half a pixel in css ?

cybmole · 02-20-2011, 11:49 AM

i am looking at source for a book where css has entries like
p {
margin-top:4.5px;
margin-bottom:4.5px;
}

if px = pixel then how is half a pixel implemented - makes no sense to me

why would anyone code in fractions of a pixel ?

Jellby · 02-20-2011, 11:54 AM

Surprisingly, a pixel might be something different than a pixel:

Normally, the pixel unit refers to physical pixels of the viewing device. However, if the pixel density of the output device is very different from that of a typical computer display, the user agent should rescale pixel values.

From the CSS3 spec

cybmole · 02-20-2011, 12:03 PM

thanks for the link & the explanation - it's not clear what they consider to be a low resolution screen though as they give no screen size examples.
thus I am none the wiser as to what 0.5px actually translates to
a) on my Kindle screen
b) on my 1920x1200 pc screen.

Another reason to avoid px & stick with em measurements, I think!

though I have to admit that the book with the 4.5px borders looks good on both screens!

theducks · 02-20-2011, 01:17 PM

Quote:

Originally Posted by cybmole

thanks for the link & the explanation - it's not clear what they consider to be a low resolution screen though as they give no screen size examples.
thus I am none the wiser as to what 0.5px actually translates to
a) on my Kindle screen
b) on my 1920x1200 pc screen.

Another reason to avoid px & stick with em measurements, I think!

though I have to admit that the book with the 4.5px borders looks good on both screens!

px is a (mostly) fixed measurement
em is a scaled measurement.
If you Zoom , em values scale.

DaleDe · 02-23-2011, 02:18 PM

A pixel is fixed for most monochome devices but on a color device a pixel is really a color triad or in the case of e-Ink a color quad consisting for a 2x2 array. In these cases a sub-pixel is defined to be used for anti-aliasing and other tasks so a .5 pixel would make sense for color devices.

Dale

02-20-2011, 11:49 AM	#1
cybmole Wizard Posts: 3,720 Karma: 1759970 Join Date: Sep 2010 Device: none	half a pixel in css ? i am looking at source for a book where css has entries like p { margin-top:4.5px; margin-bottom:4.5px; } if px = pixel then how is half a pixel implemented - makes no sense to me why would anyone code in fractions of a pixel ?

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02-20-2011, 11:54 AM	#2
Jellby frumious Bandersnatch Posts: 7,549 Karma: 19500001 Join Date: Jan 2008 Location: Spaniard in Sweden Device: Cybook Orizon, Kobo Aura	Surprisingly, a pixel might be something different than a pixel: Normally, the pixel unit refers to physical pixels of the viewing device. However, if the pixel density of the output device is very different from that of a typical computer display, the user agent should rescale pixel values. From the CSS3 spec

02-20-2011, 12:03 PM	#3
cybmole Wizard Posts: 3,720 Karma: 1759970 Join Date: Sep 2010 Device: none	thanks for the link & the explanation - it's not clear what they consider to be a low resolution screen though as they give no screen size examples. thus I am none the wiser as to what 0.5px actually translates to a) on my Kindle screen b) on my 1920x1200 pc screen. Another reason to avoid px & stick with em measurements, I think! though I have to admit that the book with the 4.5px borders looks good on both screens!

02-23-2011, 02:18 PM	#5
DaleDe Grand Sorcerer Posts: 11,470 Karma: 13095790 Join Date: Aug 2007 Location: Grass Valley, CA Device: EB 1150, EZ Reader, Literati, iPad 2 & Air 2, iPhone 7	A pixel is fixed for most monochome devices but on a color device a pixel is really a color triad or in the case of e-Ink a color quad consisting for a 2x2 array. In these cases a sub-pixel is defined to be used for anti-aliasing and other tasks so a .5 pixel would make sense for color devices. Dale