Q58105: Explanation of Tiling in BASIC; PAINTing with Patterns
Article: Q58105
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# S900114-17 B_BasicCom | mspl13_basic
Last Modified: 5-MAR-1990
The QuickBASIC PAINT statement can be used to paint a region of the
screen in a particular color or pattern. PAINTing with a pattern is
called "tiling."
The syntax for the paint statement is as follows:
PAINT [STEP] (x,y) [,[paint] [,[bordercolor] [,[background]]]
The variable labeled "paint" is used to control the color of the
painted region. If it is a numeric value, then the number must be a
valid color attribute. The corresponding color is used to paint the
area. If paint is left blank, then the foreground color attribute is
used.
However, if you use a string in this part of the PAINT statement, then
this string is used to do "tiling." This article describes how to do
tiling in different screen modes.
For an in-depth explanation of tiling, please see the following
manuals:
1. "Microsoft QuickBASIC 4.5: Programming in BASIC," Pages 181-191
2. "Microsoft QuickBASIC 4.0: Programming in BASIC: Selected Topics,"
Pages 226-239
3. "Microsoft BASIC 7.0: Programmer's Guide," Pages 179-189
4. "Microsoft BASIC Compiler 6.0: Programming in BASIC: Selected
Topics," Pages 226-239
5. "Microsoft QuickBASIC Compiler" manual for Versions 2.00, 2.01, and
3.00, Pages 380-382
This information applies to Microsoft QuickBASIC Versions 1.00, 1.01,
1.02, 2.00, 2.01, 3.00, 4.00, 4.00b, 4.50 for MS-DOS; to Microsoft
BASIC Compiler Versions 6.00 and 6.00b for MS-DOS and MS OS/2; and to
Microsoft BASIC Professional Development System (PDS) Version 7.00 for
MS-DOS and MS OS/2.
Each tile for a pattern is composed of a rectangular grid of pixels.
This tile grid can have up to 64 rows in all screen modes. However,
the number of pixels in each row depends on the screen mode. This is
because, depending on the number of color attributes per screen, each
pixel can be represented by a varying number of bits.
The formula for the number of bits per pixel is as follows:
Bits-per-pixel = Log (base 2) of the number-of-attributes
The number-of-attributes is the number of color attributes in that
screen mode.
The following table gives the bits-per-pixel and length-of-tile-row
for each screen mode:
Screen Mode Bits-per-Pixel Length-of-Tile-Row
----------- -------------- ------------------
1 1 8 pixels
2 2 4 pixels
3 1 8 pixels
4 1 8 pixels
7 4 2 pixels
8 4 2 pixels
9 2 4 pixels
10 2 4 pixels
11 1 8 pixels
12 4 2 pixels
13 8 1 pixel
The easiest way to define a tile row is with the CHR$() function. This
function defines one byte (8 bits) in the tiling string. It is also
easiest to use two hexadecimal digits per byte since they are easily
converted to 4 bits per digit. For instance, the following line
TILE$ = CHR$(&HF0)
sets up a tiling string with the value 11110000. It is used in a
PAINT statement, such as the following:
PAINT (100,100), TILE$
SCREENS 2, 3, 4, and 11
-----------------------
In screen modes with only two attributes (modes 2, 3, 4, and 11), the
tiling string is very simple since each bit equals one pixel. A one
(1) means that the pixel is on and a zero (0) means that the pixel is
off.
SCREENS 1, 9, and 10
--------------------
In screen modes with four attributes (modes 1, 9, and 10), the tiling
is only a little more complicated since a pixel is represented by two
bits. For instance, if the two bits were 01, then the pixel would be
drawn in color 1. If they were 11, then the pixel would be color 3.
One byte would equal 4 pixels. The byte 00011011 would draw 4 pixels
in color 0, 1, 2, and 3 consecutively. The tiling string for 00011011
would be the following:
TILE$ = CHR$(&H1B) '0001 = &H1, 1101 = &HB
SCREENS 7, 8, and 12
--------------------
Tiling in EGA and VGA screen modes with more than 4 colors is a lot
more complicated. In these modes, it takes more than one row (byte) to
define a tiling row. This is because each pixel is represented
three-dimensionally in a stack of "bit planes," rather than
sequentially in a single plane.
Below is an example of how to define a multicolored pattern consisting
of rows of alternating yellow and magenta in a screen mode with 16
color attributes (modes 7, 8, and 12). The following table shows how
the colors are defined for each pixel and how to arrange them into
CHR$() statements:
Column
1 2 3 4 5 6 7 8
-------------------
Row 1 1 1 0 0 0 0 1 1 = CHR$(&HC3)
Row 2 0 0 1 1 1 1 0 0 = CHR$(&H3C)
Row 3 1 1 1 1 1 1 1 1 = CHR$(&HFF)
Row 4 0 0 1 1 1 1 0 0 = CHR$(&H3C)
TILE$ = CHR$(&HC3)+CHR$(&H3C)+CHR$(&HFF)+CHR$(&H3C)
The preceding line defines 8 pixels of the colors magenta, magenta,
yellow, yellow, yellow, yellow, magenta, magenta. This combination
results in alternating stripes of magenta and yellow. You can get the
color by reading each column from BOTTOM to TOP. For instance, the
first pixel is defined in column 1 by the color number 0101. This is
color 5, which defaults to magenta. The sixth pixel is defined by the
number 1110, or color 14, which is yellow in the default palette.
To paint a circle with this pattern, you would do the following:
SCREEN 12
TILE$ = CHR$(&HC3)+CHR$(&H3C)+CHR$(&HFF)+CHR$(&H3C)
CIRCLE (100,100),50 'a circle of radius 50
PAINT (100,100),TILE$
END
SCREEN 13
---------
Screen mode 13, which has 256 attributes, works differently than the
other EGA and VGA modes. You have a lot less flexibility with screen
13 than you do with even the two-color screens. The most that you can
do with screen 13 tiling is to create horizontal lines of different
colors. This is very simple since one byte is equal to one pixel
color. Therefore, one CHR$() statement defines one line or row.
To paint a circle with alternating lines of color 5 (magenta) and
color 14 (yellow), you would do the following:
SCREEN 13
TILE$ = CHR$(&H5) + CHR$(&HE) 'colors 5 and 14
CIRCLE (100,100),50
PAINT (100,100),TILE$
END
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