Q57649: CIRCLE Statement Draws Ellipses Based on Horizontal Resolution

Article: Q57649
Product(s): See article
Version(s): 4.00 4.00b 4.50
Operating System(s): MS-DOS
Keyword(s): ENDUSER | SR# S891227-98 B_BasicCom | mspl13_basic
Last Modified: 12-JAN-1990

Circles and ellipses generated using the CIRCLE statement are based on
the current horizontal resolution of the current coordinate system.
This can result in what may appear to be incorrect circles or ellipses
for the radius value given in the CIRCLE statement.

This information applies to Microsoft QuickBASIC Versions 4.00, 4.00b,
and 4.50, to Microsoft BASIC Compiler Versions 6.00, 6.00b for MS-DOS,
and to Microsoft BASIC Professional Development System (PDS) Version
7.00 for MS-DOS.

When an ellipse is generated using the CIRCLE statement, the ellipse
will never be wider or taller than the radius value given in the
CIRCLE statement using the horizontal resolution of the current
coordinate system. The following code examples demonstrate this using
all three possibilities, using VGA SCREEN mode 12:

   X-resolution < Y-resolution: WINDOW(-320,440)-(320,440)  '640x880
   X-resolution = Y-resolution: WINDOW(-320,240)-(320,240)  '640x480
   X-resolution > Y-resolution: WINDOW(-620,240)-(620,240)  '1240x480

Each program draws a circle using the CIRCLE statement, with a center
at (0, 0) and a radius of 100. A box is then drawn using the LINE
statement, with its center at (0, 0) and sides with a length of 200.
In a coordinate system of one-to-one, the default in SCREEN 12, this
produces a circle and a box in the center of the screen with the
circle tangent to all four sides of the box. When the coordinate
system is altered using a WINDOW statement, the circle always remains
tangent to the left and right sides of the box, but either falls short
of or exceeds the top and bottom of the box.

Code Example 1
--------------

'*** X-resolution < Y-resolution ***
SCREEN 12
WINDOW(-320,440)-(320,440)
CIRCLE(0, 0), 100
LINE(-100, -100)-(100, 100),,B
END

Code Example 2
--------------

'*** X-resolution = Y-resolution ***
SCREEN 12
WINDOW(-320,240)-(320,240) 'Commenting this line out gives same result
CIRCLE(0, 0), 100
LINE(-100, -100)-(100, 100),,B
END

Code Example 3
--------------

'*** X-resolution > Y-resolution ***
SCREEN 12
WINDOW(-620,440)-(620,440)
CIRCLE(0, 0), 100
LINE(-100, -100)-(100, 100),,B
END

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