clipka <ano### [at] anonymousorg> wrote:

>
> If you use a non-native display resolution, then this can indeed happen,
> due to the non-linear blending by the monitor.

Ah, so it seems that the only way to avoid these 'blending' problems is to leave
the monitor at its native resolution. I understand now. Thanks.

It's worth mentioning that I never saw these kinds of problems on my old CRT
monitors, when adjusting gamma in POV-Ray. (Or, if present, it was subtle, or
else I never noticed.) I'm *guessing* that the reason has to do with the
arrangement of a CRT's red/green/blue phosphors (and its 'shadow mask'),
compared to a modern LCD/LED monitor. If I'm not mistaken, a CRT has something
like a triangular(?) arrangement of phosphors (depending on the brand), whereas
modern monitors have pixels in a strict linear X/Y configuration. I'm thinking
that the gamma chart's thin horizontal lines have a better chance of being
'faithfully' reproduced on a CRT (well, in some sense.) Or so my mind's eye
tells me ;-)
>
> In POV-Ray's anti-aliasing, this is already taken into account:
> Anti-aliasing nowadays works in linear colour space...

Excellent! That's a subtle bit of re-work magic that most of us would never have
thought of. (OK, maybe just me!)

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Le 17-10-16 à 15:20, Kenneth a écrit :
> clipka <ano### [at] anonymousorg> wrote:
> 
>>
>> If you use a non-native display resolution, then this can indeed happen,
>> due to the non-linear blending by the monitor.
> 
> Ah, so it seems that the only way to avoid these 'blending' problems is to leave
> the monitor at its native resolution. I understand now. Thanks.
> 
> It's worth mentioning that I never saw these kinds of problems on my old CRT
> monitors, when adjusting gamma in POV-Ray. (Or, if present, it was subtle, or
> else I never noticed.) I'm *guessing* that the reason has to do with the
> arrangement of a CRT's red/green/blue phosphors (and its 'shadow mask'),
> compared to a modern LCD/LED monitor. If I'm not mistaken, a CRT has something
> like a triangular(?) arrangement of phosphors (depending on the brand), whereas
> modern monitors have pixels in a strict linear X/Y configuration. I'm thinking
> that the gamma chart's thin horizontal lines have a better chance of being
> 'faithfully' reproduced on a CRT (well, in some sense.) Or so my mind's eye
> tells me ;-)
>>
>> In POV-Ray's anti-aliasing, this is already taken into account:
>> Anti-aliasing nowadays works in linear colour space...
> 
> Excellent! That's a subtle bit of re-work magic that most of us would never have
> thought of. (OK, maybe just me!)
> 
> 
> 
> 

On a CTR monitor, each pixel is spread over several groups of phosphore 
patches. Also, the edges of the pixels are somewhat fuzzy, making them 
blend toggether, at least, a little.
There are two way the phosphore patches are aranged : Triangular array 
of circular spots (apperture mask), and groups of rectangular "spots" 
ranged side by side (whire mesh).

The charts need to be displayed at one image pixel = one screen pixel. 
If not, you always get some interpolation that will ruin the test.

When using an LCD monitor, it's always beter to use it's native 
resolution, or an integer fraction of that.
In your case, that would be 1920 X 1280, 960 x 640, 640 x 427,...
ANY other resolution will require the use of interpolation.

If that makes things to small, then, you can adjust the PPI setting of 
your display, or use larger fonts.

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Am 16.10.2017 um 21:20 schrieb Kenneth:

> Ah, so it seems that the only way to avoid these 'blending' problems is to leave
> the monitor at its native resolution. I understand now. Thanks.
> 
> It's worth mentioning that I never saw these kinds of problems on my old CRT
> monitors, when adjusting gamma in POV-Ray. (Or, if present, it was subtle, or
> else I never noticed.) I'm *guessing* that the reason has to do with the
> arrangement of a CRT's red/green/blue phosphors (and its 'shadow mask'),
> compared to a modern LCD/LED monitor. If I'm not mistaken, a CRT has something
> like a triangular(?) arrangement of phosphors (depending on the brand), whereas
> modern monitors have pixels in a strict linear X/Y configuration. I'm thinking
> that the gamma chart's thin horizontal lines have a better chance of being
> 'faithfully' reproduced on a CRT (well, in some sense.) Or so my mind's eye
> tells me ;-)

It's not related to the arrangement of "display pixels"; with CRTs, both
the standard triangular grid as well as Sony's "trinitron" side-by-side
arrangement were fine with respect to blending.

Which is good, because controlling the cathode ray to such a precision
that it would exactly line up with the grid would be unreasonably difficult.

As a matter of fact, in a CRT there is a lot of blending going on,
because the dot projected by the cathode ray is necessarily a bit fuzzy,
and doesn't hit the grid openings precisely.

But this blending occurs on a number-of-electrons basis, to which the
physical brightness of the phosphors is (almost, I guess) directly
proportional, so it's linear.


