Re: [Devel] TrueType font-scaling

[Vadim Plessky]

On Monday 05 August 2002 5:24 pm, Owen Taylor wrote:
|  Vadim Plessky <address@hidden> writes:
|  > |
|  > |  Most Linux setups will either use the physical screen resolution
|  > |  (for Xft) or a fixed value of 75 or 100dpi (for rendering via the
|  > |  core  X protocol)
|  >
|  > Hi Owen!
|  >
|  > AFAIK several graphics adapters supported under XFree86 (ATI and Matrox,
|  > in particular) can auto-detect dpi during initialization / X startup.
|  > So, there is no need to pre-define 96dpi if user has modern graphics
|  > adapter.
|
|  A) It's very hard to tell _if_ the resolution was probed successfully,
|     and the default of XFree86 (75dpi, I think) is too low. Also,
|     sometimes the DPI will be "probed" but be wrong... we've seen cases
|     where cards will probe at 12  x 9dpi and things like that.
|
|  B) My conclusion was eventually that matching the physical screen
|     resolution is just wrong. Very few people care about how
|     many inches tall letters are on their screen. They care about:
|
|      - Angular size of the letters (think handheld or projector)
|      - If pixels are visible, how many pixels are in each letter
|
|     (Seee http://www.xfree86.org/pipermail/fonts/2002-June/001771.htmlm
|     for some detailed discussion of the issue by Keith Packard.)

Yes, I was following this discussion with great interest, but had no time to 
write valuable answer/opinion.
Now I have some time, so here it is:
***
dpi doesn't depend on actual distance from your eye to the viewing surface.
I base this statement on 200+ years of people's experience with typography & 
printing processes.
10pt Times is always has the same size on A4 (210x297mm, or 8.5"x11") paper 
sheet, and size of that text doesn't depend on eye's distance from the paper.
it would be printed at 10pt on 300dpi (old) laser printer [rounding errors may 
apply], 600dpi/1200dpi modern laser printer, or 2500dpi photo-typesetting 
machine.
8.5"x11" is 2550x3300 pixels in size at 300dpi, or 5100x6600 pixels at 600dpi
12pt = 1/6" (12/72, assuming 1"==72pt), or 50pixels in size at 300dpi.
As there is also baseline/inter-line interval, actual size would be ~10% less 
(say, 45pixels; you can check actual number of pixles by printing 12pt Times 
to PostScript file, and rendering that PostScript file in GhostScript->PNG 
image).
So, the real question for the screen/display is *how many dots it has per one 
mm (inch)".
*Number of dots* can be calculated easily - you can take it from XFree86's 
driver (or from MS Windows' Control Panel/Display Settings)
Difficult question is *what is exact viewing area* of your display.

I did following calculation for my display (15" TFT on notebook):
* width: about 31cm, or 12.205 inch (1" == 2.54cm)
horizontal resolution: 1024
dpi: 1024/12.205 = 84.8 dpi
* height: about 23cm, or 9.055 inch
vertical resolution: 768
dpi: 768/9.055 = 84.8 dpi

Hey, now I know my real dpi setting :-)
// I wrote 'about' 31cm, as I could not find ruler, neither in office or at 
home, so I was measuring with help of A4 paper sheet.

Now is the most difficult question.
Actual dpi (in my case) is 85dpi, but MS Windows defaults to 96dpi. (don't ask 
me why!)
What should typical user expect, especially newbiw coming to Linux for the 
first time (from Windows)? Same font size as in Windows.
That's why some time ago both Mozilla and Konqueror switched to *default* 
96dpi. In other cases, most web sites look like broken (you can check, for 
example, MS web sites - they will be unreadable with default 75 dpi setting 
(you can check with old Mozilla builds, or Konq 2.1).
So, for web browsers - it's ok to default at 96dpi, to keep users happy.

But for Word Processors/DTP programs - I strongly disagree with default 96dpi.
My measuremtn shows that default for 
15" TFT at 1024x768 is about 85dpi, or 
15" TFT at 1400x1050 is about 115dpi (such panels are present in high-end 
notebooks)
I think we should use XFree86's auto-detection process - but we need to check 
values on actual models first, and to see what is exact dpi range..

|
|  C) Unless padding and graphics scale along with the text, large
|     changes in the DPI used will destroy layouts.

Padding & Graphics scales in KWord 1.2beta, as KWord has very good WYSIWYG 
implementation.
On the other hand, MS Word 2000 doesn't have real WYSIWYG, and I think page 
layout canbe destroyed after zooming, especially at large scale factors (for 
example, 500%)

|
|  Regards,
|                                          Owen
|
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-- 

Vadim Plessky
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