'Effect overlay' should be subtractive/multiplied

Hi,
the current implementation of 'effect overlay' is wrong. It 'adds' the aperture 
masks that make the screen go brighter with stronger effect instead of going 
darker (subtracted/multiplied). With this implementation you cannot achieve 
intended black scanlines. 

Its interesting though. Some effects look ok with adding and don't suffer from 
losing too much brightness/luminance. Maybe you can add both worlds or even 
allow different blend modes (add/sub/multiply - see below). I think the 
'multiply' blend mode is the most important one. A global 'brightness booster' 
option could be useful here to compensate the lost brightness caused by the 
scanlines (or darkened pixels).

Multiply blend mode (see last sentence - its a generic method not just used by 
Photoshop):
"However, of the four blend modes in the Darken group, one of them stands high 
above the others, and that’s the Multiply blend mode. The Multiply blend mode 
is one of the most important and widely-used blend modes in all of Photoshop, 
whether you’re doing traditional photo retouching work or creating some wild 
and crazy special effect. It’s unique among all the blend modes in that 
it’s the only one named after the actual math that Photoshop performs behind 
the scenes when you have the Multiply mode selected. Photoshop takes the colors 
from the layer that’s set to the Multiply blend mode and multiplies them by 
the colors on the layer(s) below it, then divides them by 255 to give us the 
result."

The overscale feature is fantastic. Finally a proper integer scaling 
implementation without filtering. You might consider using this feature in 
combination with an additional bilinear fit-to-screen option. That's what some 
mame derivates do as well (prescaling). This results in a good tradeoff between 
sharp picture and fit-to-screen. The integer prescaling reduces the filter 
blurring alot.

Btw, i've created a DAT file with Romlister with stripped 'clones', 'not 
working', mahjong and quiz files. ROMs are down to 10GB (2776 files) (instead 
of 40GB+). If you're searching for a reasonable dat just let me know.

regards
Original issue reported on code.google.com by Maniaxx2...@gmail.com on 22 Dec 2014 at 12:24
Dannys9333 / imame4all

'Effect overlay' should be subtractive/multiplied #318