(Vertical lines would be a different matter on a CRT, because they
demand quick modulation of the cathode ray intensity, which may
introduce additional non-linear distortions. That's why gamma test
images favour horizontal lines over vertical ones or a checkerboard
pattern.)

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Alain <kua### [at] videotronca> wrote:
>
> On a CTR monitor, each pixel is spread over several groups of phosphore
> patches. Also, the edges of the pixels are somewhat fuzzy, making them
> blend together, at least, a little.

There's one aspect of a CRT that I still don't quite understand: The electron
gun(s)--their beams--can be *deflected* using buttons on the CRT... meaning, you
can 'squash and stretch' the image, so that it completely fills the screen (or
not!) But the acreen's aperture mask or wire mask (with its discrete holes for
the phosphors)is a fixed thing. So, I'm wondering how the squashed-and-stretched
electon beams still manage to correctly 'line up' with those phophors (and it
seems that they do, or the color mix on the screen would be completely messed
up-- which it isn't.) I had always assumed that the magnetic bending of the
beams was done in a continuously smooth way; it *appears* to be smooth. But is
the squashing-and-stretching actually done in discreet steps, so that the
individual  electron beams always *see* their own proper phosphors?

>
> The charts need to be displayed at one image pixel = one screen pixel.
> If not, you always get some interpolation that will ruin the test.
>
> When using an LCD monitor, it's always better to use its native
> resolution, or an integer fraction of that.
> In your case, that would be 1920 X 1280, 960 x 640, 640 x 427,...
> ANY other resolution will require the use of interpolation.

Yes, I see now that my use of 1600 X 900 was causing blending/interpolation (the
'incorrect' color blending, due to the non-linear 2.2-gamma of the monitor).
>
> If that makes things too small, then, you can adjust the PPI setting of
> your display, or use larger fonts.

Yes, the small text size (and overall smaller image presentation) was my
original reason for choosing a non-native resolution. It was an old habit, based
on my CRT days (and the fact that my eyes aren't what they used to be!) But I
can change at least the text size in other ways.

I used to set my CRT monitors at 800 X 600!

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On 17/10/2017 12:23, Kenneth wrote:
> Alain <kua### [at] videotronca> wrote:
>>
>> On a CTR monitor, each pixel is spread over several groups of phosphore
>> patches. Also, the edges of the pixels are somewhat fuzzy, making them
>> blend together, at least, a little.
> 
> There's one aspect of a CRT that I still don't quite understand: The electron
> gun(s)--their beams--can be *deflected* using buttons on the CRT... meaning, you
> can 'squash and stretch' the image, so that it completely fills the screen (or
> not!) But the acreen's aperture mask or wire mask (with its discrete holes for
> the phosphors)is a fixed thing. So, I'm wondering how the squashed-and-stretched
> electon beams still manage to correctly 'line up' with those phophors (and it
> seems that they do, or the color mix on the screen would be completely messed
> up-- which it isn't.) I had always assumed that the magnetic bending of the
> beams was done in a continuously smooth way; it *appears* to be smooth. But is
> the squashing-and-stretching actually done in discreet steps, so that the
> individual  electron beams always *see* their own proper phosphors?
> 
Just guessing here. The current going to the Focusing coils and 
Deflection coils are controlled digitally. I think the conversion would 
happen before the coils and use a lookup table for the different parameters.



-- 

Regards
     Stephen

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Am 17.10.2017 um 13:23 schrieb Kenneth:
> Alain <kua### [at] videotronca> wrote:
>>
>> On a CTR monitor, each pixel is spread over several groups of phosphore
>> patches. Also, the edges of the pixels are somewhat fuzzy, making them
>> blend together, at least, a little.
> 
> There's one aspect of a CRT that I still don't quite understand: The electron
> gun(s)--their beams--can be *deflected* using buttons on the CRT... meaning, you
> can 'squash and stretch' the image, so that it completely fills the screen (or
> not!) But the acreen's aperture mask or wire mask (with its discrete holes for
> the phosphors)is a fixed thing. So, I'm wondering how the squashed-and-stretched
> electon beams still manage to correctly 'line up' with those phophors (and it
> seems that they do, or the color mix on the screen would be completely messed
> up-- which it isn't.) I had always assumed that the magnetic bending of the
> beams was done in a continuously smooth way; it *appears* to be smooth. But is
> the squashing-and-stretching actually done in discreet steps, so that the
> individual  electron beams always *see* their own proper phosphors?

The trick is that there's nothing special about the squashing and
stretching.

During normal operation, the electron beam traces a zig-zag pattern
across the screen (of which only the zig is visible, as the electron gun
is effectively turned off during the zag). This would be virtually
impossible to align with the aperture mask, so no such attempt is even made.

The squashing and stretching is just a linear scaling of the beam's
current deflection on its way across the screen, so no magic is
happening there to the beam.

This regular deflection is achieved by one set of deflection elements
(either charged plates or magnets).

