IBM did not offer a color display at the launch of the PC. It was assumed that most users would connect the CGA card directly to a color composite monitor or to a TV via an RF modulator. While NTSC-standard color displays could support up to 240 visible lines without interlacing, a large portion of the visible area of the screen on these devices could be obscured by the physical shell surrounding the glass monitor. Previous home computers from Apple and Atari only supported 192 pixels as a result. IBM's 200 pixels was hardly likely to tax the newer displays of the 1980s, which showed a more rectangular viewing area than the more circular TV screens of earlier decades. In 1983 IBM released its 5153 PC Color Display, which provided official RGBI support from the Corporation. This monitor had a vertical size control, which could accomodate 240 lines quite easily. The CGA card hardware and 16KB of RAM, could not.
|Jill of the Jungle CGA 320x200x4 Mode|
IBM's EGA card also supported a 320x200x16 graphics mode, and this was by far the most frequently used graphics mode in EGA-supporting games. The EGA could also support a 640x200x16 graphics mode and was backwards compatible with CGA at the BIOS level. Tandy Graphics and EGA graphics would almost invariably look the same, but the hardware was very different. Tandy's Enhanced Graphics Adapter, introduced with the Tandy 1000TL and 1000SL also supported a 640x200x16 mode, but few programs used it as it was not EGA compatible. Amstrad's CGA adapter also supported a unique 640x200x16 mode, but few programs used it.
|Jill of the Jungle EGA 320x220x16 Mode|
In 1987, IBM introduced the VGA and new corresponding monitors. VGA supported a 320x200x256 graphics mode with a palette of 262,144 colors available. This was the mode most frequently used by games. VGA was backwards compatible with EGA at the register level and CGA at the BIOS level. It also supported a 640x480x16 mode, but far fewer DOS games used it. Windows 3.0 and above would use it for its default graphics display. A new monitor was required to display the much larger color palette of the VGA compared to the CGA and EGA. Analog color monitor outputs were used. The high resolution display supported a 31.5KHz scan rate and 70Hz vertical refresh rates for all VGA modes, including emulated modes, except for the 640x480 mode, which used 60Hz. 200-line modes would be double scanned, with each pixel being double-clocked and each vertical line being repeated to fill up the refresh rate. This gives a different kind of scan-line structure compared with earlier monitor.
|Jill of the Jungle VGA 320x200x256 Mode|
Most VGA games only supported 320x200x256 graphics mode. The BIOS mode, Mode 13h, was easy to program for but somewhat limited. Eventually programmers found out how to create custom resolutions by using the VGA hardware registers, the so-called Mode X. Mode X typically comprised of 320x240 pixels, which gave square pixels. Epic Pinball and The Last Vikings used this mode. Some games used a 320x400 graphics mode, which was easy to obtain on VGA hardware. Programmers had to be careful to ensure that their custom mode would be compatible with the wide variety of VGA adapters in the marketplace. Standard 256KB VGA can support any combination of 320 or 360 horizontal pixels by 200, 240 or 350 vertical pixels with 256 colors.
I have included screenshots of Jill of the Jungle above. Jill supports all 320x200 in all three color modes. The game does not support 320x200x16 graphics on an IBM PCjr. or Tandy 1000 Graphics adapter (few if any shareware games supported the unique graphics modes these adapters), it will use the 320x200x4 mode instead. Except for Jill's face, the graphics are virtually identical, pixel-wise, across the three modes. Many games down-convert the graphics using an algorithm to eliminate the need to have two extra sets of graphics images or tiles on the disk.
320x200 has a 1.6 pixel aspect ratio. To get truly square pixels, a 4:3 display must have letterboxing. A 16:10 1280x800, 1920x1200 or 2560x1600 widescreen monitor can display the resolution perfectly using nearest-neighbor interpolation. However, when the resolution was used all displays were 4:3, and most users would stretch the 200 vertical lines to fill up the screen. Instead of perfectly square pixels, you would get pixels 1.2 times as vertical compared with the horizontal width on a monitor where the vertical width had been stretched to the edges of the monitor. Most graphic artists assumed this and adjusted their graphics accordingly, but not all did.
An illustrative example. Look at this screenshot of Elite Plus, using the VGA 320x200x256 graphics mode.
You can see that the circle is a circle in the 1.6:1 aspect ratio. But when converted to a 4:3 aspect ratio :
The circle has become an oval. Thus it would seem that the 1.6:1 aspect ratio is correct for this game.
Let's look at another game, LOOM. Here is a screenshot with a clearly spherical object in it :
Looks a bit squat in the 1.6:1 aspect ratio. If we stretch the aspect ratio :
Now the crystal ball looks like a sphere in a 4:3 aspect ratio. Click the 4:3 images for an undistorted, pixel-perfect resize but huge (1600x1200) version of the screenshot.
Even when Windows 95 was released, most graphically intensive games for the PC were still being released for DOS. Only in 1997, with the acceptance of 3D accelerators, DirectX and the undeniable dominance of the Windows platform did high-performance games finally require Windows. Most games up to this point either only supported 320x200 (DOOM, Daggerfall) or offered it as the default resolution (Duke Nukem 3D, Quake). SVGA was not well-supported because each chipset had its own way of offering higher resolution modes, and by the time VESA modes were widely supported, Windows 95 was the gaming OS of choice. At this point, 640x400x256 and 640x480x256 graphics were the norm.