Subject: Re: [stella] need 16bit math for JoustPong? From: "Andrew Davie" <atari2600@xxxxxxxxxxxxx> Date: Thu, 29 Jan 2004 11:42:05 +1100 
Let's do this with just the Y coordinate. Assume you have one byte Y now, and you add or subtract 1 to it. If you add or subtract 1 per frame, then it moves one scanline per frame. Let's convert that to the exact 16bit equivalent... clc lda YCoord ; low byte of 16bit Y coordinate (fractional byte) adc #0 ; yeah, this isn't really doing anything... but here to show the process sta YCoord lda YCoord+1 adc #1 ; add 1 pixel to integer byte of 16bit x coordinate sta YCoord+1 OK, that's just added "1 pixel" to a 16bit coordinate which is conceptually represented as 8.8 format. That is, 8 bits of integer and 8 bits of fractional. The position of the decimal point is entirely up to us, the programmer. When we actually display the object, we use only the integer portion for its display position. SO in this case, we always use the variable "YCoord+1" Now that example showed moving 1 pixel down. How would we move 1/10th of a pixel down? Well, remember 1 pixel is 1.0 which is represented in our twobyte system as 1 for the high byte and 0 for the low byte. So what's 1/10th of this? Let's use decimal, then convert back. 1 * 256 = 256. 1/10th of this is 25.6, say 26. So if we use 0 for the high byte and 26 for the low byte (decimal) then we have roughly 1/10th of a pixel. clc lda YCoord ; low byte of 16bit Y coordinate (fractional byte) adc #26 ;OK, now this is doing something. Add 1/10th of a pixel sta YCoord lda YCoord+1 adc #0 ; caters for carry/overflow from the low fractional byte sta YCoord+1 And that's how fractional movement is done. Now let's make it general purpose. Rather than adding actual hardwired values, let's use another twobyte varable in the same format to represent the movement speed... clc lda YCoord ; low byte of 16bit Y coordinate (fractional byte) adc YSpeed ; low byte of 16bit Y movement speed sta YCoord lda YCoord+1 adc YSpeed+1 ; high byte of 16bit Y movement speed sta YCoord+1 So if the YSpeed was, say, 1/10th of a pixel... lda #26 sta YSpeed lda #0 sta YSpeed+1 then our object would work nicely. To speed it up, we modify the value in YSpeed. To slow it down, likewise. In fact, since we're doing normal binary two's complement addition here, if the YSpeed value became less than 0, instead of increasing the Y coordinate, we'd end up actually decreasing it. And this is the trick to the full 16bit movement code. Just remember you're using signed numbers here, so it's just as easy to have YSpeed hold minus 1/10th of a pixel as it is to have it hold plus 1/10th of a pixel. So the final code is... clc lda YCoord ; low byte of 16bit Y coordinate (fractional byte) adc YSpeed ; low byte of 16bit Y movement speed sta YCoord lda YCoord+1 adc YSpeed+1 ; high byte of 16bit Y movement speed sta YCoord+1 That's it! Just make sure you have the correct movement speed in YSpeed (two bytes) positive or negative... and bob's your uncle :) Cheers A  Original Message  From: <KirkIsrael@xxxxxxxxxxxxx> To: <stella@xxxxxxxxxxx> Sent: Thursday, January 29, 2004 11:07 AM Subject: [stella] need 16bit math for JoustPong? > > So if people were saying JoustPong could benefit mightily from > fractional position and/or speed...I guess I'd need 16 bit math > for all that? For a bit I thought about using a single byte, but > it probably donesn't have enough precision...is there a guide to > doing 16 bit addition/subtraction anywhere? it seems trickier than I > first guessed it would be... >  > KirkIsrael@xxxxxxxxxxxxx http://kisrael.com > Indeed, the Russians' predisposition for quiet reflection followed by > sudden preventive action explains why they led the field for many > years in both chess and ax murders. Marshall Brickman, Playboy 4/73 > > >   > Archives (includes files) at http://www.biglist.com/lists/stella/archives/ > Unsub & more at http://www.biglist.com/lists/stella/ > >  Archives (includes files) at http://www.biglist.com/lists/stella/archives/ Unsub & more at http://www.biglist.com/lists/stella/
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[stella] need 16bit math for Joust, KirkIsrael  Thread  Re: [stella] need 16bit math for J, Eric Ball 
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