How did Amiga sprites work?

[Foebane]

I'm going by the supposition that Jay Miner based his Amiga sprite hardware on the Atari 8-Bit series of computers, but more advanced, of course. So these sprites would've been the full height of the screen, were 16 colours and 16 pixels wide for OCS/ECS.

However, I don't know if sprites could be combined or not or how they were manipulated vertically (was a nightmare for players and missiles on A8, that's for sure!)

Other things are: what palette did the sprites use? From my own experience it seems to be the main colour palette for the bitplane graphics. And what about AGA sprites? I heard they were 64 pixels wide, but that sounds nuts! And how many colours?

This is the thing that I'm lacking info on about Amiga hardware, but then it's hard to tell which are the sprites and which are bobs or blitter objects.

Are there any demos that specifically show off sprites on Amiga, in both OCS/ECS and AGA?

[DanScott]

There are 8 hardware sprites

Each is 16 pixels wide, by unlimited height, in 4 colours (3+transparent)

Sprites 1&2 take their palette from Colours 16,17,18,19 (16 is ignored.. transparent)
Sprites 3&4 take their palette from Colours 20,21,22,23 (20 is ignored.. transparent)
Sprites 5&6 take their palette from Colours 24,25,26,27 (24 is ignored.. transparent)
Sprites 7&8 take their palette from Colours 28,29,30,31 (16 is ignored.. transparent)

2 sprites can be combined, by setting their positions to the same place, and setting the "attach" bit in the control word. This gives you a 16 colour sprite that uses from colours 16 to 31 (although again, 16 is treated as transparent)

If you search on google for "Amiga Hardware Sprites", you will easily find the Hardware Reference Manual official information on how they all work

[DanScott]

Here you go

http://amigadev.elowar.com/read/ADCD.../node00AE.html


There are of course some tricks you can do, using the copper (for example) to write new X positions, and data for a sprite, thereby reusing it across the screen.

Codetappers webpage explains how this was done in a selection of games:

http://www.codetapper.com/amiga/sprite-tricks/

[Toni Wilen]

Most AGA only sprite features:

- AGA sprites can be 32 pixel or 64 pixel wide (all sprites share same size).
- AGA sprites also support hires and superhires resolutions (all sprites share same resolution).
- Sprites can be visible in borders (=no bitplanes active). Also possible in OCS/ECS with BPL1DAT copper hack, with some restrictions.
- Sprite palette start offset can be moved (from OCS/ECS only offset=16) in 16 color steps. (0, 16, 32, 48, .. , 240). Separate setting for odd and even sprites.
- Sprite can be vertically doubled (Sprite scan doubling)
- Sprite can appear twice in same horizontal scanline without using copper tricks. (Side-effect of a bit hacky scan doubling implementation, very limited but can be used for some background sprite effects)

All AGA sprite features are 100% independent from bitplane resolution.

[Foebane]

Originally Posted by Toni Wilen

- Sprite can be vertically doubled (Sprite scan doubling)

As in interlace graphics modes? I always wondered about that.

All AGA sprite features are 100% independent from bitplane resolution.

I figured that already, noticing a big lo-res mouse pointer in hi-res graphics modes in WB back in the day on my A1200. But I didn't know both sprites and regular graphics could achieve the same resolutions.

If Amiga sprites have vertical positions as well as horizontal positions, then how is that possible when the sprites are capable of being as much as 256 pixels vertical (the height of the screen) and just 16 bits wide?

And if they're NOT capable of that height, then what's the deal with those glitchy graphical corruptions that sometimes span the top and bottom of the screen, usually loading demos?

[Toni Wilen]

Originally Posted by Foebane

As in interlace graphics modes? I always wondered about that.

It was meant for doublescanned modes where each bitplane line was doubled for VGA monitor compatibility. (WB monitor drivers like "Multiscan", "DBLPAL", "DBLNTSC")

If Amiga sprites have vertical positions as well as horizontal positions, then how is that possible when the sprites are capable of being as much as 256 pixels vertical (the height of the screen) and just 16 bits wide?

Height is "unlimited".

Each sprite has only 2 available DMA slots/scanline. Single slot can transfer 16 bits from chip RAM (OCS/ECS). AGA has 32-bit data bus and can do 2 data transfers/DMA cycle which enables 32 pixel/64 pixel wide sprites. (EDIT: 2 slots because single sprite is made from 2 planes = 4 colors)

And if they're NOT capable of that height, then what's the deal with those glitchy graphical corruptions that sometimes span the top and bottom of the screen, usually loading demos?

You mean random, corrupted 16-pixel wide sprites with random height appearing and disappearing? Usual cause is enabled sprite DMA (or more like someone forgot to disable sprite DMA..) without setting valid sprite pointers.

[sandruzzo]

Originally Posted by Toni Wilen

Most AGA only sprite features:

- AGA sprites can be 32 pixel or 64 pixel wide (all sprites share same size).
- AGA sprites also support hires and superhires resolutions (all sprites share same resolution).
- Sprites can be visible in borders (=no bitplanes active). Also possible in OCS/ECS with BPL1DAT copper hack, with some restrictions.
- Sprite palette start offset can be moved (from OCS/ECS only offset=16) in 16 color steps. (0, 16, 32, 48, .. , 240). Separate setting for odd and even sprites.
- Sprite can be vertically doubled (Sprite scan doubling)
- Sprite can appear twice in same horizontal scanline without using copper tricks. (Side-effect of a bit hacky scan doubling implementation, very limited but can be used for some background sprite effects)

All AGA sprite features are 100% independent from bitplane resolution.

I've heard many time about scan doubling, ho it can be used to boost if it's possible aga capability?

