[REBOL] Re: Display device DPI
From: atruter:hih:au at: 22-Oct-2002 16:41
Hi Brett,
> I'm not entirely clear what your goal is. If it is important that on
screen
> image is very accurate then you probably need a different application
from a
> word processor to render it properly.
My goal is to generate reports, which include images, that can be viewed,
edited and printed via a simple WYSIWYG editor. PDF is good, but read only,
and MS formats are obscure to say the least. HTML does not let you control
page breaks, so I have been looking at XML file format editors.
The "problem" is fairly simple to demonstrate. Use the following code to
generate a 100x100 png:
save/png %test.png to-image layout [origin 0 btn yellow 100x100
100x100
font [size: 18]]
Open a word-processor of your choice and insert this image. Look at the
properties of the image and note the dimensions (usually given in cm's or
inches). Do the same thing on an alternate display device (eg. an LCD) and
note how the same image has different dimensions. Changing the printer or
screen resolution does not effect this, it seems to be a property of the
display device itself (dot pitch?). The practical consequence of this is
that the above image saved (using one type of display device) will appear
pixelated on another. I found this out the hard way after comparing
documentation (screen-shots) written on a 17" CRT (at 1024x768) and viewed
on a 15" LCD (also at 1024x768).
> Alternatively maybe you can arrange it
> so that your documents with images are always viewed at 100% and ensure
that
> your images do not need scaling in this situation (Ie prescale them).
Exactly what I am trying to do, using code like:
; create a 6"x6" image from a "larger" image
DPI: 120 ; 96 for LCD, 100? for a notebook
image-size: to-pair (DPI * 6 DPI * 6)
save/png bin to-image layout [origin 0 image image-size %1.jpg effect
[crop 500x0 800x800]]
This works great so long as:
a) you don't generate the report on a CRT then view it on an LCD (or vice
versa)
b) you can work out what the DPI should be
This last step is easy enough. On my CRT here I load the 100x100 image from
before and observe that its dimensions are 2.11cm. (2.54 / 2.11) * 100 120 DPI. Doing
the same thing on an LCD yields 96DPI and a notebook yields
100DPI. Changing the screen res or printer does not seem to effect this.
The real answer is to use vector graphics. In the meantime, these apps
(AbiWord, MS*Word, etc) are using *something* to say "these here 100 pixels
equate to x centimetres (or inches) on this here display device". I just
want to access the same "magic number" these boys use! ;)
Regards,
Ashley
