Re: Multi monitors in OS X

AES <siegman@xxxxxxxxxxxx> wrote:

In article <1i7izsx.ci95upcuj7h0N%dempson@xxxxxxxxxxxxx>,
dempson@xxxxxxxxxxxxx (David Empson) wrote:


One mode is "mirroring". This means that all of your monitors show
exactly the same image and they must be the same number of pixels.


Just to be ultra-precise here: When you say

". . . and they must be the same number of pixels",

I think you mean the two _images_ must have exactly the same number of
(logical?) pixels. (Would you call these "image pixels"?)

Each display screen may in fact have a quite different number N by M of
physical pixels in that screen's "native resolution" -- for example, if
they're LCDs and you were look at 'em up close through a microscope, and
count the little visible rectangles. The two _monitors_, or monitor
screens, will then display very different numbers of (physical) pixels.

Agreed. The computer will be generating a single image (at a certain
number of pixels horizontally and vertically), and both monitors must be
able to display that image. This doesn't necessarily correspond to the
physical pixel size on an LCD screen.

There are several possible ways this can be handled.

(a) If the resolution chosen by the user happens to be the native one
for the display, no problem - the image will fill the screen and be
"pixel perfect".

(b) If the monitor is able to support scaling an image at a different
resolution, and it offers other resolutions to the computer, then when
one of those resolutions is chosen by the user, the computer will
generate the image at that resolution and the monitor will scale it as
appropriate. This will result in averaging effects which cause a lower
quality image. Most monitors can scale a smaller image up, and some can
scale larger images down (within a limit). This is quite common with
projectors - my user group's one supports 1024x768 as its native
resolution but will scale down a 1280x960 image (or scale up 640x480 or
800x600).

(c) Some monitors have settings (or fixed behaviour) which let it
display a lower resolution image by using black bars instead of scaling
the image to fit. In particular, this is an option with the internal LCD
screen on Apple's laptops - if you choose a resolution which has a
narrower aspect ratio than the built-in screen (4:3 instead of 16:10),
you can select between a stretched view (which fills the screen but has
wide "pixels") or an unstretched view (which fills the screen vertically
but has black bars on the sides, and has square "pixels").

(d) If there is a resolution option available on one of the monitors
which is not supported by the other one, the computer may be able to
generate a higher resolution image for the other monitor, with black
bars on all sides. This might only be possible with some video cards or
chipsets.

And, the digital signals being shuttled to and around on the wiring on
two monitors or display screens may be quite different, and in essence
consist of, or are keeping track of and interconverting, quite different
numbers of pixels.

Inside the monitor, yes. The signal being sent from the computer to the
two monitors will be identical (ignoring analog vs digital), except in
case (d) above.

[And if so, is there any standard way to refer to these different kinds
of pixels?]

I suppose "physical" and "logical" pixels are as good a name as any.

I'm fussing about this because my experience as a user goes back to the
time when my Radio Shack Model 100 laptop, or my ADM-3 terminal, had an
absolutely fixed number of pixels --- you could almost see and count 'em
on the screen with the naked eye -- and there was no futzing around with
numbers of pixels beyond that. This same fixed number was hard-wired
into every part of the connected electronics.

So I tend to mystified by modern computers that seem to be able to
magically interconvert between any number of pixels.

To pose one very specific question: Suppose my Mac laptop has NL X ML
physical pixels on its screen, and is connected to an external display
or projector which has ND X MD physical pixels -- with the ratios N/M of
these horizontal and vertical pixel numbers not being exactly the same
-- and I turn on mirroring.

The Display panel in System Prefs then offers me multiple choices of N X
M for what it calls "Resolution" (as many as 14 different choices on my
present MacBook/20" Cinema Display setup).

Which Cinema Display model is that? All of Apple's models appear to be
1.6:1 aspect ratio, except for a few lower resolution modes.

According to MacTracker, the original "Apple Cinema Display" (22" LCD
from 1999 to 2003) has a peculiar 1600x1024 maximum resolution
(1.5625:1) but the other resolutions it allows are 1.6:1.

The MacBook's internal display is 1.6:1.

In my experience some of these choices fill both screens with exactly
the same image, extending exactly to the screen boundaries -- which
means the horiz/vert (H/V) ratio of the image as seen on one monitor
must be slightly distorted from the other if the two screens have
different native N/M ratios.

That will be using pixel averaging.

Other choices seem to keep the H/V pixel ratio the same on both screens
and scale the image on one or the other screen so the larger dimension
just fits, leaving narrow dark areas on the sides, or top and bottom, of
the display.

Yep. Black bars if the monitor decides the image size isn't close enough
to its native aspect ratio.

And still other choices seem to map the N X M pixels chosen in the
Resolution setting to exactly the same number of physical pixels on both
screens -- grossly underfilling the screen on devices with higher
resolution screens.

The monitor designer may have decided that extreme cases of stretching
look too pixelated, so they chose to only do letterboxing.

At least, I think I've seen all of the above at one time or another,
with different laptops and external display devices.

It's very confusing. Does Steve Jobs personally decide what happens
with each of these resolution choices?

The monitor designer for external displays, and I expect Steve has some
say in that for Apple's ones.

The internal display behaviour has been pretty consistent for several
years. My MacBook Pro has the same general behaviour as my five year old
PowerBook G4 did.

--
David Empson
dempson@xxxxxxxxxxxxx
.

Relevant Pages

  • Re: [SLE] Need advice
    ... Pixels are display device entities. ... Are you thinking of half-tone screens? ... the principals and the engineering practice. ... jagged because of low resolution, ...
    (SuSE)
  • Re: IIgs RGB to VGA experiment
    ... So you advocate a small monitor only to ... translate 70ns to displayable monitor pixels in 39+ microseconds, ... a display listed as "1080i" would have a pixel resolution of only 1366x768, which is clearly far less than the 1920x1080 that one might ... The second motive is to make sure that high resolution digital video ...
    (comp.sys.apple2)
  • Re: LCD vs CRT resolution
    ... a CRT are not a precise match to the pixels in the digital image you're trying to show. ... With a CRT it is only an approximate match. ... The phosphor dots of course won't move; but the spread of the beam (deliberate spread, if the CRT is smart enough to notice the lower input resolution) will result in an input pixel being shown as some blend of dots around the target. ... The experiment we need involves two computer-quality screens of each technology, comparable size, and two different resolutions. ...
    (uk.tech.digital-tv)
  • Re: Auto resize forms
    ... is on the left monitor, with the form open in design view. ... The forms still have be to designed to be usable at the lowest resolution ... screens. ...
    (microsoft.public.access.forms)
  • Re: screen resolution
    ... Check the spec on the monitor, I would guess that 1440 x 900 is out of ... After installing display ... usually best to keep the resolution at the native resolution of the ... Screen res determines how many pixels of information are displayed ...
    (microsoft.public.windows.mediacenter)