[mythtv-users] Scaled BluRay video and untouched PGS subtitles wrong size

[Andre MythTV]

> On 11 Jan 2017, at 20:12, faginbagin <mythtv at hbuus.com> wrote:
> 
> Hi Andre,
> 
> On 1/11/2017 4:59 AM, Andre MythTV wrote:
>> 
>>> On 7 Jan 2017, at 20:21, faginbagin <mythtv at hbuus.com> wrote:
>>> 
>>> We got a TV series on BluRay for Christmas and I've been using
>>> makemkv to rip and handbrake to transcode down to a smaller size by
>>> scaling down the width and height in half (from 1920x1080 to
>>> 960x540, 1/4 the area).
>> 
>> That’s not a particularly standard TV frame size so you’ll be
>> triggering bugs in any player you try.
> 
> I've tried three, mythfrontend, mplayer and vlc. Only mythfrontend has this problem.

Fair enough, mythfrontend needs fixing, in the mean time I was just suggesting to stick to frame sizes that are standard so there’s a better chance players have been tested with them.
>> 
> 
> I've got both 720p and 1080p TVs, mythfrontend demonstrates the bug on both.

I suggested a 720p encode to (potentially) avoid the bug, not that a 720p or 1080p playback would fix it.

> 
>> Resizing should always be a last resort with compression, especially
>> if the source material is already compressed like Bluray or DVD.
> 
> This is a TV show, Prime Suspect, not a recent release movie filmed with Hi-Def digital cameras. Most episodes are copyright 1991-1996, the last two seasons were 2003 and 2006 respectively. According to this link:
> http://www.blu-ray.com/movies/Prime-Suspect-The-Complete-Collection-Blu-ray/72157/
> it was shot on Super 16 film, an analog technology.

Super 16 is borderline HD standard, it’s usually used to save money on productions destined for SDTV. With decent scanning, pre-processing and low compression like a bluray it’s usually nice enough to be an upgrade over DVD or SDTV.


> I have a hard time seeing any real difference in quality between the 1920x1080 and the 960x540 versions, even on the 1080p TV.

Very surprised at that, LCD TV a long way away I guess? Or maybe a plasma TV with a very good internal scaler?

> To me, it makes a lot of sense to do spatial scaling in whole numbers rather than to scale to a standard resolution involving scaling fractional pixels. I doubt that scaling to 1280x720 would have produced better quality or compression.

Doesn’t matter about whole numbers if the scaling is done well, if it’s done badly it will look bad regardless of the maths, the main things to avoid are small adjustments in size or ratio. This is why TVs with overscan turned on look so bad, a ~5% resize is very difficult to do well, anything over 20% change is usually good enough.  Anyway the point was you are scaling twice, once from 1920x1080 to 960x540 then back again in the TV to 1280x720 or 1920x1080 in the other TV. You are going to do a fractional resize do it properly over enough time in re-encoding software rather than in real time in a TV.

In fact if you drive the 720p TV from Myth it’s entirely possible the frontend runs at 1080p and there’s three lots of scaling going on!

I do many projects where the customer wants material scaled and encoded to precicely match their display technology, we always get the best results when frame sizes match exactly, it’s a general best practice thing in the industry. It’s becoming less important now that source material is coming along at at least HD and more likely 2k,4k,5k or better.

> 
>> Alternatively you could keep the makemkv rips and just enjoy the show
>> at the quality you have paid for… Bluray is already a seriously
>> compressed format and re-compressing already compressed material
>> doesn’t usually work too well.
>> 
>> If you wanted 960x540 you could have saved some money and bought the
>> DVDs!
> 
> There wasn't a big difference in price, and the link above says:
> "anyone who has only experienced Prime Suspect on DVD will find these Blu-ray presentations a quantum leap in image quality, though obviously less than one would expect from a contemporary production shot on hi-def video." So I opted to start with better quality source material and see if I could compress it enough so as to not exceed my current storage capacity without degrading video quality too much. I'm happy with the results scaling the resolution 4 to 1.

Fair enough, the comment was a little facetious, apologies, it’s always best to start with the best material available.


> 
> The subtitle issue is a nuisance, but not enough to re-transcode the entire series. If time permits, I might experiment with scaling to 1280x720 and see if mythfrontend still screws up the subtitles. I'm keeping one BluRay rip of one episode for tinkering and to get other opinions. Or, I might dig into the code and see if I can fix what I think is a bug. A fix that would probably never be reviewed and accepted if I'm the only one the the problem.

I work a lot with commercial software where finding a way to avoid the bug is the only possible solution in the available time frame, it’s great using open source software things get fixed much much faster and easier. If you are able to find the bug and fix it you are a better programmer than me.


Andre