Subject: Re: [stella] Light gun/pen From: Eckhard_Stolberg@xxxxxxxxxxxxxxxxxxxxx (Eckhard Stolberg) Date: Wed, 1 Oct 1997 20:37:49 +0200 |
>The Atari 8-bit has discrete hardware registers for reading a light pen >(light pens work better than guns for obvious reasons). It does a fairly >rough job on the horizontal ;) The important thing isn't whether or not a >light gun detects light (which will just give you on or off), it is >finding a way for the computer to determine the X and Y coordinates of >where the beam is hitting the TV, and THEN cross referencing this with >your objects. You are right. I took a look at the Sentinel code and it seems the lightgun just sets the joystick button signal low, when it detects enough light, and high otherwise. Sentinel checks the coordinates like this: When the guntrigger (joystick up signal) is pressed, it sets all colour registers to white for one frame. During that frame it does the following for the visible part of every scanline: BIT INPT4 BPL pos1 BIT INPT4 BPL pos2 BIT INPT4 BPL pos3 ... pos1 STX ypos ; X-reg contains the current scanline LDA #70 STA xpos BNE end pos2 STX ypos LDA #85 STA xpos BNE end pos3 STX ypos LDA #100 STA xpos BNE end ... That way it gets a horizontal resolution of 11 different spots and a vertical resolution of 192 or smaller depending on the sensitivity of the gun. Ciao, Eckhard Stolberg -- Archives updated once/day at http://www.biglist.com/lists/stella/archives/ Unsubscribing and other info at http://www.biglist.com/lists/stella/stella.html
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