[mythtv-users] OT: How to calculate displaysize

[Michael T. Dean]

Brian C. Huffman wrote:

>Discussion about mythtv <mythtv-users at mythtv.org> wrote: 
>  
>
>>2) You can calculate the appropriate "DisplaySize" based on the 
>>resolution you're using:
>>
>>Given 1 inch = 25.4millimeters
>>DisplaySize is specified in millimeters
>>You're using a resolution of X x Y:
>>
>>DisplaySize should be set to the truncated integer value (note, you have 
>>to do the math yourself, don't specify the equation in your X 
>>configuration file):
>>
>>DisplaySize: ((X / 100) * 25.4) ((Y / 100) * 25.4)
>>
>>So, for 1024x768:
>>
>>1024 / 100 = 10.24 * 25.4 = 260.096 = 260
>>768 / 100 = 7.68 * 25.4 = 195.5072 = 195
>>
>>DisplaySize  260 195
>>
>>Some round instead of truncating.  I've found that truncating gives best 
>>results...
>>    
>>
>This doesn't make sense to me (probably why I'm asking the question in the first
>place).  You're calculating displaysize based on the resolution rather than the
>physical dimensions of the display.
>
Yep.  (Keep reading...)

>The man page for xorg.conf says the following:
>
>DisplaySize width height
>    This optional entry gives the width and height, in millimetres, of the
>picture area of the monitor. If given this is used to calculate the horizontal
>and vertical pitch (DPI) of the screen.
>
>If this option is calculated this way, the Xserver could do it by itself?
>
Only if the X server has access to that information.  If you're using a 
computer monitor (i.e. not a TV), it will almost definitely have access 
to that information (which is why you don't typically specify 
DisplaySize for your desktop system).  If you're using an NTSC/PAL TV, 
it almost definitely won't.  If you're using an "HDTV monitor" it may or 
may not depending on which output you're using (i.e. will with VGA/DVI, 
won't with Composite).  If you're using a converter box (i.e. VGA->NTSC 
or VGA->Component or something) it almost definitely won't.

>Honestly, logic would tell me to get out a tape rule and measure my TV, but
>obviously the information that I'm reading is telling me something different. 
>Still confused.  :-(
>  
>
That would work if you want your fonts to display at the appropriate 
sizes for your DisplaySize--as if it were being used as a computer 
monitor 2 feet in front of the user.  Since most people watch TV from 
farther away than 2 feet (i.e. at least 5 to 10 feet), the calculated 
(or measured) DisplaySize isn't appropriate--fonts will be unreadably 
small.

Also, the whole reason it's important to specify exactly 
100dpi--regardless of your actual DisplaySize--is because Myth themes 
were designed to use 100dpi.  To design the themes, it was critical to 
have a way to convert dimenstions between font sizes (i.e. "points" = 
1/72 inch) and pixels so that graphic images (which use pixels) line up 
with text.  Since position information is specified in the themes at 
pixel locations--and not calculated dynamically at runtime by Myth--we 
have to pick one DPI setting to use.  The actual value is unimportant; 
however, there can be only one value to ensure all themes line up 
properly.  The value that was chosen is 100dpi.

So why didn't the Myth devs choose a DPI setting that was more "natural" 
and use huge font sizes so you could read the text from 5-10 feet away?  
If they had done this, then those who do use their monitors that are 2 
feet away from them for watching TV with Myth would find the fonts way 
too large.  For example, I set my DisplaySize on my Myth box to 100dpi 
and output to my TV, but I also use X redisplay to change frontend 
settings/etc. sometime and I know some people actually install MythTV on 
their desktops to watch TV.  With the font-sizes that are used in the 
themes, fonts look good on my 27in TV from 5 feet away and in a window 
on my 19in monitor from 2 feet away...

All in all, I'd say JDS (and everyone else involved in thinking this 
stuff through) did an excellent job of making it work everywhere for 
everyone.  :)   (At least, it works for everyone who configures their 
system appropriately.)

Note that many people find that they don't like their font sizes when 
using 100dpi, so they go back and change their DisplaySize until they 
get the font size they want.  The right way to do this is to a) make 
sure you have the requested font installed (as the X font server will 
give a "close-enough" font if the requested font doesn't exist, and the 
given font metrics may not be anything like the requested font in real 
life) and b) modify the ui.xml for your theme to specify the desired 
font sizes (and/or font names).

Mike