[mythtv] Cropping

Michael T. Dean mtdean at thirdcontact.com
Fri Sep 16 20:43:24 UTC 2005

Tim McClarren wrote:

> I had posted a while back about a problem I was having with video 
> capture.
> Basically, I get about 10 black pixels on the left edge of captured 
> stream, and about 20 pixels at the bottom, which are mostly black, but 
> with some grey blocks in it.  I'm pretty sure it's entirely an issue 
> with my very cheap SAA7130 based capture card.

No.  It's an issue with NTSC (or PAL).  These analog formats have the 
unfortunate problem of having "ragged edges," sometimes with "rainbow 
stripes."  Therefore, when designing the specifications, the solution 
that was chosen was to factor in an amount of "overscan"--extra image 
size that would be shown outside the visible area of the screen.  
Because of this designed in overscan--which had a bit of an engineering 
factor built in--some of the extra lines of information were used to 
include non-visible data (i.e. closed-captions, etc.).  Therefore, you 
may see gray lines on the top of your video, too, that aren't a problem 
with your capture card.

In the DVD specification, there are 720x(whatever--480 for NTSC or 576 
for PAL) resolutions called "full D1", which are allowed to contain the 
ragged edges and are meant to be displayed overscanned.  There are also 
704x(whatever) resolutions, often called "cut D1", which are required to 
have clean edges and should not be overscanned.  Players are supposed to 
take this into account.

> ...
> and I don't have to use the crop filter on playback (which was no good 
> anyways, because I want the captured stream to be right

I would argue that it is right...

> , not to correct it in the viewer on playback,

Although that's how the specification was designed--to "correct" these 
ragged edges with the player by covering part of the picture tube with a 
bezel.  Therefore, correcting it on a computer outputting to a 
non-overscanned screen should be done by the player.

> which means I have to configure each front end seperately, and I have 
> to set up the transcoder to crop out those annoying bits if I want to 
> watch my stuff in some other format).

But each frontend typically has different playback characteristics 
requiring different settings, anyway.  If you crop the video at capture, 
it looks great when displayed on a digital projector or computer monitor 
without overscan, but you lose part of the image when displayed on a TV 
with overscan...

> Am I the only one who has this issue?  Do other cards capture pretty 
> much perfectly, right out of the box?

Only if they're not capturing NTSC/PAL.  :)  Note, though, that the 
source format (i.e. analog cable, digital cable/satellite with a 
receiver usign S-Video output, etc.) you're using has a big effect on 
how noticeable the issue is.

> Would it be worth having the UI in the "Capture Card" settings dialogs 
> to be able to specify a crop rectangle?

Not for me, but it wouldn't hurt me, either.  Just make sure you 
consider cards that don't support cropping.  If you do, and create the 
patch with defaults that work the same way as current recordings (i.e. 
no cropping), it's quite possible it will be accepted into the trunk.


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