Chapter 21 - Qbasic Tutorial - Qbasic Tutorial - Qbasic Tutorial - ^^JPS^^
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Chapter 21 - Qbasic Tutorial

So far, we've only been dealing with text (words and numbers). How do we do pictures in QBASIC? First, we need to use SCREEN to change from text mode to graphics mode.

SCREEN

SCREEN lets you select a "graphics" screen instead of the "text" screen we've been using. This will let you draw pictures. In the next program, we'll use DRAW to draw a square on the screen in SCREEN 12 graphics mode.

 SCREEN 12
CLS
DRAW "D100 R100 U100 L100"

There are many other SCREEN numbers you can use, but 12 is probably the easiest to work with. It gives you a lot of space and the color numbers are familiar. QBASIC Help explains all the possible values of SCREEN. You can always try them and see what happens.

DRAW

DRAW is kind of like the turtle in the programming language Logo. With DRAW, you can move around the screen and draw lines along the way. In the above example we used the following DRAW commands:

  • D100 - Go down 100 units
  • R100 - Go right 100 units
  • U100 - Go up 100 units
  • L100 - Go left 100 units

DRAW can do a lot more than that. It is like PLAY. It's a small programming language inside of QBASIC. Look at QBASIC Help for a complete description of everything it can do. Here's a filled in box:

 SCREEN 12
CLS
DRAW "C15 D100 R100 U100 L100 BF1 P15,15"

"C15" sets the color to bright white. "BF1" moves into the square, then "P15,15" fills it with bright white. Finally, here's something very Logo-like:

 SCREEN 12
CLS
FOR I = 0 TO 360 STEP 10
DRAW "D100 R100 U100 L100 TA" + STR$(I)
NEXT I

"TA" means to turn to a specific angle. STR$ converts the value in I to a string. This lets DRAW turn to the angle in the variable I. It's not quite as easy as Logo, but it's still pretty impressive.

LINE

QBASIC also lets you draw using a coordinate system. It's like drawing graphs on graph paper. Try this:

 SCREEN 12
CLS
LINE (0, 0)-(320, 240), 15

LINE lets you draw a line between two points. The points are specified in (x, y) coordinates. You may have seen this when learning about graphs in math class. In QBASIC, the coordinates are almost the same. The only thing that is different is the Y coordinate. In QBASIC, the Y coordinate is upside down. "0" is at the top, and bigger numbers go toward the bottom of the screen.

"LINE (0, 0)-(320, 240), 15" draws a line starting at coordinate (0, 0) which is the upper left corner of the screen. The line ends at (320, 240) which is the center of the screen. The last number is the color (15 which is bright white).

Box

By adding a "B" to the end of a LINE statement, you can draw a box. Try this:

 SCREEN 12
CLS
LINE (0, 0)-(320, 240), 15, B

The first coordinate is the upper left corner while the second coordinate is the lower right.

Try "BF" instead of "B". Interesting?

CIRCLE

QBASIC can also draw circles using the CIRCLE statement:

 SCREEN 12
CLS
CIRCLE (320, 240), 100, 15

The coordinate (320, 240) tells the computer where to put the center of the circle. "100" is the radius, or how big the circle will be. "15" is the color number (bright white again).

PAINT

Notice how that circle was not filled in. LINE has a "BF" option that will let us fill in the boxes it draws. CIRCLE has no such option, so we have to use PAINT:

 SCREEN 12
CLS
CIRCLE (320, 240), 100, 15
PAINT (320, 240), 15, 15

PAINT fills an area with a color. It stops painting when it runs into a certain color on the screen. The coordinate (320, 240) tells PAINT where to start filling in, and the first "15" tells PAINT to use bright white as the paint color. The second "15" tells PAINT to stop painting when it runs into anything that is bright white.

Circle Art

Concentric circles are very easy to draw:

 SCREEN 12
CLS
FOR I = 5 TO 200 STEP 5
CIRCLE (320, 240), I, 15
NEXT I

With CIRCLE, PAINT and some random numbers, we can make some interesting pictures:

 SCREEN 12
CLS
FOR I = 1 TO 50
X = INT(RND * 640)
Y = INT(RND * 480)
R = INT(RND * 100)
Color1 = INT(RND * 16)
CIRCLE (X, Y), R, Color1
PAINT (X, Y), Color1, Color1
NEXT I


Source: http://jpsor.ucoz.com
Category: Qbasic Tutorial | Added by: JPSor (2009-02-26) | Author: JPSor
Views: 2574 | Rating: 4.2/6 |
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