Re: useful units (ACTION-27)

On  3-Feb-2009, at 12:14 , Silvia Pfeiffer wrote:

>
> On Tue, Feb 3, 2009 at 9:09 PM, Yves Lafon <ylafon@w3.org> wrote:
>> On Tue, 3 Feb 2009, Silvia Pfeiffer wrote:
>>
>>> I agree with the use of pixels and cm for spatial fragment  
>>> specifications.
>>> Maybe points, too, but I don't really see that as necessary.
>>
>> Meaning you are against using percentages ?
>
> Oh no, no. I am happy for them, too. As long as it is clear what they
> are calculated against. It should be the explicit or implicit width
> and height of the video viewport.


Pixels: definitely.

Percentage: definitely, with respect to video pixel sizes.

Centimeters and other real-world sizes (inches, points, etc): this  
would be good to have from a user point of view, but it has an  
interpretation problem. I don't know of any video formats that have a  
DPI setting. Or, at the very least: I know that there are a lot of  
video formats that don't have a DPI setting. This means that the  
mapping of centimeters to pixels will often be done on a whim of the  
server (such as: every video is 72 DPI). This is misleading at the  
very least, and may turn out to be wildly wrong in the future (if  
video formats acquire a DPI setting). This has happened with images in  
the past: lots of software that was DPI-agnostic just filled in "72  
DPI" or sometimes "100 DPI", leading to all sorts of undesirable  
results if you later used these images in software that is resolution- 
aware. In other words: we may cause more problems down the road by  
pretending to support centimeters than we solve today. So, I wouldn't  
be in favor of allowing centimeters.

That is, unless someone can refute my argument/provide a good use case/ 
etc,
--
Jack Jansen, <Jack.Jansen@cwi.nl>, http://www.cwi.nl/~jack
If I can't dance I don't want to be part of your revolution -- Emma  
Goldman