BBC Basic Sprite Question

II have a small question.

Can someone give me an example code?

I would like to adapt a sprite to the resolution of the desktop.

Desktop is e.g. 1280×600 the Sprite 1024×300.

It should then be at the height of 600, width then accordingly…
And how can you read the height and width of a Sprites?

The simplest way is to use OS_SpriteOp.

• Get the length of the sprite file and add four.
• Allocate that much memory.
• Initialise it as a sprite area (SpriteOp 256+9).
• Load the sprite file into it (SpriteOp 256+10).
• Get the sprite information for width and height in pixels (SpriteOp 256 + 40).

https://www.riscosopen.org/wiki/documentation/show/OS_SpriteOp

Note that as you’ll be providing your own sprite area and referring to the sprite by name, you should add 256 to all of the calls, as indicated above. The linked page explains this.

[ the other way is to load the sprite header into memory and pull it apart, but note that sprites don’t store a width, they record how many words the scanline takes, along with left/right wastage (to indicate how many bytes are unused on either side) and the effective width is a combination of that and the bits per pixel of the image; it’s a lot more complicated ]

Get the length of the sprite file and add four.

Have you a short sample for that ?
I have read the documentation.
But I don’t really understand that.

A sprite area is very similar in content to a sprite file – the length of the sprite area is the first word (corresponding to the file length) then the data from the file follow. Once a sprite area is set up, you can use SpriteOp SWIs on it.

An alternative approach is to embed the sprite in a Draw file and then Render the Drawfile to the screen. That will display any sprite stretched to fit the area you choose, e.g. the whole screen.

Example using MakeDraw:

      10 : REM >ProgName
      20 : bufsize%=10001240:REM Nearly 10 Mbytes needed for draw buffer
      30 : REM This programme requires a minimum of 2Mbytes of memory
      40 : REM in addition to the bufsize% setting
      50 : DIM qbuf% 300
      60 : memrq%=2
      70 : IF memrq% THEN memrq%+=bufsize%/1000000:IF memrq%<3 THEN memrq%=3
      80 : REM If this much memory is not available then abort with an error
      90 : IF END-PAGE<memrq%*1000000 THEN END=PAGE+memrq%*1000000
     100 : DIM bufmd% bufsize%
     110 : makedraw%=TRUE
     120 : LOCAL ERROR
     130 : ON ERROR LOCAL makedraw%=FALSE
     140 : IF makedraw% THEN LIBRARY "<MD$Dir>.MakeDraw"
     150 : RESTORE ERROR
     160 : PROCmdInitialise(bufsize%-10000, bufmd%)
     170 : P=OPENIN("SpriteFile")
     180 : filelen%=EXT#P
     190 : IF P THEN CLOSE#P:P=0
     200 : PROCmdSprite(0,0,"SpriteFile",1,0):REM scale=1
     210 : PROCmdDiagramInfo(s%,len%)
     210 : PROCmdSaveDiagram("DrawFile"):REM saving is optional
     220 : SYS "DrawFile_Render",0,bufmd%,len%
     230 : DIM tranmat% 20
     240 : tranmat%!0=1<<16:REM x magnification
     250 : tranmat%!4=0
     260 : tranmat%!8=0
     270 : tranmat%!12=2<<16:REM magnify y by  factor 2
     280 : tranmat%!16=256*0:REM x=0 OS units
     290 : tranmat%!20=256*0:REM y=0 (0,0) is bottom left of screen
     300 : SYS "DrawFile_Render",0,bufmd%,len%,tranmat% 

https://github.com/gerph/riscos-examples/blob/master/graphics/sprites/PlotSprite%2Cfd1 covers many different ways.

Example using MakeDraw

170 : P=OPENIN("SpriteFile")
180 : filelen%=EXT#P
190 : IF P THEN CLOSE#P:P=0

The (sensible) test for P being zero at line 190 seems a little pointless¹, because absent any error handling which hasn’t been included in your example, you’ll get a crash at line 180 with “Handle is either illegal or has been closed” in the event that P is zero…

¹ Yes, it’s not wrong to include, because CLOSE# should never be called with a handle of zero. However, it would probably be better to do something like

file% = OPENIN("SpriteFile")
IF file% <> 0 THEN
  filelen% = EXT#file%
  CLOSE#file%
  file% = 0
ELSE
  filelen% = 0
ENDIF

and then gracefully exit if filelen% is zero at the end of it all.

Just to chuck another example into the mix, you’ll find a more Wimp oriented example of loading a sprite area here:

https://github.com/steve-fryatt/wimplib/blob/d5d4b0e280441b83d77e8e8ea674d673c112c4d6/src/WimpSprite.bbt#L28C1-L57C7