Aspect ratio

Vanilla Doom runs in VGA 320x200 mode, which is an 16:10 aspect ratio. This causes problems on some modern computers which only support 4:3 ratio graphics modes. Some source ports support aspect ratio correction, which attempts to remedy this.

Almost all monitors have a physical size where the width and height of the screen are in a 4:3 ratio. However, Vanilla Doom used a 320x200 screen resolution. Because of the difference in aspect ratio between the screen resolution and the physical dimensions of the screen, the 320x200 screen is stretched. The effect of this is that the pixels displayed on the screen are not square: they are each slightly taller than they are wide.

Some modern graphics cards and monitors only support 4:3 screen resolutions, and so Doom's native 320x200 resolution is not supported. In these cases, the screen may be displayed within a higher screen resolution, resulting in black borders at the top and bottom of the screen. For example, 320x200 mode games may actually run in 320x240 mode (a 4:3 resolution).

The effect of running with borders like this is that the screen appears squashed compared to the original Vanilla Doom, as everything physically appears 16% shorter. It appears that the doom graphics have been specifically designed for the 320x200 resolution; for example, the appearance of the title screen better matches the Doom box art. Similarly, the shape of the status bar face better matches the dimensions of a real human face when run in the stretched mode.

Aspect ratio correction
Some source ports include aspect ratio correction. This performs stretching of the screen's contents when running in a 4:3 screen mode, so that the graphics appear as they appeared in Vanilla Doom. This is generally accomplished in one of two ways:


 * Graphical scaling : In this approach, the individual components of the screen are scaled to fit the 4:3 mode. For example, the renderer is run in a way such that the game view is stretched to fit the screen.  Other graphics, such as the status bar, are drawn stretched.  A disadvantage to this approach is that some of the graphics (such as the status bar) may appear distorted due to the scaling. This is the approach taken by ZDoom.


 * Whole-screen correction : With whole-screen correction, the screen is rendered to a 16:10 size. The low-level graphics code then stretches the entire screen to fit the 4:3 graphics mode.  A disadvantage of this approach is that the screen may appear blurry due to the stretching.  This is the approach used by Chocolate Doom for aspect ratio correction.

Incorrect aspect ratio
Some source ports do not handle 4:3 screen modes correctly. An example is Doom 95, which renders the screen with the wrong aspect ratio when running in 640x480 mode. The result is that the player can see more when running in 640x480 mode than in 320x200 or 640x400 mode.

Widescreen modes
Widescreen modes add an extra level of complication to the aspect ratio issue. Some modern monitors have a physical 16:10 aspect ratio, rather than the "traditional" 4:3 aspect ratio. In this scenario, the aspect ratio correction should still be performed. The extra horizontal space can be dealt with either through horizontal borders, or through increasing the horizontal viewing angle.