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Why no SNES homebrew scene?
http://forums.nesdev.com/viewtopic.php?f=12&t=10957
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Author:  tepples [ Fri Aug 25, 2017 12:11 pm ]
Post subject:  Re: Why no SNES homebrew scene?

Haunted: Halloween '85 for NES does something similar with its sprite CHR. It emulates a sprite system with 256 16x16 CHRs, uploading two per frame to one of two buffers allocated to each entity and switching to the next animation frame only once all needed 16x16s are in place. On Super NES, it should be easier, as you don't need two buffers if no single cel needs more than 5K.

But you still need someone to draw plausible sprite cels with which to test a sprite system including this sort of OBJ-ExGrafix simulator, so that you don't end up over-relying on peculiarities of your synthetic test tiles.

Author:  psycopathicteen [ Fri Aug 25, 2017 12:44 pm ]
Post subject:  Re: Why no SNES homebrew scene?

Or you can just make the code not peculiar.

Author:  tepples [ Fri Aug 25, 2017 1:15 pm ]
Post subject:  Re: Why no SNES homebrew scene?

You might not know your architecture is peculiar until you plug in real data and discover that it's not as flexible or efficient as you thought it'd be. It doesn't have to be final data, just representative data.

Author:  psycopathicteen [ Fri Aug 25, 2017 2:18 pm ]
Post subject:  Re: Why no SNES homebrew scene?

Well, I guess I would have to come up with a separate version for 8x8 & 16x16 sprites, and 16x16 & 32x32 sprites.

EDIT:
I'm guessing that using 16384 16x16 would be limiting to games larger than 16-megabits. So you can fix that problem with extra SRAM, because you need a table to store the VRAM location of every sprite.

If you want to use a lot of SRAM, then lorom is better.

Author:  psycopathicteen [ Sat Aug 26, 2017 8:33 pm ]
Post subject:  Re: Why no SNES homebrew scene?

Okay, I just wrote an example code for a simple "convert arcade-style sprite attributes to SNES-style sprite attributes" system. I wonder where Espozo is, he might be interested in this.

Code:
// "CHR" is the name for the table of 14-bit (or 15-bit) ROM CHR numbers for 128 sprites.
// The routine converts the 14/15-bit CHR values to the SNES's native 9-bit CHR values.
// "CHR_allocation" is a table of where each 14/15-bit CHR is mapped to what VRAM slot.
// Each slot has a hardcoded 9-bit CHR value
// "slot_CHR" says what 14/15-bit CHR value each slot cooresponds to.
// "active_slot" flags if the slot is being used or not.  If $ffff, then the slot needs to be DMA'd.
// "sprite size" table declares if a sprite is 16x16 or 32x32

manage_VRAM:
ldx #$007e
-;            //clear active slot list
stz {active_slot},x
dex #2
bmi -

ldy #$00fe
-;            //check all active slots
ldx {CHR},y
lda {CHR_allocation},x
and #$00ff
tax
inc {active_slot},x
dey #2
bmi -

ldy #$00fe
-;            //this loop allocates missing sprites and converts CHR values
ldx {CHR},y
lda {CHR_allocation},x
and #$00ff
bne +
jsr allocate_CHR      //allocate missing sprites
+;
tax
lda VRAM_slots,x
sta {CHR},y         //replace 14-bit (or 15-bit?) ROM CHR value with
dey #2            //SNES's native 9-bit CHR value
bmi -
rts

allocate_CHR:
phy
lda {sprite_size},y
bne allocate_large_sprite
ldy #$0040         //small sprites are allocated in slots 32-63
-;
lda {active_slot},y
beq +            //open slots have "active_slot" as #0
iny #2
cpy #$0080
bne -
ldy #$0000         //VRAM full error gets NULL slot number
bra +

allocate_large_sprite:
ldy #$0010         //large sprites are allocated in slots 8-31
-;
lda {active_slot},y
beq +            //open slots have "active_slot" as #0
iny #2
cpy #$0040
bne -
ldy #$0000         //VRAM full error gets NULL slot number

+;
lda #$ffff
sta {active_slot},y      //#$ffff marks slots to DMA
tya
ora {CHR_allocation},x
sta {CHR_allocation},x      //table entry is expected to have a low byte of #0
txa
ldx {slot_CHR},y
sta {slot_CHR},y
sep #$20
stz {CHR_allocation},x
rep #$20
tya
ply
rts

VRAM_slots:
dw $ffff,$ffff,$ffff,$ffff,$ffff,$ffff,$ffff,$ffff   //NULL slots (only 0 is used)
dw $0000,$0004,$0008,$000c,$0040,$0044,$0048,$004c   //32x32 slots
dw $0080,$0084,$0088,$008c,$00c0,$00c4,$00c8,$00cc
dw $0100,$0104,$0108,$010c,$0140,$0144,$0148,$014c
dw $0180,$0182,$0184,$0186,$0188,$018a,$018c,$018e   //16x16 slots
dw $01a0,$01a2,$01a4,$01a6,$01a8,$01aa,$01ac,$01ae
dw $01c0,$01c2,$01c4,$01c6,$01c8,$01ca,$01cc,$01ce
dw $01e0,$01e2,$01e4,$01e6,$01e8,$01ea,$01ec,$01ee

Author:  Espozo [ Sun Aug 27, 2017 8:16 pm ]
Post subject:  Re: Why no SNES homebrew scene?

Don't worry, I'm still alive. :lol: Could you give a summary of what this code does? I read the description at the top, and glanced at the code, but I don't understand what is going on. It sounds like you want to somehow increase the CHR size of OAM to 15 bits instead of 9, but this is a hardcoded limitation. The end of your comment says something about 16x16 and 32x32 sized slots; is this related to the system we're doing of dynamically looking for space in VRAM for sprite graphics?

Author:  psycopathicteen [ Mon Nov 13, 2017 12:20 pm ]
Post subject:  Re: Why no SNES homebrew scene?

I thought more about this idea, and I thought that it might be easier to do 16x16 sprites in groups of 2, for a lot of reasons:

-half of the SNES's memory space can be used for sprites.
-32x32 and 16x16 sprites would have the same alignment.
-All sprite pairs have a page aligned address.
-DMA routine can be made more efficient. Possibly around 5kB at once without bars.

The downsides are slightly less flexible 16x16 sprites, and doing 48x48 rotating sprites would have a much more confusing mapping.

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