I've noticed in videos of computers using slow ISA video (think Trident 8900), or PCem with video set to slow 16-bit animations redraw in a "flipping page from top to bottom" (or opposite) kind of way even when the framerate is relatively fast (meaning ~20 fps). I never saw this with PCI/AGP/PCIe card even at 1 fps. What's the deal?
How weird, I'd guess extreme screen tearing, absurdly low refresh rate of slow RAMDAC.
On a slow ISA VGA card it can take longer to write a full buffer of data than it does to display it. For example, in 320x200x256 64kb needs to be copied per frame. A slow card, such as a TVGA9000B, only allows 2mb/sec of bandwidth. On such a card it will take 31.25 msec to write a frame to the card, while the card displays a frame in 1/70th of a second (~14 msec). In this case at least two partial frames will be seen - showing as tearing.
So games using double buffering and drawing the entire screen would perform horribly, but text-based stuff and tile-based games (when not scrolling) should perform well enough? Seems fair.
To provide examples:
http://m.youtube.com/watch?v=dyUmny3wj-
http://m.youtube.com/watch?v=1-TTW3fPypg
Wouldn't 2 MB/s speed mean the Trident is running in 8-bit mode? The tearing seems to be more evident in SVGA than VGA. 640x480 in 256 colors would mean 6.5 fps at 2 MB/s and the framerate here is obviously higher than that.
They are probably not redrawing the entire screen, just the modified bits and even then the cards are just too slow that you see the redrawing of the changed parts. If you had more videoram and the game supported some fancier double buffering in videoram solution you shouldn't be seeing any screen tearing. Same as games in dos that make better use of the vga memory to have double buffering videoram instead of burst copying the frame from main memory to video memory each frame.
Thanks, very interesting 😀 . I never saw such an effect on our family 386 many years ago with a TVGA 8900, but those were VGA 320x200x256 games, not SVGA. Is there any list that has bandwidth values for videocards?
http://advsys.net/ken/utils.htm#KPEG (DOS)
For old VGA cards, KPEG will display more than 256 colors by TIME DITHERING (320*200*8 mode only) By time dithering,
I mean that KPEG averages the colors across 32 display pages and quickly flipping between them. (Doesn't support "progressive" JPEG yet.)
since 320*200*8 mode uses A000:0000 ram, is the KPEG shows tearing on your system?
-fffuuu
