Backcoupling When you aim a camera at a screen which is displaying the picture of this camera, pictures of backcoupling are created. On the way through camera, amplifier electronics, cables etc., out of every screen dot is created a new one. In the beginning the picture can vary strongly, but often a stationary picture will be established after some time. With this method interesting and nice pictures can be created, especially when the camera is in relation to the screen rotated with a constant angle. What kind of pictures are generated depends on a multitude of factors which are often difficult to analyse (for example non-linearity of the video electronics, sharpness of the screen/camera, contrast settings, color unbalaces etc). With this program it is possible to simulate some aspects of optical backcoupling. You can alter the sharpness, the rotation angle of camera to screen, and the workings of the signal transportation electronics. With this you should be able to imitate a lot of typical backcoupling pictures. The pictures can have up to eight colors. 1. Program start und main menu The program is started by doubleclicking the appropriate icon on the workbench or from dos-shell by typing Backcoupling. A main menu with the following options will appear: 1. Editor: With the editor you define the starting picture. Later you can modify backcoupling pictures, too. You can draw freely on the screen window. On the right is a small menu with which you can set the paint mode (draw or fill), clear the whole picture, and set the paint color. Furthermore, you can reset the picture as it was when the editor was called. 2. Sharpness: Here you set the degree of sharpness. The amplifieing factorses of the center and of the four neighbouring points can be set freely in the interval of -2 to 2. At program start the center point has the factor 1, and the neighbouring ones have the factors 0. This means a sharp 1:1-copy of the original picture. More details below. 3. Rotation: With this menu item you can set the angle which has the camera in relation to the screen. This affects the rotation symmetry of the picture. With an angle of 180 degree the symmetry is 2-counting, with 90 degree it is 4-counting etc. A rotation will increase the computing time for one picture. 4. Amplifier: When you select this menu item you get into an editor for a distribution table. With the sharpness factors the programm derives from five old screen dot colors a new value, and with this distribution table the new dot color is chosen by this value. More details below. 5. Next picture: With this option the next backcoupling picture can be computes. 6. Continuous pictures: This is like the option before, but it will be computed endlessly until you click on the Stop-button. When you click on this button the actual picture will be finished; so the reappearance of the main menu can be delayed. 7. Change colors: Here you get into a color menu with which you can change the eight picture colors. Of all colors you can adjust the three RGB-Values. So every one of the eight color registers can be set to one of the 4096 Amiga colors. 8. Quit: To quit the program. 2. Picture generation To further explain the parameters sharpness, rotation and amplifier let us go through the generation of a backcoupling picture. First you need a starting picture, because from an empty picture only an empty picture can be generated. For that you go into the editor and put some lines into the picture. Normally it is not so deciding what you paint into the picture, because it can be altered strongly by the backcoupling process. When the editor is left and "Next picture" is called, and nothing has been altered in the backcoupling parameters since program start, you will see that nothing is changed on the picture. This depends on the fact that no special sharpness has been adjusted and a neutral amplifier table is active. So in the sharpness menu you can set the sharpness with five prop-gadgets. You decide with which factors the color values of the old screen dots and their neighbours are weighed (linear combination). If you for example have set 0.5 on all five factors, the new color value is computed in this way: New color = 0.5*color(center dot) +0.5*color(upper dot) +0.5*color(lower dot) +0.5*color(dot left) +0.5*color(dot right) As described earlier, every factor can freely be set in the range of -2 to 2. The color can have integer values of 0 to 7 (eight colors). If all five factors are set to the maximum of 2, the new color can have a value between 0 and 70 (color 7 times 5 dots times factor 2). But this new color value must not exeed the maximum of 7, so that the brightness won`t rise into infinity. This is established by the amplifier table. If you call the amplifier table, a graphics similar to an equalizer will appear, in which the whole distribution table is shown. On the horizontal axis are drawn the values which result from the sharpness (0 to 70), and in the vertical axis are drawn the new color values which are distributed to the computing results of the sharpness. With the mouse you can freely change this distribution. A maximum sharpness computing result of 70 can appear, as described above, when all sharpness factors are set to the maximum of 2. The actually possible maximum result depends only on the settings of the five sharpness factors. The program computes the actually possible maximum and displays relevant distribution values in white. From a distinct value on the distribution graph has another color, and these marked distributions are irrelevant with the momental sharpness setting. With this amplifier editor you lastly decide, whether a distinct color can be stable or will change into another in the next backcoupling step. The distributions on the right side of the graph, being for big computed values, are especially important if the picture has a lot if white parts (high brightness). On the left side of the graph are small computed values, which often appear in the beginning of a backcoupling series. If you here set bad distributions, the picture can be destroyed very quickly, e.g. become completely one color. The parameters sharpness and amplifier are admittedly not so easy to learn to use; in spite of that the third parameter is very easy: the rotation. You can freely change the angle which the camera has to the screen. This has the effect, that in every backcoupling step the picture will be rotated with this angle and interferes with the straight picture. This results in the creation of symmetries. With a rotation angle of 180 degree the picture has reached the original position after two steps, with 90 degree after four steps etc. With the rotation parameter you can easily create symmetrical pictures. Symmetry looks good an most pictures. For speed reasons it is better first to compute to pictures with rotation, until the complete rotation symmetry is reached. Then you can turn of the rotation (angle of 0 degree). The symmetry will be mostly sustained. 3. Some examples With values of 1 or more for all five sharpness factors the pictures will "grow" slowly. This has some similarity to biological growth. With adequate colors such pictures look like moss. If more than one sharpness factor is strongly negative, the pictures normally get fissured because full areas are instable. When all five factors have values of about 0.5, the opposite effect takes place. Small patterns will merge to greater, smoother areas. On this way you can create pictures which look like cheese or wood. For the setting of the amplifier you should be a bit cautious, otherwise good looking pictures can become junk very quickly. Best you try a new amplifier setting with computing just one picture before running a series, to see if something unwanted happens. If you choose a "smooth" distribution graph, larger stable or alternating (continuously changing the color) areas can be created, while with a "wild" setting the picture easily can become chaotic and uninteresting. You should keep in mind that there is nothing like a "final" picture. If a especially nice picture has been generated, you should thinking of ending the computing process. Maybe some pictures later only uninteresting pictures appear.