The Plus ASIC hardware sprite format is different to the standard CPC sprite format, so sprites have to be converted if you want to use them as hardware sprites on the Plus.
The formats
Plus hardware sprites are 16x16 pixels in size and have 16 colours, with PEN 0 always being transparent. Therefore, when converting standard CPC sprites it is easiest to convert them from MODE 0 because this mode supports a colour depth of 16 and two pixels are stored in one byte.
The pixel data in MODE 0 on the standard CPC is stored in an awkward manner and a byte is organised as follows:
The top row shows the two pixels and the bottom row shows how they are arranged in one byte on the standard CPC.
The pixel data on the Plus is organised in a straight forward manner being that each byte is a pixel and its value is the PEN colour. For example, the first two lines of a Plus sprite could be:
<geshi lang=Z80> 00,01,02,03,04,05,06,07,08,09,10,11,12,13,14,15 01,02,03,04,05,06,07,08,09,10,11,12,13,14,15,00 </geshi>
Conversion
To convert a sprite, you can use the converter utility on the DSK image (to load type RUN”DISC”) which can be downloaded below.
To use the converter utility, you must have first capture your standard CPC sprite (you can use the SPRITER utility on the disc) which must be 16 x 16 in size (8 bytes wide by 16 lines high). The converter utility will then display the sprites and convert them before saving the converted sprite to the disk.
Download
Media:Convspr.dsk - DSK image containing the converter utility
Source code
The source code for the converter (which will assemble in WinApe and Maxam) is as follows:
<geshi lang=Z80>
- Convert CPC sprite to Plus sprite
- by Redbox, February 2010
- Public Domain
org &8000
ld b,16 ;loop counter, sprite is 16 lines high
mainloop: push bc ;preserve loop counter
call doline ld hl,(scraddr) ;load HL with screen address call nextline ;work out next screen line down ld (scraddr),hl ;and store the screen address
pop bc ;restore loop counter djnz mainloop ;decrease B and jump back to loop if not 0
ret ;exit
doline: ld b,8 ;loop counter, sprite is 8 bytes wide = 16 pixels
lineloop: push bc ;store counter
ld hl,(scraddr) ;get screen location ld a,(hl) ;and load data from it into A
bit 7,a ;does bit equal 1? if so, reset Z ld b,&1 ;pixel 0, bit 0 = &1 call nz,addbit ;add the bit to our pixel
bit 3,a ;does bit equal 1? if so, reset Z ld b,&2 ;pixel 0, bit 1 (%0010) = &2 call nz,addbit ;add the bit to our pixel
bit 5,a ;does bit equal 1? if so, reset Z ld b,&4 ;pixel 0, bit 2 (%0100) = &4 call nz,addbit ;add the bit to our pixel
bit 1,a ;does bit equal 1? if so, reset Z ld b,&8 ;pixel 0, bit 3 (%1000) = &8 call nz,addbit ;add the bit to our pixel
call storeit ;store pixel at sprite address
ld a,&0 ;reset pixel ld (pixel),a ;and store reset
ld hl,(scraddr) ;get data from current screen location ld a,(hl) ;and load it into A
bit 6,a ;does bit equal 1? if so, reset Z ld b,&1 ;pixel 1, bit 0 (%0001) = &1 call nz,addbit ;add the bit to our pixel
bit 2,a ;does bit equal 1? if so, reset Z ld b,&2 ;pixel 1, bit 1 (%0010) = &2 call nz,addbit ;add the bit to our pixel
bit 4,a ;does bit equal 1? if so, reset Z ld b,&4 ;pixel 1, bit 2 (%0100) = &4 call nz,addbit ;add the bit to our pixel
bit 0,a ;does bit equal 1? if so, reset Z ld b,&8 ;pixel 1, bit 3 (%1000) = &8 call nz,addbit ;add the bit to our pixel
call storeit ;store pixel at sprite address
ld a,&0 ;reset pixel ld (pixel),a ;and store reset
ld hl,(scraddr) ;get screen address inc hl ;increase it ld (scraddr),hl ;store it for next time
pop bc ;restore loop counter djnz lineloop ;decrease B and jump back to loop if not 0
ret ;else return
addbit: push af ;preserve screen data ld a,(pixel) ;load A with pixel add b ;add what we've found to it ld (pixel),a ;store it again pop af ;restore screen data ret
storeit: ld hl,(spraddr) ;load HL with sprite address ld a,(pixel) ;load pixel into A ld (hl),a ;load sprite address with pixel value inc hl ;increase sprite address ld (spraddr),hl ;and store it for next time ret
nextline: ld a,l ;load A with L (low byte of screen address) sub 8 ;subtract the 8 bytes we've already done ld l,a ;and load it back into L ld a,8 ;load A with 8 add a,h ;which means we add &800 for the next pixel line down ld h,a ;write new address back to H ret nc ;and return if no overflow
ld de,&C050 ;else loop back round to top of screen plus one pixel line add hl,de ;by adding &C000+&50 to HL ret
scraddr: dw &C000 ;screen address to grab CPC sprite from
pixel: db &0 ;pixel store
spraddr: dw &9000 ;address Plus sprite is stored (&100 bytes in length) </geshi>