Artwork problems

[wpcmame]

Tried to ask this on the mame forum but din't get any reply.

My problem is that when I display space invaders with correct aspect ratio on the monitor the planet on the backdrop is very elliptical.

My theory is that this is because different pixel aspect ratios are used.

Here is my reasoning:

Space invaders got a game resolution of 224x240 displayed on a 3:4 monitor. This means that the game pixels got an aspect ratio of 7:10 (i.e. they are higher than wide).

The backdrop is created from a digital camera image which usually got square (1:1) pixels.

When the backdrop is displayed with 7:10 pixels it gets compressed horizontally (since the pixels are thinner).

If I run space invaders on my PC monitor (which got 1:1 pixels) the backdrop looks fine but the space invaders game display is no longer 3:4 aspect ratio (since 224x240 with 1:1 pixels becomes 14:15).

I tried to prove my theory by looking at pictures on the real game. Didn't find anything good but I found this flyer: http://pixeleye.de/spaceinvaders/flyers/spaceinvaders_1.jpg

If you look at the small picture at the lower left you can see that the backdrop planet meets the game edge at approximatly the level where the 30 point invader is. When running invaders in mame the planet meets the game edge further down at approximatly the 10 point invader.

To me this means that to much of the planet is visible within the game area in mame.

To test it I changed the values in the invaders.art file from
   position   = 0.0,-0.14,1.29,1.15
to
   position   = 0.0,-0.39,1.29,1.40

and now the backdrop looks fine.

Does anyone who know who created the invaders.art file or how the original values were created?