Sprites

SNES programming tutorial. Example 5.

https://github.com/nesdoug/SNES_05

 

Sprites are the graphic objects that can move around the screen. Nearly all characters are made of sprites… Mario, Link, Megaman, etc. The OAM RAM controls how each sprites appear.

Mario

You will notice that Mario is made of 2 16×16 sprites. It is common to use more than 1 sprite for a character. Rex is also made of 2 16×16 sprites, with the lower sprite several pixels to the right of the top one. You can also layer sprites on top of each other, but with 15 colors to choose from, you shouldn’t have to.

You could increase the large sprite size to 32×32, but that would end up wasting more VRAM space on blank spaces. 8×8 and 16×16 are more common. I call it a “metasprite” when it is a collection of multiple sprites to make up 1 character. The SPEZ sprite editor I wrote saves these as tables of numbers (metasprite/save options). And the tiles save as 4bpp CHR files.

https://github.com/nesdoug/SPEZ

I actually used SPEZ to draw the example sprites, but you may choose to use YY-CHR or some other tile editor. But let’s go over how sprites work.

spez

 

OAM

The official docs call sprites “objects”. You need to write data to the OAM RAM to get them to show up on screen.

There are 2 tables in the OAM, and you need to write both of them, usually a DMA during v-blank or forced blank. (v-blank is the vertical blank period, the slight pause after each frame is drawn to the TV where the PPU isn’t doing anything, and can be updated for the next frame).

Low Table

The low table (512 bytes) is divided into 4 bytes per Sprite, with sprite #0 using bytes 0,1,2,3 and sprite , #1 using bytes 4,5,6,7, etc… up to sprite #127. 4 x 128 = 512 bytes.
Those bytes are, in this order…
x, y, tile #, attributes.
X and Y are screen relative, in pixels (for the top left of the sprite).

Attributes

vhoopppN
v vertical flip
h horizontal flip
oo priority
ppp palette
N 1st or 2nd set of tiles (you can have up to 512 tiles for sprites).

The High Table

There are 32 bytes in the high table for 128 sprites. That’s 2 bits per sprite, and it can be very tedious to manage. Lots of bit shifting. The bits are

sx (s upper bit, x lower bit)
s= size (small or large)
x = 9th bit for x

The extra X bit is so you can smoothly move a sprite off the left side of the screen. With that bit set and the regular X set to $ff, that would be like -1. Whereas, without the extra X bit, $ff would be the far right of the screen, with only 1 pixel wide showing.

How are the 2 bits put together?
Let’s say,
Sprite 0 = aa
Sprite 1 = bb
Sprite 2 = cc
Sprite 3 = dd
The the first byte of the high table is
ddccbbaa
or (dd << 6) + (cc << 4) + (bb << 2) + aa

Palettes

Sprites use the second half of the CGRAM (palette). It is 15 colors + transparency for each palette. Sprite palette #0 uses indexes 128-143. Sprite palette #1 uses indexes 144-159. And so forth.

Priorities

I like to set sprite priority to 2. That would be in front of bg layers (but behind layer 3 if it’s set as super-priority in front of everything). Higher sprite priority would be in front of sprites with lower priority.

Besides priorities…Low index sprites will go in front of higher index ones. Sprite #0 would be in front of Sprite #1. Sprite #1 would be in front of Sprite #2. Sprite #2 would be in front of Sprite #3. Etc.

There is a limit to how many sprites can fit on a horizontal line. And using larger sprites doesn’t improve that, internally it splits sprites up into 8×1 slivers, and only 32 slivers can fit on a line. The 33rd one disappears. Because of this, you could shuffle the sprites every frame. That’s a lot of sprites, so I see most games just ignore this problem, and try not to put too many sprites on each line. Space shooter games (lots of sprites on screen at once) re-order the sprites in the OAM manually every frame. Some kind of shuffling algorithm, to make sure no bullets hit you that you couldn’t see.

