[stella] Studying River Raid Source.

[Thom Cherryhomes]

why is it done this way?

-Thom


On 3/16/07, SeaGtGruff at aol.com <SeaGtGruff at aol.com> wrote:
>
>  In a message dated 3/15/2007 9:13:48 PM Eastern Daylight Time,
> thom.cherryhomes at gmail.com writes:
>
> I am in the middle of studying the River Raid source code, and I am
> wondering on something maybe relatively simple, but I do have to
> ask....
>
> I see the following in the graphic segment data:
>
> ...
>
> ; the data is stored for interlaced display:
> FuelA:
>        .byte $FE ; |XXXXXXX |
>        .byte $DE ; |XX XXXX |
>        .byte $DE ; |XX XXXX |
>
> ...
>
> what is meant by "interlaced display" in this case? I know what an
> interlaced picture is, but i'm not sure of the details for what is
> being done in this context?
>
> -Thom
>
>  I've never looked at this disassembly before. Wow! :)
>
> If you look carefully at the data for FuelA and FuelB, you'll notice that
> they must be combined in order to get the complete image (I added the
> commented numbers to the side):
>
> ; the data is stored for interlaced display:
> FuelA:
>        .byte $FE ; |XXXXXXX | ; A01
>        .byte $DE ; |XX XXXX | ; A02
>        .byte $DE ; |XX XXXX | ; A03
>        .byte $FE ; |XXXXXXX | ; A04
>        .byte $DE ; |XX XXXX | ; A05
>        .byte $DE ; |XX XXXX | ; A06
>        .byte $FE ; |XXXXXXX | ; A07
>        .byte $D6 ; |XX X XX | ; A08
>        .byte $D6 ; |XX X XX | ; A09
>        .byte $DE ; |XX XXXX | ; A10
>        .byte $CE ; |XX  XXX | ; A11
> FuelB:
>        .byte $C6 ; |XX   XX | ; B01
>        .byte $DE ; |XX XXXX | ; B02
>        .byte $DE ; |XX XXXX | ; B03
>        .byte $C6 ; |XX   XX | ; B04
>        .byte $CE ; |XX  XXX | ; B05
>        .byte $C6 ; |XX   XX | ; B06
>        .byte $C6 ; |XX   XX | ; B07
>        .byte $D6 ; |XX X XX | ; B08
>        .byte $FE ; |XXXXXXX | ; B09
>        .byte $DE ; |XX XXXX | ; B10
>        .byte $DE ; |XX XXXX | ; B11
>        .byte $7C ; | XXXXX  | ; B12
>
> And here's a blowup of the actual image to help us understand:
>
>
> By examining the image and the data-- and noticing that FuelB has one more
> line of data than FuelA does-- we see that the image is drawn as follows:
>
>        .byte $7C ; | XXXXX  | ; B12
>        .byte $C6 ; |XX   XX | ; B01 <-- Think of this as A12 :)
>        .byte $DE ; |XX XXXX | ; B11
>        .byte $CE ; |XX  XXX | ; A11
>        .byte $DE ; |XX XXXX | ; B10
>        .byte $DE ; |XX XXXX | ; A10
>        .byte $FE ; |XXXXXXX | ; B09
>        .byte $D6 ; |XX X XX | ; A09
>        .byte $D6 ; |XX X XX | ; B08
>        .byte $D6 ; |XX X XX | ; A08
>        .byte $C6 ; |XX   XX | ; B07
>        .byte $FE ; |XXXXXXX | ; A07
>        .byte $C6 ; |XX   XX | ; B06
>        .byte $DE ; |XX XXXX | ; A06
>        .byte $CE ; |XX  XXX | ; B05
>        .byte $DE ; |XX XXXX | ; A05
>        .byte $C6 ; |XX   XX | ; B04
>        .byte $FE ; |XXXXXXX | ; A04
>        .byte $DE ; |XX XXXX | ; B03
>        .byte $DE ; |XX XXXX | ; A03
>        .byte $DE ; |XX XXXX | ; B02
>        .byte $DE ; |XX XXXX | ; A02
>        .byte $C6 ; |XX   XX | ; B01
>        .byte $FE ; |XXXXXXX | ; A01
>
> So in this case, "interlaced" means the data in table FuelA is interleaved
> with the data in table FuelB to get the complete image. It has nothing to do
> with interlacing (or interleaving) two fields to get a complete frame.
>
> Michael
>
>
>
>
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