Q48058: Graphics Algorithm Different in QuickBASIC Versus GW-BASIC

Article: Q48058
Product(s): See article
Version(s): 1.00 1.01 1.02 2.00 2.01 3.00 4.00 4.00b 4.50
Operating System(s): MS-DOS
Keyword(s): ENDUSER | SR# S890811-64 B_BasicCom B_GWBasicI | mspl13_basic
Last Modified: 15-DEC-1989

When constructing an image with multiple-line statements, the image
may not appear as fully symmetrical as the x and y coordinates
indicate. The algorithm used by QuickBASIC to draw the various graphic
images executes the starting and ending coordinates in a specific
order to produce the fastest results. This algorithm is different than
that of GW-BASIC, where each line is drawn literally.

This information applies to Microsoft QuickBASIC Versions 1.00, 1.01,
1.02, 2.00, 2.01, 3.00, 4.00, 4.00b, and 4.50 for MS-DOS, to Microsoft
BASIC Compiler Versions 6.00 and 6.00b for MS-DOS, to Microsoft BASIC
PDS Version 7.00, and to Microsoft GW-BASIC Versions 3.20, 3.22, and
3.23 for MS-DOS.

The algorithm used by QuickBASIC to draw the various graphic images
determines the fastest way to draw the image and then constructs that
image on the screen. The LINE statement is the best example, with the
following syntax:

   LINE (50,0) - (100,200)
   LINE (150,0) - (100,200)

Under QuickBASIC, the algorithm draws the initial line, from (50,0) to
(100,200), then evaluates the next statement and draws the next line
from (100,200) to (150,0). GW-BASIC draws the first line similar to
QuickBASIC, but then draws the next line from (150,0) to (100,200).

This is not considered a problem, but is a feature of the QuickBASIC
graphics algorithm that makes it faster and more efficient than the
interpreted sequential method of graphics. This difference is
illustrated when using a medium to low resolution graphics screen, as
in the following code example. The two lines drawn do not look
symmetrical under QuickBASIC, but are drawn identically under
GW-BASIC. The two lines are exactly the same, but one is inverted from
the other.

Code Example
------------

10 SCREEN 1
20 LINE (160, 90)-(140, 1), 1
30 LINE (162, 90)-(182, 1), 1

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