Aspect ratio

Vanilla Doom runs in VGA 320x200 mode. However, the pixels in this mode are not square: each pixel is 20% taller than it is wide. The game screen therefore fills the entire screen on a monitor with a 4:3 aspect ratio, although the dimensions of the screen mode itself are not themselves in a 4:3 ratio.

Most of the Doom in-game graphics appear to have been specifically designed for the 320x200 resolution; for example, the appearance of the title screen better matches the Doom box art when using the rectangular pixels unique this hardware video mode. Similarly, the shape of the status bar face better matches the dimensions of a real human face.

When the Doom graphics are displayed in a square pixel video mode (640x480 or 800x600, for example), they appear "squashed" relative to how they appear in the non-square pixel 320x200 VGA mode that Doom runs in. Problems arise on some modern computers that only support square pixel graphics modes. Some source ports support aspect ratio correction, which attempts to remedy this.

Viewing the 320x200 game, or screenshots thereof, inside a square-pixel resolution will result in the graphic appearing to be inappropriately flattened relative to its appearance on the original intended hardware.

If Doom's native 320x200 resolution is not supported, the screen may be displayed within a higher screen resolution, resulting in black borders at the top and bottom of the screen ("letterboxing"). For example, 320x200 mode games may actually run in 320x240 mode (a 4:3 resolution).

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 drawn at the correct aspect ratio.  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 the original 320x200 size. 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. The corresponding square-pixel resolution which appears correct for 320x200 is 320x240.

Note that the automap is not generated by the rendering engine, so it does not automatically inherit the aspect ratio correction of a given port (see figure).

Different aspect ratios
Some source ports handle 4:3 screen modes in a different fashion. An example is Doom 95, which renders the screen with a different type of aspect ratio correction when running in 640x480 mode. Instead of stretching the pixels, it expands the viewable area. 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 either be used to increase the horizontal viewing angle, or simply filled as a black border ("pillarboxing").