ScrollVPort double buffer problem with garbage pixels

[balrogsoft]

Hello.

I'm working on a system friendly game engine for Amiga, from AmigaOS 2.0 to the new gen. compatible systems, I have tested my engine on MorphOS, EUAE and my Amiga 600. But I have a problem with my double buffer implementation, it uses ScrollVPort trick with a doubled screen height, it show some garbage pixels at the end of one of the buffers on real Amiga or emulators. I think that it could be related with overscan, but I figure out where is the problem, I can't find the reason, maybe someone can help me with my problem, this is a simple code showing my problem (compiled with SAS/C 6.58):

#include <stdlib.h>
#include <stdio.h>
#include <string.h>

#include <libraries/lowlevel.h> 
#include <libraries/asl.h>
#include <proto/asl.h>
#include <proto/lowlevel.h> 
#include <proto/exec.h>
#include <proto/intuition.h>
#include <proto/graphics.h>
#include <proto/dos.h>
#include <exec/memory.h>    

#define DEPTH   8

struct Library *AslBase =NULL;
						
UWORD__chip EmptyPointer[] = {0, 0, 0, 0};	

int main(void)
{
	struct ScreenModeRequester *screenReq;
	struct Screen *screen;
	struct Window *window;
	LONG width = 0, height = 0, frame = 0, frameOffset = 0, key = 0, x, y, n;
	AslBase =OpenLibrary("asl.library",38);
	if (AslBase)
	{
		
		screenReq = AllocAslRequest(ASL_ScreenModeRequest, NULL);
	
		AslRequestTags(screenReq, NULL); 
		
		width = screenReq->sm_DisplayWidth;
		height=screenReq->sm_DisplayHeight;
		
		screen= OpenScreenTags(NULL,
					   SA_DisplayID, screenReq->sm_DisplayID,
					   SA_Left,		 0,
					   SA_Top,		 0,
					   SA_Width,     width,
					   SA_Height,    height<<1,
					   SA_Depth,     DEPTH,
					  // SA_Type,	     CUSTOMSCREEN, 
					   SA_Quiet,     TRUE,
					   SA_Overscan,  OSCAN_STANDARD,
					   SA_AutoScroll, FALSE,
					   TAG_DONE);
					   
		window =OpenWindowTags(NULL,
									WA_CustomScreen, screen,
									WA_Title, NULL,
									WA_Flags, WFLG_BORDERLESS | WFLG_REPORTMOUSE | WFLG_ACTIVATE,
									WA_IDCMP, 0,
									TAG_DONE);
									
		FreeAslRequest(screenReq);         
				
		CloseLibrary(AslBase);
	}
	if (screen && window)
	{	
		SetPointer(window, EmptyPointer, 1L, 1L, 0L, 0L);

		do
		{   
			key = GetKey();
			
			RectFill(&screen->RastPort, 0, frameOffset, width, frameOffset+height);
			
			
			frame ^= 1;
			frameOffset = frame*height;
			screen->ViewPort.RasInfo->RyOffset = frameOffset;
			
			ScrollVPort(&screen->ViewPort);
			WaitBOVP(&screen->ViewPort);
			
		} while (key != 69);
		
		CloseWindow(window);
		CloseScreen(screen);
	}

	return RETURN_OK;
}

This is a screenshot of EUAE showing this problem, it appears at the end of one buffer, on the last 5-6 rows of pixels.

[balrogsoft]

I have found the solution on CyberAnim13.lha from aminet, it uses a ScrollVPort double buffer, and overscan was the problem, here is the correct source for a ScrollVPort, it works on MorphOS, EUAE, and my Amiga 600.

#include <stdlib.h>
#include <stdio.h>
#include <string.h>

#include <libraries/lowlevel.h> 
#include <libraries/asl.h>

#include <proto/asl.h>
#include <proto/lowlevel.h> 
#include <proto/exec.h>
#include <proto/intuition.h>
#include <proto/graphics.h>
#include <proto/dos.h>

#include <exec/memory.h>    

#define DEPTH   5

struct Library *AslBase =NULL;
						
UWORD__chip EmptyPointer[] = {0, 0, 0, 0};	

int main(void)
{
	struct ScreenModeRequester *screenReq;
	struct Screen *screen;
	struct Window *window;
	LONG width = 0, height = 0, frame = 0, frameOffset = 0, key = 0, x, y, n;
	AslBase =OpenLibrary("asl.library",38);
	if (AslBase)
	{
		struct DimensionInfo dimsinfo;
		LONG oscan_height;
		void *handle;
		
		screenReq = AllocAslRequest(ASL_ScreenModeRequest, NULL);
		
