HLSL is an acronym for “High-Level Shader Language”.
HLSL creates image effects calculating how each pixel should look like, using the graphics card of the computer.
Some of the effects are scaling, scanlines, bloom, focus, glass reflection and curving of the edges of the screen, which are usually used to simulate old CRT monitors.
The HLSL settings are on
mame.ini file that is on the same folder of the MAME binary.
It's also possible to change the settings on the fly, with sliders: when running a game, press TAB and select the "Slider controls" option.
Another option is creating
INI files with the names of the games in the
INI subfolder, for example,
mslug.ini for the "Metal Slug" game.
Many settings can be found on the Internet with accompanying screenshots of the result, so it's not necessary to create your own.
To create a screenshot, press ALT and F12 simultaneously. If only F12 is pressed, the screenshot saved won't have HLSL effects.