The real magic is that the three electron guns in the CRT are carefully
arranged (and possibly equipped with additional tunable deflection
elements and associated electronics), such that each beam hits the
aperture mask at a specific well-defined angle. The mask is carefully
aligned with the phosphor dot matrix and has only one aperture per
phosphor dot triplet, with the angles of the beams chosen in such a
manner that each hits its corresponding dot from the triplet as
precisely as technically feasible.

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clipka <ano### [at] anonymousorg> wrote:
>
> During normal operation, the electron beam traces a zig-zag pattern
> across the screen (of which only the zig is visible, as the electron gun
> is effectively turned off during the zag)...

Right-- the 'blanking signal', to allow the elctron gun(s) to zip back to their
start positions.

> ...This would be virtually impossible to align with the aperture mask
> so no such attempt is even made.
>
> ...The mask is carefully
> aligned with the phosphor dot matrix and has only one aperture per
> phosphor dot triplet, with the angles of the beams chosen in such a
> manner that each hits its corresponding dot from the triplet as
> precisely as technically feasible.

So the aperture mask has only ONE hole per color triplet.  :-0   THANKS, Clipka;
you've cleared up my LONG-standing misconception--I've always assumed the mask
had a hole for each and every phophor! Funny thing: I've been doing some
research about this-- 'CRTs', 'color television', 'Trinitron tubes', 'aperture
masks' etc. And not ONE of the sources I looked at mentioned this
single-aperture-per-triplet fact. So now, the slight 'angle-ing' of each
electron beam makes perfect sense, as to how it hits its proper phosphors on the
screen through that single hole.

A Eureka moment!

And getting back to the subject of gamma 2.2 non-linear blending of colors: It
occured to me that POV-ray's focal_blur camera might be mistaken by some users
for actual 'blurring'-- when it's really just lots of discreet 'randomized'
camera rays, shot out in a stochastic fashion. I.e., there's no actual
*blurring* of scene colors to cause the gamma 'darkening' effect.

But that raises a question in my mind: If a focal_blur camera has it
blur_samples set at a high value (like 500, for example), do any of those
discrete samples actually overlap? IF so, then I would imagine that there's some
color 'averagaing' going on behind-the-scenes, of the overlapped rays. Does that
relate to the gamma 2.2 color-blending problem? (Hmm, probably not-- because the
'averaging'-- if done-- is done *within* POV-ray, before the resulting pixel
ever reaches the monitor.)

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Just because I always happen to serendipitously stumble upon things that are
related:

http://makeanddo4d.com/spreadsheet/
https://www.andrewt.net/megapixel/

( Gamma!  :D )


Which I discovered through
https://www.youtube.com/watch?v=UBX2QQHlQ_I

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"Kenneth" <kdw### [at] gmailcom> wrote:

> And getting back to the subject of gamma 2.2 non-linear blending of colors: It
> occured to me that POV-ray's focal_blur camera might be mistaken by some users
> for actual 'blurring'-- when it's really just lots of discreet 'randomized'
> camera rays, shot out in a stochastic fashion. I.e., there's no actual
> *blurring* of scene colors to cause the gamma 'darkening' effect.
>
> But that raises a question in my mind: If a focal_blur camera has it
> blur_samples set at a high value (like 500, for example), do any of those
> discrete samples actually overlap? IF so, then I would imagine that there's some
> color 'averagaing' going on behind-the-scenes, of the overlapped rays. Does that
> relate to the gamma 2.2 color-blending problem? (Hmm, probably not-- because the
> 'averaging'-- if done-- is done *within* POV-ray, before the resulting pixel
> ever reaches the monitor.)

The focal blur feature /is/ potentially problematic with respect to gamma; but
as it is performed well within POV-Ray's render engine, it is fine as long as
`assumed_gamma 1.0` is used.

As a matter of fact the potentially problematic operations aren't limited to
blurring, but encompass all operations where brightness values are added,
because the equation

    a = b + c

is /not/ equivalent to

    a^G = b^G + c^G

(except for a few special cases).

Blurring is problematic because it is an averaging operation, which in turn is
essentially an addition combined with a constant multiplication.

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SO... getting back to POV-Ray's built-in gamma set-up chart...

As that chart was made many eons ago, I had a desire to see if it is still
'accurate' in the modern world ;-)  Leaving no stone unturned, I copied that
image and took it into both my (older) Photoshop, and a recent version of GIMP--
 to check not the thin horizontal bars, but the gray 'gamma' swatches that they
are compared against.  (BTW, both of my apps are working in sRGB color/gamma
space.) Using the 'eyedropper' tools there, I measured the gamma 2.2 swatch. Its
brightness value in both apps reads as 186/255, or 0.72941    And 0.72941^2.2 =
0.49950, or almost exactly 0.5

I can't say that I know exactly what I'm doing with this experiment-- ha!-- but
is  0.72941  the correct value for the gamma 2.2 swatch? (I assume it is, of
course, but I wanted an expert's opinion.)

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