[mythtv-users] Resolution / Aspect Ratio Mismatch - Proper Setup?

Michael T. Dean mtdean at thirdcontact.com
Thu Sep 21 19:43:43 UTC 2006

On 09/21/06 15:12, David Rees wrote:

>On 9/20/06, Michael T. Dean <mtdean at thirdcontact.com> wrote:
>>Current SVN can handle non-square pixels.  However, you must specify a
>>DisplaySize such that the calculated aspect ratio is that of your
>>physical display (i.e. if you have a 16:9 TV, make sure the DisplaySize
>>you provide has a 16:9 ratio).  Any DPI (even different DPI's on the X
>>and Y axes) works.
>Interesting, though that doesn't help me much as I'm running 0.20...
>Happen to know if that feature will be folded into 0.20 fixes?

It depends.  It's really a new feature, so it shouldn't be.  However, 
it's possible that Isaac may get tired of people talking about 
DPI/square-pixel constraints on MythTV and put it in there, anyway.  
(That's why he fixed it in SVN.)

>>If you're using NVIDIA display drivers, be very careful--as they will
>>ignore DisplaySize and DPI by default.
>I found that the one I'm using will use either DisplaySize or DPI
If NVIDIA drivers 8756+, only if you disable EDID detection of display 
size/DPI or if your display doesn't report EDID information about 
display size/DPI.

> just
>fine, but not both (if DPI is specified DisplaySize is ignored).
Right.  Let's put it this way:

If you have a 32" x 18" display and are running it at 1920 x 1080 pixels 
(=display dots), how many dots are there in an inch?

You can't specify--and use--both.  DisplaySize should be specified such 
that it's proportional to the actual display size (i.e. same aspect 
ratio as the physical display) and--once using a version of MythTV 
without a DPI dependence--DPI should /not/ be specified.  If you want to 
specify DPI, you have to do the math to ensure that the resultant 
calculated DisplaySize is the proper ratio (basically the inverse of the 
calculation described above).

This will be the most fun part of 0.21:  many distros have /broken/ X 
start scripts which pass the -dpi argument to X (which will override the 
DisplaySize/DPI settings they put in their X config files); recent 
NVIDIA drivers completely ignore -dpi, DisplaySize, and DPI if the 
display reports EDID information unless that capability is explicitly 
disabled; older NVIDIA drivers (and, TTBOMK, all other drivers) ignore 
DisplaySize if DPI is specified, and since people have gotten used to 
specifying 100x100DPI and generally don't research the changes necessary 
to successfully use the new version...  And, somehow, we'll have to 
teach all the MythTV users to do things the right way now.  (Although, 
even if they do things wrong, they won't notice any difference in 
before/after behavior--which means they won't gain the benefit of losing 
the dependence on DPI (properly calculated aspect ratios from software 
players--even with non-square pixels).)


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