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Chapter 4
Coordinate Systems
60 Fastgraph User's Guide
Overview
Fastgraph uses three coordinate systems to perform text and graphics
output. These coordinate systems are character space, screen space, and
world space. The world space coordinate system is not available with
Fastgraph/Light.
Character Space
The coordinate system used for displaying characters is called character
space. Fastgraph uses character space for displaying characters in both text
and graphics video modes. It can be thought of as a grid of rows and
columns, with each cell in the grid holding one character. Each cell is
identified by its unique (row,column) integer coordinates. The rows and
columns are numbered starting at zero; the origin is always the upper left
corner of the screen. For example, in the 80-column by 25-row text modes (2,
3, and 7), the default (row,column) coordinates of the screen corners are
shown in the following diagram.
(0,0) (0,79)
(24,0) (24,79)
The number of rows and columns depends on the video mode, as shown in the
following table. For graphics modes, the table also includes the width and
height in pixels of a character cell.
Mode Char. Char.
Number Columns Rows Width Height
0 40 25
1 40 25
2 80 25
3 80 25
4 40 25 8 8
5 40 25 8 8
6 80 25 8 8
7 80 25
9 40 25 8 8
11 80 25 9 14
12 40 25 8 8
13 40 25 8 8
14 80 25 8 8
15 80 25 8 14
16 80 25 8 14
17 80 30 8 16
18 80 30 8 16
19 40 25 8 8
20 40 25 8 8
21 40 50 8 8
22 40 30 8 8
Chapter 4: Coordinate Systems 61
23 40 60 8 8
24 80 25 8 16
25 80 30 8 16
26 100 37 8 16
27 128 48 8 16
28 100 37 8 16
29 128 48 8 16
Fastgraph includes two routines, fg_getmaxx and fg_getmaxy, that
respectively return the maximum column and row numbers in text modes.
Example 4-1 demonstrates these two routines in a text mode. The program uses
fg_getmaxx and fg_getmaxy to obtain the maximum column and row numbers in
mode 3. It then displays these values (79 and 24).
Example 4-1.
#include <fastgraf.h>
#include <stdio.h>
void main(void);
void main()
{
int max_col;
int max_row;
int mode;
mode = fg_getmode();
fg_setmode(3);
max_col = fg_getmaxx();
max_row = fg_getmaxy();
fg_setmode(mode);
fg_reset();
printf("Last col = %d\n",max_col);
printf("Last row = %d\n",max_row);
}
Screen Space
Screen space is one of two available coordinate systems in graphics
modes. It uses the physical device coordinates. Screen space can be thought
of as a grid of rows and columns, with each unit in the grid holding one
pixel. Each pixel is identified by its unique (x,y) integer coordinates.
The pixel rows and columns are numbered starting at zero; the origin is
always the upper left corner of the screen. For example, in the 320 by 200
graphics modes, the (x,y) coordinates of the screen corners are shown in the
following diagram.
(0,0) (319,0)
(0,199) (319,199)
62 Fastgraph User's Guide
The Fastgraph routines fg_getmaxx and fg_getmaxy return the maximum x
and y screen coordinates when used in graphics modes, as shown in example
4-2. The program uses fg_getmaxx and fg_getmaxy to obtain the maximum x and
y coordinates in the standard CGA four-color graphics mode (mode 4). It then
displays these values (319 and 199).
Example 4-2.
#include <fastgraf.h>
#include <stdio.h>
void main(void);
void main()
{
int maxx;
int maxy;
int mode;
mode = fg_getmode();
fg_setmode(4);
maxx = fg_getmaxx();
maxy = fg_getmaxy();
fg_setmode(mode);
fg_reset();
printf("(%d,%d)\n",maxx,maxy);
}
World Space
World space is the other available coordinate system in graphics modes.
It utilizes user-defined floating point coordinates. Fastgraph translates
world space coordinates into physical device coordinates (screen space), and
because of this it is somewhat slower than using screen space. World space
can be thought of as a standard cartesian plane extending from the lower left
corner of the screen.
Any program that uses world space coordinates must first initialize
Fastgraph's internal world space parameters. The Fastgraph routine fg_initw
is provided for this purpose. The fg_initw routine has no arguments and must
be called before any other routine that uses world space coordinates.
