Graphing; The Graph Screen - Texas Instruments TI-89 Developer's Manual

17.

Graphing

This chapter discusses the Graph application on the TI-89 / TI-92 Plus
calculators and how to interface with it.
17.1.

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
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Beta Version January 26, 2001