REAL amount of image data in BluRay movies displayed with high aspect ratio.

[Elroc]

Let me phrase this in as many ways possible to ensure no confusion (I hope)

Short version:
When watching a BluRay movie, which is 1080p, but due to aspect ratio contains black bars at the top and bottom of the screen, how much ACTUAL 'scanlines' of IMAGE data is present (or rather available on disc, regardless of TV set).

Long version:
1080p refers to the amount of 'scanlines' (1080) as well as how they are filled (progressive), this I get.
When watching 'widescreen' movies, its common to get black bars, as the aspect ratio is different from that of the screen. This I ALSO get
BUT
When watching a 1080p widescreen movie, does 1080p include X amount of scanlines which are effectively black, OR, does the original image STILL contain 1080p lines of image data, but the lines are just longer (Higher horizontal resolution)

Just to make sure the question is understood, heres putting it a different way:
Native 1080P tv's USUALLY have a resolution of 1920x1080 (HorizontalxVertical). In the case mentioned above, does the true picture encoded on disc have the resolution:
a) 1920x(less than 1080) OR
b) (more than 1920)x1080

Bottom line, lets say I dont care about losing some image along the sides of the screen but want to zoom it to fill the screen, will the widescreen picture have enough horizontal scanlines (1080) to fill the screen truley, or will I be stretching a lower amount of scanlines?