Updating Objects

BOB CLEAR
instruction: clear all Bobs from the screen
Bob Clear

BOB DRAW
instruction: re-draw all Bobs on screen
Bob Draw

This pair of commands is used to synchronise Bob updates with complex screen movements, and generate superbly smooth scrolling screen effects. The technique is achieved by the following steps.

• Remove all Bobs from the logical screen display with BOB CLEAR. Background areas are copied from their invisible hiding places in memory, and the display is returned to its original condition.
• Each Bob is now examined in turn, and checked to see if it has been repositioned. If so, the area beneath the new coordinates are copied invisibly, as they will be needed to return the screen back to normal, when the Bob is next moved. You can now perform your drawing operations as required, and move your Bobs to any point on the screen.
• Now use BOB DRAW to re-draw any Bobs that have moved at their new screen coordinates, using the appropriate image from the Object Bank.

Note that BOB CLEAR and BOB DRAW will only work on the current logical screen, so if DOUBLE BUFFER has been activated, a SCREEN SWAP command will be needed to call the relevant display, as follows:

X> Screen Swap : Wait Vbl

Also remember to turn off the automatic updating system completely before use. Here is the correct procedure.

• Turn off the AUTOBACK system to stop the synchronisation between your graphics and Bobs, like this:

Autoback 0

• Now that all graphical operations have been forced to work with the logical screen, turn off the standard updating system, with BOB UPDATE OFF.
• Next add a BOB CLEAR command at the start of your main loop. You can now draw your graphics on screen, and move your objects as required.
• Finally, re-draw your objects at their new positions using BOB DRAW.