Caution. Don’t put sprites at X position 0x100. (with the 9th bit 1 and the regular X at 00) They will be off screen, but will somehow count towards the 32 sprites per line limit.

Clearing Sprites

If you leave the OAM zeroed, it will display sprites at X=0, Y=0, Tile=0, palette=0… and the top left of the screen would have 128 sprites on top of each other. If you just want ALL sprites off screen, you could just turn them off from the main screen ($212c). But to put an individual sprite off screen, you should put its Y value at 224 (assuming screens are left to the default 224 pixel height). This would put 8×8,16×16, and 32×32 sprites off screen, but 64×64 sprites would wrap around to the top of the screen… so it’s a good idea to also reset the sprite size bit to 0.
.

Let’s go over the code.

Code

We need to change a few settings, first.
$2101 sets the sprite size and the location of the sprite tiles.
sssnnbbb
sss = size mode*
nn = offset for 2nd set of sprite tiles. leave it at zero, standard.
bbb = base address for the sprite tiles.
Again, the upper bit is useless. So, each b is a step of $2000.

* size modes are

000 = 8×8 and 16×16 sprites
001 = 8×8 and 32×32 sprites
010 = 8×8 and 64×64 sprites
011 = 16×16 and 32×32 sprites
100 = 16×16 and 64×64 sprites
101 = 32×32 and 64×64 sprites

.

lda #2
sta $2101 ; sprite tiles at VRAM $4000, sizes are 8×8 and 16×16

And we need to make sure sprites show up on the main screen.

lda #$10
sta $212c ; main screen

https://wiki.superfamicom.org/sprites

https://wiki.superfamicom.org/registers

From here on out, I am going to use BUFFERS. Buffers are temporary locations in local RAM that will be copied (DMA) each frame to the actual memory (the OAM RAM)… during the v-blank period. Well, next time we will do that. In this example, we are doing it once during forced blank (2100 bit 7 set), which is also fine.

We are using a block move macro to copy from the ROM to the BUFFER.

BLOCK_MOVE 12, Sprites, OAM_BUFFER

to set up a MVN operation (to copy a block of data from the ROM to the RAM). See macros.asm for details.

And I’m writing just one byte to the high table. We only need 3 sprites in this example, so we will only need 2×3=6 bits, setting the size of each to large (16×16).

lda #$2A ;= 00101010
sta OAM_BUFFER2

Now I will DMA both tables at once. A DMA to the OAM looks like this…

; DMA from OAM_BUFFER to the OAM RAM
ldx #$0000
stx oam_addr_L ;$2102 (and 2103)

stz $4300 ; transfer mode 0 = 1 register write once
lda #4 ;$2104 oam data
sta $4301 ; destination, oam data
ldx #.loword(OAM_BUFFER)
stx $4302 ; source
lda #^OAM_BUFFER
sta $4304 ; bank
ldx #544
stx $4305 ; length
lda #1
sta $420b ; start dma, channel 0

That’s 544 bytes being copied to the $2104 (OAM DATA register) after we zeroed the OAM address registers ($2102-3). We need to write the whole thing. I recommend always writing to the OAM with a 544 byte DMA. Even if you don’t want to set any of the high table bits, write both tables every time. The first demo I made didn’t work on a real SNES because I failed to write to the high table. DMA all 544 bytes to the OAM, and you won’t have problems.

The data we are transferring looks like this…

Sprites:
;4 bytes per sprite = x, y, tile #, attribute
.byte $80, $80, $00, SPR_PRIOR_2
.byte $80, $90, $20, SPR_PRIOR_2
.byte $7c, $90, $22, SPR_PRIOR_2

With the top left sprite at x = $80 and y = $80. We are using tiles 00,20,22, and all of the sprites use palette #0 and priority #2 (above BG layers).

And this is what it looks like.

example5

Try drawing your own sprite, and getting it to show up on screen.

 

SNES main page

 

 

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