How does VLC make 720x480 snapshots?

[markfilipak]

I have a nerdy question that may be of interest to other video nerds.

I've made many, many video frame captures in order to determine raw frame size & aspect ratio of film telecines.

Imagine a DVD (made from a film telecine) having a non-black, video frame image that is not full 720x480 either horizontally or vertically. Such an image will have black areas all around the movie image, left-right & top-bottom. The edges are not abrupt. They show gradual luminance edges all around, left-right & top-bottom. The question is: What is the cause of the gradual luminance edges? This is important to me because I seek the best possible numbers for the horizontal and vertical frame size of the original telecine. Directly related to this issue is how VLC is set up to take snapshots.

VLC setting in order to capture screen frames:
'Advanced Preferences' 'Video' 'Video snapshot width' [ -1 ]
'Advanced Preferences' 'Video' 'Video snapshot height' [ -1 ]
For 16:9-480i30 DVDs, this captures 853x480 snapshots.

VLC setting in order to capture video frames:
'Advanced Preferences' 'Video' 'Video snapshot width' [ 0 ]
'Advanced Preferences' 'Video' 'Video snapshot height' [ 0 ]
For 16:9-480i30 DVDs, this captures 720x480 snapshots.

Of interest to me is 720x480 snapshots. Because the 720x480 capture should be directly from a video buffer, it should have abrupt luminance edges, however, it has gradual luminance edges. Why is that?

I can think of 4 reasons:
[1] The 720x480 luminance gradient is smoothed as a consequence of limited telecine bandwidth (prior to MPEG-2 encoding).
[2] The 720x480 luminance gradient smoothing is a property of the telecine lab's MPEG-2 encoder.
[3] The 720x480 luminance gradient is smoothed by VLC's MPEG-2 decoder as the video frame buffer is filled.
[4] VLC actually captures the screen, then scales the image to 720x480.

The answer is probably the sum of [1]+[2]+[3], but I'm only speculating.
The answer may be [4].

What is the answer? How does VLC make 720x480 snapshots?

-Factors I've contemplated-
Case [1] can explain luminance gradients of left-right edges, but not of top-bottom edges.
Case [2] can explain luminance gradients of left-right edges & top-bottom edges, but only if blocking is clearly evident.
Case [3] can explain luminance gradients of left-right edges & top-bottom edges, and interlace may be a strong factor for top-bottom luminace gradient.
Blu-ray captures always have abrupt luminance edges. That cannot be a consequence of better codec (MPEG-4 AVC v. MPEG-2) or higher resolution (1920 pix/line v. 720 pix/line), but it can be (and probably is) a consequence of progressive frames (24p) v. interlaced fields (30i).
Or, Case [4], VLC may actually capture the screen, then scale the image to 720x480.

Warm Regards,
Mark.