		AslRequestTags(screenReq, NULL); 
		
		width = screenReq->sm_DisplayWidth;
		height= screenReq->sm_DisplayHeight;

		handle = FindDisplayInfo (screenReq->sm_DisplayID);
		GetDisplayInfoData (handle, (UBYTE *)&dimsinfo,
                                   sizeof(struct DimensionInfo), DTAG_DIMS,
                                   NULL);
		
		oscan_height =  dimsinfo.MaxOScan.MaxY - dimsinfo.MaxOScan.MinY + 1;
		
		if (oscan_height > height)
			height =oscan_height;
		
		screen= OpenScreenTags(NULL,
					   SA_DisplayID, screenReq->sm_DisplayID,
					   SA_Left,		 0,
					   SA_Top,		 0,
					   SA_Width,     width,
					   SA_Height,    height<<1,
					   SA_Depth,     DEPTH,
					   SA_Type,	     CUSTOMSCREEN,
					   SA_Draggable, FALSE,
					   SA_Exclusive, TRUE,
					   SA_AutoScroll,FALSE,
					   TAG_DONE);
					   
		window =OpenWindowTags(NULL,
									WA_CustomScreen, screen,
									WA_Title, NULL,
									WA_Flags, WFLG_BORDERLESS | WFLG_REPORTMOUSE | WFLG_ACTIVATE,
									WA_IDCMP, 0,
									TAG_DONE);
									
		FreeAslRequest(screenReq);         
				
		CloseLibrary(AslBase);
	}
	if (screen && window)
	{	
		SetPointer(window, EmptyPointer, 1L, 1L, 0L, 0L);

		do
		{   
			key = GetKey();
				
			RectFill(&screen->RastPort, 0, frameOffset, width, frameOffset+height);
			
			ScrollVPort(&screen->ViewPort);
			WaitBOVP(&screen->ViewPort);
			
			frame ^= 1;
			frameOffset = frame*height;
			screen->ViewPort.RasInfo->RyOffset = frameOffset;
		
			
		} while (key != 69);
		
		CloseWindow(window);
		CloseScreen(screen);
	}

	return RETURN_OK;
}

[Thorham]

You should get rid of WaitBOVP, because it uses busy waiting. Better to setup a copper list that generates a copper interrupt at the bottom of the screen, and have an interrupt server signal your program.

[Toni Wilen]

If you don't want to bother with user copperlists and chipset specific stuff, just use WaitTOF(), it does not busy wait and difference between WaitTOF() and WaitBOVP() will be only few scanlines in overscan modes. (and probably there is no difference in RTG modes)

[balrogsoft]

Originally Posted by Thorham

You should get rid of WaitBOVP, because it uses busy waiting. Better to setup a copper list that generates a copper interrupt at the bottom of the screen, and have an interrupt server signal your program.

I think that it won't work on new gen systems like MorphOS or AmigaOS4.

Originally Posted by Toni Wilen

If you don't want to bother with user copperlists and chipset specific stuff, just use WaitTOF(), it does not busy wait and difference between WaitTOF() and WaitBOVP() will be only few scanlines in overscan modes. (and probably there is no difference in RTG modes)

I will give a try, I have tested my engine with a vertical scroller with soft sprites and it works at very decent speed using WaitBOVP on my Amiga 600 with ACA 630@25mhz.

My engine is not aimed to low end Amigas, it uses system functions for drawing operations to make it compatible with new gen systems. Its features are:

Screen management (up to 8 bits depth only) with primitive gfx functions, chunky 2 planar support (via kalms c2p and WritePixelArray8 for CGFX).
Software sprites, with background restoration, and simple sprite mask generation for blitting.
Mod music support via ptreplay.library.
Joystick support via lowlevel.library.

I want to port my Engine to Android and other platforms, and develop games for Amiga and other platforms like Android and iPhone with the same source code.

This is one example for my engine, I think it is pretty easy to use:

    if (bitmap_init() & joy_init() & music_init())
    { 
        Bitmap *bm = bm_create(WIDTH,HEIGHT,DEPTH);
        FrameBuffer *fb = fb_init(320, 256, 8);
        GraphicContext *gc = gc_createWithFrameBuffer(fb);
        GraphicContext *bm_gc = gc_createWithBitmap(bm);
        int x = 100, y = 100, fr = 0, key = 0, i, m = 0, 
            cwidth = fb->width, cheight = fb->height;

        UBYTE *chunky = (UBYTE*)malloc(WIDTH*HEIGHT);

        for (y = 0; y < HEIGHT; y++) 
        {
            for (x=0; x < WIDTH; x++)
            {
                chunky[y*WIDTH+x] = (x-y)&63;
            }
        }
        
        m=0;
        do
        {     
            key = GetKey();
            // C2P to bitmap
            gc_c2p(bm_gc, chunky, 0, 0, WIDTH, HEIGHT, TRUE);
            // C2P to screen bitmap
            gc_c2p(gc, chunky, 100, 130, WIDTH, HEIGHT, TRUE);
            
            gc_drawBitmap(gc, bm, 0, 0, 0);
            
            gc_setColor(gc, 11);
            
            gc_fillRect(gc, 10+(m%64),20, 10, 10);
            m++;
            
            fb_flush(fb);
            
        } while (key != 69); 
        
        free(chunky);
        gc_dealloc(gc);
        gc_dealloc(bm_gc);
        fb_dealloc(fb);
        bm_dealloc(bm);
    }

    joy_dealloc();
    bitmap_dealloc();
    music_dealloc();

[Thorham]

Originally Posted by balrogsoft

I will give a try, I have tested my engine with a vertical scroller with soft sprites and it works at very decent speed using WaitBOVP on my Amiga 600 with ACA 630@25mhz.

Yeah, try it. WaitBOVP is a huge waste of CPU cycles, and should not ever be used on Amiga hardware.