So, I got a bunch of DOS games and have been enjoying them immensely.

However, I don't really understand how DosBox stretches the image on my screen. I have a widescreen 16:10 monitor and DosBox games don't fill the vertical slice of the screen. To illustrate, I've composed a little image that can explain the situation a lot better.

I expect the game to fill the screen the way the green box would, but instead games fill the red box area, leaving a substantial slice on the bottom. If I switch DosBox's renderer to OpenGL, the blank slice moves to the top and the game's picture moves to the bottom, vertically.

I think ScummVM runs DOS 4:3 games in a way that fills the screen vertically and keeps the original ratio. Why do DosBox games have an extra horizontal black bar at the bottom of the picture?

Thanks in advance!

change aspect=false with aspect=true into .conf file

This is the single most misunderstood setting in DOSBox, all because of the name DOSBox developers chose.

"Aspect ratio" doesn't affect in any way the resolution display ratio of your game. All this setting does is make sure games with non-square pixels (e.g. those meant to run in 640x400 resolution, back when CRT monitors could stretch such pixels to make them non-square and thus fill the whole screen) are accounted for and displayed properly.

EDIT: Sorry, I made a mistake and thought you meant that your game is stretched all the way across the black screen. If it is red vs. green then it could be a case where for example 640x400 doesn't stretch to 640x480 and so aspect ratio set to true would help. If not, check my solution below and just set fullresolution to whatever you want.

The easiest way would be to edit the dosbox file and set "fullresolution" to whatever you want. Alternatively, check you graphic's card control panel and make sure "maintain aspect ratio" is set there.

https://www.dosbox.com/wiki/Dosbox.conf

A lot depends on what video output you're using, surface, overlay, ddraw, opengl(nb).
What I generally use in my dosbox configurations is:

[sdl]
fullscreen=true
fulldouble=false
fullresolution=desktop
windowresolution=original
output=ddraw

[render]
aspect=true

For instance surface video output doesn't scale properly when you're using fullscreenresolutions.