Texas Instruments TI89 Developer Guide - Page 231
Graphing, The Graph Screen
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189 17. 17.1. Graphing This chapter discusses the Graph application on the TI-89 / TI-92 Plus calculators and how to interface with it. The Graph Screen The graph screen always has an odd number of pixels vertically and horizontally, even if the window itself has an even number of pixels. In this case, the rightmost column and/or the bottom row will not be used for graphing. The odd number of pixels insures that there will always be a pixel in the center of the graph, which is where the origin of the axis is with the default window settings. The Window variable, xmin, corresponds to the value at the center of the leftmost pixel column, while xmax is the center of the rightmost pixel column used for graphing. ymin and ymax values correspond to the center of the pixels of the top and bottom rows respectively. ∆x and ∆y are measured from the center of one pixel to the center of the next and are computed when the graph screen is displayed. If the Window variables or screen size has changed since the last time the graph was displayed, ∆x and ∆y may not have been recomputed yet and may be invalid. Figure 17.1 shows the four pixels in the upper left corner of the graph screen and the relationship between the x, y viewing window coordinates and the row, column pixel coordinates. Some system routines and TI-BASIC calculator commands use pixel values and others require viewing window coordinates. For drawing objects such as lines, dots, and circles on the screen, the system routines described in section 11.2. Windows are provided. These routines all expect pixel coordinate inputs. Several system routines, listed in section 17.6. Available Graph System Routines and Global Variables, convert between pixel and viewing window coordinates. It is important to remember that when using system routines or calculator commands with x, y viewing window coordinates as inputs that the outer half of the first and last columns and the outer half of the top and bottom rows are outside the viewing window. If the TI-BASIC command PtOn xmin-∆x/4, ymax is entered on the Home screen, the upper left pixel on the graph screen will not be set. Even though xmin-∆x/4 is within the range covered by the first column (xmin-∆x/2 to xmin+∆x/2), it is less than xmin and therefore outside the viewing window. TI-89 / TI-92 Plus Developer Guide Not for Distribution Beta Version January 26, 2001
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