[html5] a question about high-definition

Ian Hickson ian at hixie.ch
Mon Apr 27 15:08:54 PDT 2009

On Mon, 20 Apr 2009, caoyipeng wrote:
> there is a description at canvas in html5 :"On high-definition displays, 
> for instance, the user agent may internally use a bitmap with two device 
> pixels per unit in the coordinate space, so that the rendering remains 
> at high quality throughout."
> Why a bitmap with two device pixels per unit is high-definition 
> displays? I think it is a low-definition displays.for example:there is a 
> picture on the screen and the resolution is 1600*1200,and then we change 
> the resolution as 800*600,we will find that the picture will be 
> larger,because when the resolution is 1600*1200,we supposed the picture 
> is a device pixel per unit;when change the resolution as 800*600,the 
> picture is two device pixels per unit,so the picture will be larger,but 
> the picture is in a low resolution,so i think a bitmap with two device 
> pixels per unit is low-definition displays..

In your example, the two device pixels per unit are always the same value. 
Thus they are one logical device pixel per unit, and it is in fact the 
same resolution (in terms of uniquely addressable device pixels per CSS 
pixel) as the "higher resoution" display.

If you actually had a 1600x1200 display but each unit could have more than 
one colour (i.e. because it was actually two device pixels per unit) then 
that would be a higher-resolution display.

Ian Hickson               U+1047E                )\._.,--....,'``.    fL
http://ln.hixie.ch/       U+263A                /,   _.. \   _\  ;`._ ,.
Things that are impossible just take longer.   `._.-(,_..'--(,_..'`-.;.'

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