Star Wars vector emulation missing “overdrive → defocus” effect in Death Star explosion

[rebroad]

MAME version

0.280

System information

Ubuntu 24.04
OpenGl/BGFX/Soft

INI configuration details

# Use BGFX vector renderer chain
video                   opengl
bgfx_screen_chains      vector

# Visual tuning
brightness              1.0
contrast                1.0
gamma                   1.3
waitvsync               1
syncrefresh             1

# Input devices
joystick                1
mouse                   1

Emulated system/software

Atari Star Wars (1983, vector arcade)

Incorrect behaviour

In the original Atari Star Wars arcade hardware, the Death Star explosion was not just a set of concentric circles. As described by original developer Jed Margolin (), the vector generator was given the ability to overdrive the monitor’s color inputs. This overdrive caused the CRT to draw more current than normal, which in turn dragged down the high voltage. That shift changed the ratio of Focus Grid to Anode voltage, defocusing the beam and producing a filled, “blooming” explosion.

Currently, MAME’s vector core and renderers do not emulate this coupling:

• Beam width is interpolated from intensity only, via a static sigmoid curve.
• The vector core and OpenGL/software renderers draw lines with fixed width/alpha, no HV/focus interaction.
• BGFX/HLSL have optional static bloom/defocus passes, but these are user-configurable post-processes, not tied to instantaneous current draw or segment intensity.
• Star Wars’ AVG intensity scaling is also reduced, which makes the explosion appear dim and overly crisp compared to original hardware.

The result is that the Death Star explosion looks too faint and too sharp in MAME, lacking the characteristic “defocused bright fill” of the arcade monitor.

Expected behaviour

It would be more accurate if MAME’s vector emulation supported intensity-driven defocus/bloom coupling, where high instantaneous beam current (overdrive) causes transient defocus. This would capture the unique look of the Star Wars explosion and other effects relying on CRT overdrive behavior.

Even an approximate simulation (e.g. intensity-driven defocus shader) would significantly improve visual fidelity for Star Wars and potentially other vector titles.

Steps to reproduce

1. Launch Atari Star Wars in MAME.
2. Play until the Death Star trench run.
3. Destroy the Death Star and watch the explosion sequence.
4. Compare against reference footage from original hardware (or descriptions from original developer Jed Margolin).