Explanation of Tiling in Basic; PAINTing with Patterns (58105)

   PAINT [STEP] (x,y) [,[paint] [,[bordercolor] [,[background]]]
				

MORE INFORMATION

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 example, 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 example, 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.

Listed 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, and 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 example, 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:

Example

To try this example in VBDOS.EXE:

1. From the File menu, choose New Project.
2. Copy the code example to the Code window.
3. Press F5 to run the program.

  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:

Example

To try this example in VBDOS.EXE:

1. From the File menu, choose New Project.
2. Copy the code example to the Code window.
3. Press F5 to run the program.

  SCREEN 13
  TILE$ = CHR$(&H5) + CHR$(&HE)   ' Colors 5 and 14.
  CIRCLE (100,100),50
  PAINT (100,100),TILE$
  END