The next step in using world space is to use the Fastgraph routine
fg_setworld to define the world space coordinates of the screen edges. The
fg_setworld routine has four floating-point arguments -- the minimum x
coordinate (left edge), the maximum x coordinate (right edge), the minimum y
coordinate (bottom edge), and the maximum y coordinate (top edge). For
example, if you define the world space coordinates with the statement
fg_setworld(-10.0,10.0,0.0,2.5);
Chapter 4: Coordinate Systems 63
the (x,y) coordinates of the screen corners would be defined as shown in the
following diagram.
(-10.0,2.5) (10.0,2.5)
(-10.0,0.0) (10.0,0.0)
Fastgraph includes a routine fg_getworld that returns the world space
extremes as defined in the most recent call to fg_setworld.
Example 4-3 uses fg_setworld and fg_getworld to illustrate an
interesting application of world space. This program calls another routine
named redraw (not shown) that erases the screen and draws a certain image
using world space coordinates. The program draws the image, waits for a
keystroke, reduces the world space by a factor of two in each direction, and
then draws the image again. This produces a zoom effect in which the image
appears twice as large as it was originally.
Example 4-3.
#include <fastgraf.h>
#include <stdio.h>
#include <stdlib.h>
void main(void);
void redraw(void);
void main()
{
int new_mode, old_mode;
double xmin, xmax, ymin, ymax;
old_mode = fg_getmode();
new_mode = fg_automode();
if (new_mode == 0) {
printf("This program requires graphics.\n");
exit(1);
}
fg_setmode(new_mode);
fg_initw();
fg_setworld(0.0,40.0,0.0,30.0);
redraw();
fg_waitkey();
fg_getworld(&xmin,&xmax,&ymin,&ymax);
fg_setworld(0.0,xmax*0.5,0.0,ymax*0.5);
redraw();
fg_waitkey();
fg_setmode(old_mode);
64 Fastgraph User's Guide
fg_reset();
}
Conversion Routines
Sometimes it's necessary to convert coordinates between character space,
screen space, and world space. Fastgraph includes eight conversion routines,
four for x coordinates and four for y coordinates, to perform such
conversions. These routines return the translated coordinate as their
function value.
The fg_xalpha and fg_yalpha routines convert screen space coordinates to
character space. The fg_xalpha routine converts a screen space x coordinate
to the character space column that contains the coordinate. Similarly, the
fg_yalpha routine converts a screen space y coordinate to the character space
row that contains the coordinate.
The fg_xconvert and fg_yconvert routines convert character space
coordinates to screen space. The fg_xconvert routine converts a character
space column to the screen space coordinate of its leftmost pixel.
Similarly, the fg_yconvert routine converts a character space row to the
screen space coordinate of its top (lowest-numbered) pixel.
The fg_xscreen and fg_yscreen routines convert world space coordinates
to screen space. The fg_xscreen routine translates x coordinates, while the
fg_yscreen routine translates y coordinates. Conversely, the fg_xworld and
fg_yworld routines convert screen space coordinates to world space. The
fg_xworld routine translates x coordinates, while the fg_yworld routine
translates y coordinates.
Summary of Coordinate Routines
This section summarizes the functional descriptions of the Fastgraph
routines presented in this chapter. More detailed information about these
routines, including their arguments and return values, may be found in the
Fastgraph Reference Manual.
FG_GETMAXX returns the maximum x coordinate in screen space when used in
a graphics mode. It returns the maximum column number in character space
when used in a text mode.
FG_GETMAXY returns the maximum y coordinate in screen space when used in
a graphics mode. It returns the maximum row number in character space when
used in a text mode.
FG_GETWORLD returns the current world space limits, as defined in the
most recent call to fg_setworld.
FG_INITW initializes Fastgraph's internal parameters for world space.
This routine must be called once, before any other routine that uses world
coordinates.
FG_SETWORLD defines the world space coordinates that correspond to the
physical edges of the screen.
Chapter 4: Coordinate Systems 65
FG_XALPHA and FG_YALPHA convert screen space coordinates to character
space.
FG_XCONVERT and FG_YCONVERT convert character space coordinates to
screen space.
FG_XSCREEN and FG_YSCREEN convert world space coordinates to screen
space.
FG_XWORLD and FG_YWORLD convert screen space coordinates to world space.
66 Fastgraph User's Guide