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Old 29 May 2020, 17:30   #1
cbmeeks
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Question on dual playfield mode

I think I already know the answer to this question, but I can't seem to find a reference.


In dual playfield mode, you have 6 bitplanes total. Three for the background and three for the foreground.


So you have 8 colors in each layer. Well, you loose a color due to transparency.


My question is, can you arrange the 6 bitplanes differently? Can you have the foreground have 4 bitplanes and the background 2? Seems like the bandwidth would be the same but I'm guessing the hardware isn't capable of that.


Thanks!
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Old 29 May 2020, 17:39   #2
Samurai_Crow
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You are correct. You cannot rearrange to get more than 3 bitplanes on dual playfield mode except on AGA where 4 is the maximum for both.
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Old 29 May 2020, 17:39   #3
roondar
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Nope, you can't.

You can select fewer bitplanes (for instance, 5 instead of 6) and select which playfield has priority, but you can't select how the bitplanes are divided. The odd bitplanes always go in one playfield, the even ones in the other.
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Old 29 May 2020, 18:15   #4
ross
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Well, actually you can do it to some extent, but you lose several fundamental property [EDIT: because actually you are in single playfield mode with disjoint bitplanes]:

- you cannot scroll independently the two (or more) 'virtual' playfields (this too can somehow be resolved, but with terrible headaches and the response require so much effort that I give up )
- color tables for the two 'natural' playfield live happily with sprite bank colors (OCS/ECS specificity); in single playfield mode color table is linear and can cross sprite colors bank
- your generic x+y playfields must manage the colors to avoid clashes due to how the selection occurs for the different bplanes

Having said that, and remembering that the blitter is your friend, various transparency/overlay effects are possible (this is where the Amiga shine for its bitplanes architecture)

Last edited by ross; 29 May 2020 at 18:26.
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Old 29 May 2020, 18:19   #5
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Indeed Ross is correct.

In Rygar i split the playfields to have a 5 planes in front and 3 in back. It came at a cost though.
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Old 29 May 2020, 20:00   #6
roondar
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I'm going to do something now I don't normally do... I'm going to (kind of) disagree with ross & mcgeezer

So here's my position:
  • Yes, you can use bitplane graphics and cleverly set up palettes to generate a display which creates an end result that looks like Dual Playfield with a different split in bitplanes than 3/3 or 4/4.
  • No, these clever software splits are not Dual Playfield mode. Dual Playfield mode is a very specific hardware feature that is not being used in these splits.
Doesn't mean they're not very cool (used them myself), or that they're not useful (they can be very useful), but IMHO they're not the same as Dual Playfield mode. Which is to say: if it ain't got the Dual Playfield bit set, it ain't DPF for me

Now this might seem overly pedantic, but all I'm really trying to do is keep clear what's what ... So don't worry ross/mcgeezer, I still love you guys, you be my Amiga coding heros


Edit: so I kind of misread ross's post, he's not calling those different splits Dual Playfield mode. Which means mcgeezer isn't do this either. So, err... This post of mine here is not an accurate portrayal of what they said. But I still think it contributes to our "common coding knowledge", so I'm not going to alter it.

Instead, I'll just point out that he does call it single playfield (which it is).
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Old 29 May 2020, 21:11   #7
ross
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Eheheh, so in the end we don't disagree
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Old 29 May 2020, 21:13   #8
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Yup. We agree to agree
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