F-18 interceptor alternating between 5 and 4 bitplanes

[Rotareneg]

I was just playing around with the visual DMA debugger and I noticed something I can't figure out: F-18 interceptor alternates between using 4 and 5 bitplanes every other rendered frame (not video frame.) Anyone know why they would do that?

4 bit planes:



5 bit planes:

[jotd]

maybe one plane is for HUD, and isn't updated every frame to save CPU time? easy to check by turning planes off and check if HUD goes away.

[Rotareneg]

The only evidence I can see for the 5th bit plane being used at all is on the title screen. Everywhere else I just get a blank screen if I isolate only 5 and no visible difference if only 1-4 are enabled. It even alternates between 4 and 5 planes with each letter as it "prints" the credit text.

[roondar]

That is very odd indeed. It would waste a whole bunch of cycles to achieve nothing... Makes me wonder what the idea was here. Mysterious!

[ross]

Originally Posted by Rotareneg

The only evidence I can see for the 5th bit plane being used at all is on the title screen. Everywhere else I just get a blank screen if I isolate only 5 and no visible difference if only 1-4 are enabled. It even alternates between 4 and 5 planes with each letter as it "prints" the credit text.

It's been a long time since I last loaded F18, and I haven't done it now, so I can be totally wrong.
I can only guess why it does it (or at least I would do it for this reason )

Let's say you have to do some blinking/shadow effects using colors that are not in the initial 16 color palette.
Since they have to flash I can activate a fifth bpl alternately every other frame, but also only when I need them.
But there is a problem: in those situations I would have a slowdown in the rendering of the graphics engine.
So to keep the FPS consistent I can constantly alternate the frames to keep the speed constant in any case.

[DanScott]

But the frame rate is so low, that the blinking would be like 4 or 5 times a second maximum... that would look awful.

I am wondering if he has a double buffered copperlist, and for some reason possibly even forgot to change one of the copperlists from setting 5 bitplanes active down to 4

[ross]

Originally Posted by DanScott

But the frame rate is so low, that the blinking would be like 4 or 5 times a second maximum... that would look awful.

Why?
The blink could be totally unrelated with the rendering... (if double buffered)

I am wondering if he has a double buffered copperlist, and for some reason possibly even forgot to change one of the copperlists from setting 5 bitplanes active down to 4

This could be too, but I wanted to hope it wasn't

[jotd]

it means that if we remove the 5th plane the game will be faster? I doubt it, but if it's the case, it would be a quickwin

[DanScott]

Originally Posted by ross

Why?
The blink could be totally unrelated with the rendering... (if double buffered)

Original poster wrote:

"alternates between using 4 and 5 bitplanes every other rendered frame (not video frame.)"

So i assume he means that the 4/5 bitplane switch does not happen at 50hz

[ross]

Originally Posted by jotd

it means that if we remove the 5th plane the game will be faster? I doubt it, but if it's the case, it would be a quickwin

If it is really unused then it can be removed .
Which goes faster depends on how the frame change / rendering code is structured.

[ross]

Originally Posted by DanScott

Original poster wrote:

"alternates between using 4 and 5 bitplanes every other rendered frame (not video frame.)"

So i assume he means that the 4/5 bitplane switch does not happen at 50hz

ahh, you are right!

Then this makes even less sense..

[Rotareneg]

The first screenshots were with some fast RAM to reduce CPU "noise" in the DMA debugger, here it is with only chip RAM:





Both shots are the same number of video frames from the last display update. In that particular scene it took 7 PAL frames to render when 4 bp were active and 8 PAL frames with 5 bp, so there is some performance hit.

On another note, it does unfortunately tie the simulation speed to the frame rate, so if you go all out with JIT fastest possible and immediate blitter it'll run super smooth but also far too quickly.

[Old_Bob]

I remember fiddling about with this a while ago. IIRC, it only behaves this way when running the intro demo sequence. In gameplay, I think it's fine.

Originally Posted by DanScott

I am wondering if he has a double buffered copperlist, and for some reason possibly even forgot to change one of the copperlists from setting 5 bitplanes active down to 4

It does switch between two lists so, this seems like the most likely explanation.

B

[Rotareneg]

It does it while playing too, the screenshots in the post above are from the free flight mode. Programmer oversight aka bug sounds good to me.

[redblade]

The palette to this game seemed dark to me, do you think they use a 512 palette instead of a 4096 palette?