BB Tute #12 - More Serendipity

Blitz Basic Tute #12 - More Serendipity

In preparation for our next exciting project, I set up this dot that bounced off the walls. Then I decided to randomly change the colour at each bounce - this was the result! Perhaps you should try for yourself before reading on - you'll need the new word Plot. For example,

Plot 10,20

will draw a dot at x=10, y=20 in the current colour.

Graphics 640,480
SeedRnd MilliSecs()
x=Rand(320)
y=Rand(240)
dx=1
dy=1
Repeat
  Plot x,y
  x=x+dx
  If x<0 Then
    x=Rand(0,1):dx=1:change_colour()
  End If
  If x>320 Then x=320:dx=-1
  y=y+dy
  If y<0 Then
    y=Rand(0,1):dy=1:change_colour()
  End If
  If y>240 Then y=240:dy=-1
Until KeyHit(1)
End

Function change_colour()
  Color Rand(255),Rand(255),Rand(255)
End Function

Notice that I haven't used the Back Buffer here cos I'm just trying to develop the "bouncing" idea - a bit of flicker won't matter. Notice also that I'm only using the top left quarter of the screen - you'll see why later.

We start with a random point (x,y) and set up the increments dx and dy.

After plotting the point we add dx to x and check whether x is now outside the "box". If it is, we take appropriate action then we do the same for y. The rand(0,1) is necessary to add a little randomness to the path otherwise the dot will just retrace the same path and will probably leave gaps.

My program.

So where is this heading?

What is this "next exciting project" mentioned at the top?

Well, we're going to use that "travelling dot" to specify the top-left corner of a 320x240 grab from an image.

Now you see why I restricted (x,y) to the top-left corner - our grab image will not run off the edge of the main image.

Then we're going to display that grab in the top left corner so it will be like we have a camera scanning over the image.

Recall that GrabImage grabs from the current buffer so we'll need to have a way to swap from the back buffer (yes ... we don't want any flicker this time!) and the buffer where the image is. Suppose the image is loaded into img then these are the lines you'll need:

...
SetBuffer BackBuffer()
...
SetBuffer ImageBuffer(img)
...

Another little bonus for you - when you have finished, change DrawBlock to TileBlock in your program.

Ex. 5: Use this image in a new program to produce a fullscreen 640x480 scanning effect. My solution.