation as it is panned across the screen [for example a man running or the Unicorn shown above]. This gives the illusion that the animation is more complex as it occupies more screen area. Laserists can also combining simple short background animations with a more detailed foreground animation to create a more complex image.

In Database 3D, an image with three dimensions is created in the computer system either by hand drawing or importing data from other computer 3D programs. The image can then be rotated, scaled and displayed with perspective and 'Back clipping'. With more advanced systems, "image masking" is used to remove parts of objects that fall behind other objects to enhance the 3D illusion.
It is still a flat 2D image projected onto a surface but the image follows the rules of 3D and fools the eye into believing it is seeing a 3D image - this is a similar process to that used to make 3D logos and objects on TV. No glasses or special viewing equipment is required to see database 3D images.

Stereoscopic 3D

    In Stereoscopic 3D, two slightly different images of the object, called views, must be projected, one for the left eye and one for the right eye version of the object. Often only one object is created and the computer system then calculates the two different views to be projected in real time.
   When the show is projected, generally two scanning systems (and sometimes two laser projectors) must be used; one to project the left eye view and one to project the right eye view. These images are separated by glasses worn by the spectators to deliver the correct image to each eye. The eye sees the two slightly different flat images and the brain integrates them into a full 3D image.
The two most common techniques in use are Anaglyph and orthagonally polarized 3D. Anaglyph 3D uses glasses with a red filter and a blue filter. The images are projected in red and blue so full colour images are not possible.
   Orthogonal 3D uses polarized lenses that are set at right angles to each other. The left eye image is polarized at right angles to the right eye image so that the glasses pass only the appropriate image to each eye. The polarized lenses appear light gray and do not interfere with colour perception allowing full colour 3D images to be projected.

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