screen banks

I have written a little programme to display two pages of a manual side by side and to ‘turn the page’ as an animated graphic.

I thus need to wite to one screen bank whilst displaying the other.

There is still a little flicker and if I press f12 then return whilst it is turning a page, the second screen bank disappears. Once the graphic is finished, I revert to one screen bank.

What is the right protocol for dealing with a mode change in a multi-tasking BASIC prgramme?

So far I have added the commands:

    Initialise:
    
     100 bank%=1

    In the multitasking 'null poll' bit:
    
    4060 : REM Change VDU output to alt bank
    4070 : SYS "OS_Byte",112,bank%
    4090 : bank%=bank% EOR 3
    4100 : REM change hardware to alt bank
    4110 : SYS "OS_Byte",113,bank%
    
    Once the animation is done:        
      
    4200 :   REM change VDU & hardware to default bank
    4210 :   SYS "OS_Byte",112,0
    4220 :   SYS "OS_Byte",113,0
    4230 :   REM current size = scnorm% , max size= r2%
    4240 :   SYS "OS_ChangeDynamicArea",2,-scnorm%
    
    When starting to turn the page:
    
    7420 :           REM Set up dual screen banks
    7430 :           SYS "OS_ReadDynamicArea",2+128 TO ,scnorm%,r2%
    7440 :           REM current size = scnorm% , max size= r2%
    7450 :           SYS "OS_ChangeDynamicArea",2,scnorm%
    7460 :           bank%=bank% EOR 3
    7470 :           REM Change VDU output to alt bank
    7480 :           SYS "OS_Byte",112,bank%
    7490 :           SYS "Wimp_ForceRedraw",-1,0,-2047000,2047000,2047000

When the control returns to the Wimp from the F12 CLI, anything could have happened on screen, so the Wimp just changes MODE.

I’d be tempted to look for a F12 keypress event, stop your animation, then pass it on.

Best check in 7450 : SYS “OS_ChangeDynamicArea”,2,scnorm% that you have actually changed that D.A. by enough to have the second bank of screen memory.

You could reduce tearing by using OS_Byte 19 to wait for VSync to change the hardware bank (though on many systems these days I believe it is faked)

As part of the documentation for the ZapRedraw module (v0.41) there is BASIC example, R-Example, which deals with mode changes. I am not sure if this is any help. I do not know whether it is relevant to your case, but another tack is to use the Drawfile module which I guess automatically copes with mode changes. There is an example, !Codex, in the RiscLua distribution, that uses the toolbox and displays pages taken from a list of drawfiles. Clicking Select or Adjust on the iconbar icon moves you forward or backward through the list.

I do use DrawFile_Render as part of the redraw code, see example below:

    2120 :         IF theta>PI/2 THEN
    2130 :           stX%=ddReg%!(oo%+16*page%-8)
    2140 :           fnX%=ddReg%!(oo%+16*page%-4)
    2150 :           a=-(1<<16)*COS(theta)*zoom
    2160 :           b=-0.1*(1<<16)*SIN(theta)*zoom
    2170 :           c=0
    2180 :           d=(1<<16)*zoom
    2190 :           e=-(ddReg%!24)*zoom+256*cx%+pw%*zoom+mm%*zoom
    2200 :           f=-(ddReg%!36)*zoom+256*cy%
    2210 :           e+=pw%*zoom*COS(theta):REM xc - r COS theta
    2220 :           f+=0.1*pw%*SIN(theta):REM yc + 0.1 r SIN theta
    2230 :         ELSE
    2240 :           stX%=ddReg%!(oo%+16*page%-8-16)
    2250 :           fnX%=ddReg%!(oo%+16*page%-4-16)
    2260 :           a=(1<<16)*COS(theta)*zoom
    2270 :           b=(1<<16)*0.1*SIN(theta)*zoom
    2280 :           c=0
    2290 :           d=(1<<16)*zoom
    2300 :           e=-(ddReg%!24)*zoom+256*cx%+pw%*zoom+mm%*zoom
    2310 :           f=-(ddReg%!36)*zoom+256*cy%
    2320 :         ENDIF
    2330 :       ENDIF
    2340 :     ENDIF
    2350 :     REM Now render it
    2360 :     !ddtran%=&10000*zoom
    2370 :     ddtran%!4=0
    2380 :     ddtran%!8=0
    2390 :     ddtran%!12=&10000*zoom
    2440 :     ddtran%!16=-(ddReg%!24)*zoom+256*cx%
    2450 :     ddtran%!20=-(ddReg%!36)*zoom+256*cy%
    2460 :     REM brown background
    2470 :     SYS "DrawFile_Render",0,bufmd%,ddEDraw%-bufmd%,ddtran%
    2480 :     REM LH page
    2490 :     IF stL%<>0 THEN
    2500 :       av$=STR$~(stL%)
    2510 :       ddtran%!16=-(ddReg%!24)*zoom+256*cx%+mm%*zoom
    2520 :       ddtran%!20=-(ddReg%!36)*zoom+256*cy%
    2530 :       SYS "DrawFile_Render",0,bufmd%+stL%,fnL%-stL%,ddtran%
    2540 :     ENDIF
    2550 :     ddtran%!16=-(ddReg%!24)*zoom+256*cx%+pw%*zoom+mm%*zoom
    2560 :     ddtran%!20=-(ddReg%!36)*zoom+256*cy%
    2570 :     SYS "DrawFile_Render",0,bufmd%+stR%,fnR%-stR%,ddtran%
    2580 :     IF stX%<>0 THEN
    2590 :       ddtran%!0=a
    2600 :       ddtran%!4=b
    2610 :       ddtran%!8=c
    2620 :       ddtran%!12=d
    2630 :       ddtran%!16=e
    2640 :       ddtran%!20=f
    2650 :       SYS "DrawFile_Render",0,bufmd%+stX%,fnX%-stX%,ddtran%
    2660 :     ENDIF
    2670 :     :
    2680 :     SYS "Wimp_GetRectangle",,block% TO flag
    2690 :   ENDWHILE

Your program is evidently doing a lot more than mine, with zooming and rotating. I have just tried changing mode with !Codex displaying a page and everything goes fine, so I guess that DrawFile_Render really does take care of mode changes automatically. This

local Drawfile_Render = \ (buf, buf_size)
       => \ (@, x, y)
   local b in @
   local u, v = 1<<16, 1<<8
   ![b + 256] = u
   ![b + 260] = 0
   ![b + 264] = 0
   ![b + 268] = u
   ![b + 272] = v*x
   ![b + 276] = v*y
   sys (0x45540, 0, buf, buf_size, b + 256, b + 28)     -- DrawFile_Render
   end end

What is the right protocol for dealing with a mode change in a multi-tasking BASIC prgramme?

You can detect a mode change (including caused by a press of F12) by looking out for Wimp message &400c1

I don’t know if that’s relevant for the flicker on desktop, as I’m only familiar with non desktop coding: Jeffrey told and showed me (and it’s implemented in some of my demo related code and his) to use tripple buffering. As far as I remember especially on the RPI’s it’s needed.

Many thanks for the help. The application !RingBound is now available from !Store.