[Michael Parent]

Do you know if there's any possible copper type trick to repeat sprites across the entire screen like with ECS Amiga's (aside from the ability to clone it once at 256 pixels farther to the right)? Without that ability, in a scrolling game, creating a back layer going across the entire screen is impossible, as there would be a 64 pixel gap between each repeat of the sprite pattern.

Also, I have a vague memory about sprites being set wider at by making its pixels twice as wide. Am I confused, was that for the c64? When you set all sprites to 64 pixels wide, do you still have 8 per scan line and are their pixels not horizontally doubled?

Originally Posted by Toni Wilen

Most AGA only sprite features:

- AGA sprites can be 32 pixel or 64 pixel wide (all sprites share same size).
- AGA sprites also support hires and superhires resolutions (all sprites share same resolution).
- Sprites can be visible in borders (=no bitplanes active). Also possible in OCS/ECS with BPL1DAT copper hack, with some restrictions.
- Sprite palette start offset can be moved (from OCS/ECS only offset=16) in 16 color steps. (0, 16, 32, 48, .. , 240). Separate setting for odd and even sprites.
- Sprite can be vertically doubled (Sprite scan doubling)
- Sprite can appear twice in same horizontal scanline without using copper tricks. (Side-effect of a bit hacky scan doubling implementation, very limited but can be used for some background sprite effects)

All AGA sprite features are 100% independent from bitplane resolution.

[Samurai_Crow]

Originally Posted by Michael Parent

Do you know if there's any possible copper type trick to repeat sprites across the entire screen like with ECS Amiga's (aside from the ability to clone it once at 256 pixels farther to the right)? Without that ability, in a scrolling game, creating a back layer going across the entire screen is impossible, as there would be a 64 pixel gap between each repeat of the sprite pattern.

Also, I have a vague memory about sprites being set wider at by making its pixels twice as wide. Am I confused, was that for the c64? When you set all sprites to 64 pixels wide, do you still have 8 per scan line and are their pixels not horizontally doubled?

Rearming the sprites during the display portion of the screen for horizontal multiplexing takes enough time out of the Copper list that the sprites cannot be tiled side by side. Reusing the image data so it repeats every 4 sprites is cheaper and can be done.

BTW, good to hear from you again!

[Michael Parent]

Hi Samual_Crow. It's great to reconnect with you too. I'm not sure I understand your answer. Are you verifying AGA can not ever tile 64 pixel wide 16 color sprites across the screen and also have horizontal screen scrolling?

Do we lose a sprite if we want hardware scrolling, which would leave the possibility for three 16 color sprites (X64 pixels=192) and one 3 color sprite (X64 pixels) which is a total of 256 pixels (4 sprites) so theoretically, for a full screen back "playfield" created with sprites, it would have to be made entirely out of 3 color sprites or 3 16 color sprites with 1 3 color sprite for one 64 pixel column?

Another question: Do Risky Woods and Jim power run properly on AGA Amiga's? If so, how?

Thanks for your time and knowledge

Originally Posted by Samurai_Crow

Rearming the sprites during the display portion of the screen for horizontal multiplexing takes enough time out of the Copper list that the sprites cannot be tiled side by side. Reusing the image data so it repeats every 4 sprites is cheaper and can be done.

BTW, good to hear from you again!

[Samurai_Crow]

AGA can but ECS only reuses the sprite image data because there is not time for the Copper to reload new image data and also move the sprite in 16 pixels' time.

On AGA it is another story. The manual mode of feeding data into the sprite with the Copper can't happen but sprites can be duplicated horizontally by abusing the scan double circuit to make each sprite appear twice on the same row of pixels. As such, three page-fetched sprites can cover the screen width if they're low resolution. (Page fetch mode is the one with 64 pixel wide sprites.)

[Daedalus]

Originally Posted by Michael Parent

Also, I have a vague memory about sprites being set wider at by making its pixels twice as wide. Am I confused, was that for the c64?

I can't remember if the C64 had that as well, but there was definitely a sprite pixel width setting on the Atari 8-bits, where you defined a sprite as being x1, x2 or x4 width, doubling or quadrupling the pixels in the sprite. It was used to good? effect in Frogger and the likes.

When you set all sprites to 64 pixels wide, do you still have 8 per scan line and are their pixels not horizontally doubled?

No, the pixels are still 1:1 at 64 pixels wide, and you still have eight sprites, but from memory I think the display width has to be reduced to be able to use them all at once.

[Samurai_Crow]

Originally Posted by Daedalus

I can't remember if the C64 had that as well, but there was definitely a sprite pixel width setting on the Atari 8-bits, where you defined a sprite as being x1, x2 or x4 width, doubling or quadrupling the pixels in the sprite. It was used to good? effect in Frogger and the likes.

No, the pixels are still 1:1 at 64 pixels wide, and you still have eight sprites, but from memory I think the display width has to be reduced to be able to use them all at once.

C64 had 2 modes for sprites high resolution and multicolor. The multicolor mode had half the resolution. Also you could double the pixel dimensions horizontally or vertically in each mode.

The only reason you need to narrow the display on AGA page mode was for the scroll offsets of the playfields. Sprites could globally be super high resolution, high resolution or low resolution. This was a pixel doubler or quadrupler relative to the AGA 1280x200 super high resolution mode.

[Daedalus]

Originally Posted by Samurai_Crow

The only reason you need to narrow the display on AGA page mode was for the scroll offsets of the playfields.

Hmmm, that must be what I'm remembering so.

Sprites could globally be super high resolution, high resolution or low resolution. This was a pixel doubler or quadrupler relative to the AGA 1280x200 super high resolution mode.

Yes, true - that's multiplying the width, but that's independent of the width of the sprite image data, which is still 16, 32 or 64 pixels.